diff --git a/config/sync/core.entity_form_display.node.ligne_comptable.default.yml b/config/sync/core.entity_form_display.node.ligne_comptable.default.yml
index 0bacc0e..0a31aa3 100644
--- a/config/sync/core.entity_form_display.node.ligne_comptable.default.yml
+++ b/config/sync/core.entity_form_display.node.ligne_comptable.default.yml
@@ -12,6 +12,7 @@ dependencies:
     - field.field.node.ligne_comptable.field_notes
     - field.field.node.ligne_comptable.field_numero_facture
     - field.field.node.ligne_comptable.field_repartition
+    - field.field.node.ligne_comptable.field_tva
     - field.field.node.ligne_comptable.field_type_ligne
     - node.type.ligne_comptable
   module:
@@ -47,7 +48,7 @@ content:
     third_party_settings: {  }
   field_entree_liee:
     type: entity_reference_autocomplete
-    weight: 2
+    weight: 4
     region: content
     settings:
       match_operator: CONTAINS
@@ -57,7 +58,7 @@ content:
     third_party_settings: {  }
   field_flag:
     type: entity_reference_autocomplete_tags
-    weight: 8
+    weight: 10
     region: content
     settings:
       match_operator: CONTAINS
@@ -67,21 +68,21 @@ content:
     third_party_settings: {  }
   field_montant_ht:
     type: number
-    weight: 3
+    weight: 5
     region: content
     settings:
       placeholder: ''
     third_party_settings: {  }
   field_montant_ttc:
     type: number
-    weight: 4
+    weight: 7
     region: content
     settings:
       placeholder: ''
     third_party_settings: {  }
   field_notes:
     type: string_textarea
-    weight: 6
+    weight: 9
     region: content
     settings:
       rows: 5
@@ -97,7 +98,7 @@ content:
     third_party_settings: {  }
   field_repartition:
     type: paragraphs
-    weight: 5
+    weight: 8
     region: content
     settings:
       title: Répartition
@@ -113,6 +114,13 @@ content:
         collapse_edit_all: collapse_edit_all
         duplicate: duplicate
     third_party_settings: {  }
+  field_tva:
+    type: number
+    weight: 6
+    region: content
+    settings:
+      placeholder: ''
+    third_party_settings: {  }
   field_type_ligne:
     type: options_select
     weight: 1
diff --git a/config/sync/core.entity_view_display.node.ligne_comptable.default.yml b/config/sync/core.entity_view_display.node.ligne_comptable.default.yml
index a015afe..2d5cfb8 100644
--- a/config/sync/core.entity_view_display.node.ligne_comptable.default.yml
+++ b/config/sync/core.entity_view_display.node.ligne_comptable.default.yml
@@ -12,6 +12,7 @@ dependencies:
     - field.field.node.ligne_comptable.field_notes
     - field.field.node.ligne_comptable.field_numero_facture
     - field.field.node.ligne_comptable.field_repartition
+    - field.field.node.ligne_comptable.field_tva
     - field.field.node.ligne_comptable.field_type_ligne
     - node.type.ligne_comptable
   module:
@@ -55,7 +56,7 @@ content:
     settings:
       link: true
     third_party_settings: {  }
-    weight: 8
+    weight: 10
     region: content
   field_montant_ht:
     type: number_decimal
@@ -66,7 +67,7 @@ content:
       scale: 2
       prefix_suffix: true
     third_party_settings: {  }
-    weight: 3
+    weight: 5
     region: content
   field_montant_ttc:
     type: number_decimal
@@ -77,14 +78,14 @@ content:
       scale: 2
       prefix_suffix: true
     third_party_settings: {  }
-    weight: 4
+    weight: 7
     region: content
   field_notes:
     type: basic_string
     label: above
     settings: {  }
     third_party_settings: {  }
-    weight: 6
+    weight: 9
     region: content
   field_numero_facture:
     type: string
@@ -102,7 +103,18 @@ content:
       view_mode: default
       link: ''
     third_party_settings: {  }
-    weight: 5
+    weight: 8
+    region: content
+  field_tva:
+    type: number_decimal
+    label: above
+    settings:
+      thousand_separator: ''
+      decimal_separator: .
+      scale: 2
+      prefix_suffix: true
+    third_party_settings: {  }
+    weight: 6
     region: content
   field_type_ligne:
     type: list_default
diff --git a/config/sync/field.field.node.ligne_comptable.field_tva.yml b/config/sync/field.field.node.ligne_comptable.field_tva.yml
new file mode 100644
index 0000000..2501fe3
--- /dev/null
+++ b/config/sync/field.field.node.ligne_comptable.field_tva.yml
@@ -0,0 +1,25 @@
+uuid: 9e9fc15c-9f7b-466a-bab0-fc6fe802a22f
+langcode: en
+status: true
+dependencies:
+  config:
+    - field.storage.node.field_tva
+    - node.type.ligne_comptable
+id: node.ligne_comptable.field_tva
+field_name: field_tva
+entity_type: node
+bundle: ligne_comptable
+label: 'TVA (%)'
+description: ''
+required: false
+translatable: true
+default_value:
+  -
+    value: 0.0
+default_value_callback: ''
+settings:
+  min: null
+  max: null
+  prefix: ''
+  suffix: ''
+field_type: decimal
diff --git a/config/sync/field.storage.node.field_tva.yml b/config/sync/field.storage.node.field_tva.yml
new file mode 100644
index 0000000..87c88ca
--- /dev/null
+++ b/config/sync/field.storage.node.field_tva.yml
@@ -0,0 +1,20 @@
+uuid: 92ad1651-ab19-4c60-ad8c-4824fd97ecc3
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+id: node.field_tva
+field_name: field_tva
+entity_type: node
+type: decimal
+settings:
+  precision: 8
+  scale: 4
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false
diff --git a/web/modules/custom/figli_compta_ledger/css/ledger-form.css b/web/modules/custom/figli_compta_ledger/css/ledger-form.css
index 5efddd6..587bc28 100644
--- a/web/modules/custom/figli_compta_ledger/css/ledger-form.css
+++ b/web/modules/custom/figli_compta_ledger/css/ledger-form.css
@@ -59,12 +59,14 @@ html.gin--dark-mode .figli-ledger-form {
   display: none;
 }
 
-/* Row layout. Everything not listed here (Montant HT / N° Facture /
-   Montant TTC) is left to grid auto-placement, which -- given they're
-   the only three single-column items appearing consecutively in DOM
-   order -- lines them up as one row of three for free. */
+/* Row layout, fully explicit (not left to grid auto-placement) --
+   there are now two separate 1-col-plus-2-col row groupings (Client +
+   N° Facture, then Montant HT + TVA + Montant TTC) and relying on DOM
+   order alone to keep them from bleeding into each other got fragile
+   once a 4th financial field (TVA) entered the picture. This must stay
+   in sync with the field weights set in
+   figli_compta_ledger_update_8004()/_figli_compta_ledger_create_node_type_ligne_comptable(). */
 .figli-ledger-form > .field--name-title,
-.figli-ledger-form > .field--name-field-client,
 .figli-ledger-form > .field--name-field-entree-liee,
 .figli-ledger-form > .field--name-field-repartition,
 .figli-ledger-form > .field--name-field-notes,
@@ -77,6 +79,21 @@ html.gin--dark-mode .figli-ledger-form {
 .figli-ledger-form > .field--name-field-type-ligne {
   grid-column: 2 / 4;
 }
+.figli-ledger-form > .field--name-field-client {
+  grid-column: 1 / 3;
+}
+.figli-ledger-form > .field--name-field-numero-facture {
+  grid-column: 3 / 4;
+}
+.figli-ledger-form > .field--name-field-montant-ht {
+  grid-column: 1 / 2;
+}
+.figli-ledger-form > .field--name-field-tva {
+  grid-column: 2 / 3;
+}
+.figli-ledger-form > .field--name-field-montant-ttc {
+  grid-column: 3 / 4;
+}
 
 /* Field basics */
 .figli-ledger-form .form-item__label {
@@ -118,6 +135,14 @@ html.gin--dark-mode .figli-ledger-form {
   outline: 2px solid var(--flform-accent);
   outline-offset: 1px;
 }
+/* Montant TTC: readonly, computed from HT + TVA (see
+   figli_compta_ledger_node_presave()) -- a faint fill instead of the
+   plain white/transparent of an editable field reads as "there but not
+   for you to type in" without looking disabled/greyed-out. */
+.figli-ledger-form input[readonly].form-element {
+  background: var(--flform-bg-subtle);
+  cursor: default;
+}
 .figli-ledger-form input[type="date"].form-element {
   width: auto;
   min-width: 9.5rem;
diff --git a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.install b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.install
index a43b1a2..85280c1 100644
--- a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.install
+++ b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.install
@@ -211,6 +211,13 @@ function _figli_compta_ledger_create_node_type_ligne_comptable() {
   _figli_field('node', 'ligne_comptable', 'field_numero_facture', 'N° Facture', 'string', ['max_length' => 64]);
 
   _figli_field('node', 'ligne_comptable', 'field_montant_ht', 'Montant HT (€)', 'decimal', ['precision' => 12, 'scale' => 2], TRUE);
+  // Scale 4 (not 2) so that reconstructing an existing invoice's exact
+  // TTC from HT * (1 + TVA/100) round-trips to the centime for the
+  // overwhelming majority of migrated lines -- see
+  // figli_compta_ledger_update_8005()'s docblock for the empirical check
+  // behind that choice. A user typing a plain rate like "20" is
+  // unaffected either way.
+  _figli_field('node', 'ligne_comptable', 'field_tva', 'TVA (%)', 'decimal', ['precision' => 8, 'scale' => 4, 'min' => 0]);
   _figli_field('node', 'ligne_comptable', 'field_montant_ttc', 'Montant TTC (€)', 'decimal', ['precision' => 12, 'scale' => 2]);
   _figli_field('node', 'ligne_comptable', 'field_notes', 'Notes / détail', 'string_long');
 
@@ -235,11 +242,12 @@ function _figli_compta_ledger_create_node_type_ligne_comptable() {
       ->setComponent('field_type_ligne', ['type' => 'options_select', 'weight' => 1])
       ->setComponent('field_client', ['type' => 'entity_reference_autocomplete', 'weight' => 2])
       ->setComponent('field_numero_facture', ['type' => 'string_textfield', 'weight' => 3])
-      ->setComponent('field_montant_ht', ['type' => 'number', 'weight' => 4])
-      ->setComponent('field_montant_ttc', ['type' => 'number', 'weight' => 5])
-      ->setComponent('field_repartition', ['type' => 'paragraphs', 'weight' => 6, 'settings' => ['title' => 'Répartition', 'title_plural' => 'Répartitions', 'edit_mode' => 'open', 'add_mode' => 'button']])
-      ->setComponent('field_notes', ['type' => 'string_textarea', 'weight' => 7])
-      ->setComponent('field_flag', ['type' => 'entity_reference_autocomplete_tags', 'weight' => 8])
+      ->setComponent('field_montant_ht', ['type' => 'number', 'weight' => 5])
+      ->setComponent('field_tva', ['type' => 'number', 'weight' => 6])
+      ->setComponent('field_montant_ttc', ['type' => 'number', 'weight' => 7])
+      ->setComponent('field_repartition', ['type' => 'paragraphs', 'weight' => 8, 'settings' => ['title' => 'Répartition', 'title_plural' => 'Répartitions', 'edit_mode' => 'open', 'add_mode' => 'button']])
+      ->setComponent('field_notes', ['type' => 'string_textarea', 'weight' => 9])
+      ->setComponent('field_flag', ['type' => 'entity_reference_autocomplete_tags', 'weight' => 10])
       ->save();
   }
 
@@ -254,11 +262,12 @@ function _figli_compta_ledger_create_node_type_ligne_comptable() {
       ->setComponent('field_type_ligne', ['type' => 'list_default', 'weight' => 1])
       ->setComponent('field_client', ['type' => 'entity_reference_label', 'weight' => 2])
       ->setComponent('field_numero_facture', ['type' => 'string', 'weight' => 3])
-      ->setComponent('field_montant_ht', ['type' => 'number_decimal', 'weight' => 4])
-      ->setComponent('field_montant_ttc', ['type' => 'number_decimal', 'weight' => 5])
-      ->setComponent('field_repartition', ['type' => 'entity_reference_revisions_entity_view', 'weight' => 6])
-      ->setComponent('field_notes', ['type' => 'basic_string', 'weight' => 7])
-      ->setComponent('field_flag', ['type' => 'entity_reference_label', 'weight' => 8])
+      ->setComponent('field_montant_ht', ['type' => 'number_decimal', 'weight' => 5])
+      ->setComponent('field_tva', ['type' => 'number_decimal', 'weight' => 6])
+      ->setComponent('field_montant_ttc', ['type' => 'number_decimal', 'weight' => 7])
+      ->setComponent('field_repartition', ['type' => 'entity_reference_revisions_entity_view', 'weight' => 8])
+      ->setComponent('field_notes', ['type' => 'basic_string', 'weight' => 9])
+      ->setComponent('field_flag', ['type' => 'entity_reference_label', 'weight' => 10])
       ->save();
   }
 }
@@ -353,3 +362,122 @@ function figli_compta_ledger_update_8003() {
     $view_display->setComponent('field_flag', ['type' => 'entity_reference_label', 'weight' => 8])->save();
   }
 }
+
+/**
+ * Adds field_tva (TVA %) to ligne_comptable and switches Montant TTC from
+ * manual entry to an automatically computed value (see
+ * figli_compta_ledger_node_presave(): TTC = round(HT * (1 + TVA/100), 2)
+ * from now on, whenever the line isn't a skip_validation-flagged
+ * migration save). Existing content is backfilled separately, see
+ * figli_compta_ledger_update_8005().
+ *
+ * Also reweights field_numero_facture (now next to Client, both
+ * identifying "which invoice/client" this line is about) and
+ * field_entree_liee (pushed just after it) so the HT/TVA/TTC trio can
+ * form its own clean three-column row afterwards -- see
+ * css/ledger-form.css's explicit grid-column rules, which this weight
+ * order must stay in sync with.
+ */
+function figli_compta_ledger_update_8004() {
+  _figli_field('node', 'ligne_comptable', 'field_tva', 'TVA (%)', 'decimal', ['precision' => 8, 'scale' => 4, 'min' => 0]);
+
+  // New lines default to 0% rather than blank -- the common case (every
+  // type other than charge/entrée/achat/hébergement with a real invoice)
+  // has no VAT at all, so this is one less field associates need to
+  // touch for most lines.
+  $field_tva = FieldConfig::loadByName('node', 'ligne_comptable', 'field_tva');
+  if ($field_tva && $field_tva->getDefaultValueLiteral() === []) {
+    $field_tva->setDefaultValue(0)->save();
+  }
+
+  // Weights are plain integers -- entity_display component config
+  // schema coerces anything else (a fractional weight silently rounds/
+  // truncates to its integer part on save), so the row-grouping can't be
+  // expressed as "insert field_tva between HT and TTC" via a fractional
+  // weight the way it might be elsewhere; every field in this block gets
+  // an explicit new integer instead.
+  $form_display = EntityFormDisplay::load('node.ligne_comptable.default');
+  if ($form_display) {
+    $form_display->setComponent('field_numero_facture', ['type' => 'string_textfield', 'weight' => 3]);
+    $form_display->setComponent('field_entree_liee', ['type' => 'entity_reference_autocomplete', 'weight' => 4] + (array) $form_display->getComponent('field_entree_liee'));
+    $form_display->setComponent('field_montant_ht', ['type' => 'number', 'weight' => 5] + (array) $form_display->getComponent('field_montant_ht'));
+    $form_display->setComponent('field_tva', ['type' => 'number', 'weight' => 6] + (array) $form_display->getComponent('field_tva'));
+    $form_display->setComponent('field_montant_ttc', ['type' => 'number', 'weight' => 7] + (array) $form_display->getComponent('field_montant_ttc'));
+    $form_display->setComponent('field_repartition', ['weight' => 8] + (array) $form_display->getComponent('field_repartition'));
+    $form_display->setComponent('field_notes', ['weight' => 9] + (array) $form_display->getComponent('field_notes'));
+    $form_display->setComponent('field_flag', ['weight' => 10] + (array) $form_display->getComponent('field_flag'));
+    $form_display->save();
+  }
+
+  $view_display = EntityViewDisplay::load('node.ligne_comptable.default');
+  if ($view_display) {
+    $view_display->setComponent('field_tva', ['type' => 'number_decimal', 'weight' => 6] + (array) $view_display->getComponent('field_tva'));
+    $view_display->setComponent('field_numero_facture', ['weight' => 3] + (array) $view_display->getComponent('field_numero_facture'));
+    $view_display->setComponent('field_montant_ht', ['weight' => 5] + (array) $view_display->getComponent('field_montant_ht'));
+    $view_display->setComponent('field_montant_ttc', ['weight' => 7] + (array) $view_display->getComponent('field_montant_ttc'));
+    $view_display->setComponent('field_repartition', ['weight' => 8] + (array) $view_display->getComponent('field_repartition'));
+    $view_display->setComponent('field_notes', ['weight' => 9] + (array) $view_display->getComponent('field_notes'));
+    $view_display->setComponent('field_flag', ['weight' => 10] + (array) $view_display->getComponent('field_flag'));
+    $view_display->save();
+  }
+}
+
+/**
+ * Backfills field_tva for existing ligne_comptable content from the
+ * existing Montant HT / Montant TTC pair -- never touches Montant TTC
+ * itself, so every historical value stays exactly as migrated (same
+ * "never correct historical data" rule as everywhere else in this
+ * module). Left blank where there's nothing to derive from (no TTC ever
+ * recorded, or HT is zero).
+ *
+ * Why scale 4 on field_tva: checked empirically against all 1556
+ * existing lignes_comptables before picking it. Of the 946 lines with a
+ * real (non-zero, HT != TTC) rate, rounding the derived rate to 2
+ * decimals and reconstructing TTC = HT * (1 + rate/100) mismatched the
+ * real historical TTC (by more than a centime) for 74 of them --
+ * unacceptably lossy for what's supposed to be a faithful backfill. At 4
+ * decimals that drops to 9 (likely genuine blended/multi-rate invoices
+ * collapsed into a single ligne_comptable, where no single "TVA %" can
+ * be perfectly exact) -- an acceptable residual, and still purely
+ * informational since Montant TTC itself is left untouched here either
+ * way.
+ */
+function figli_compta_ledger_update_8005() {
+  $storage = \Drupal::entityTypeManager()->getStorage('node');
+  $nids = $storage->getQuery()
+    ->accessCheck(FALSE)
+    ->condition('type', 'ligne_comptable')
+    ->execute();
+
+  $filled = 0;
+  $skipped = 0;
+  // Only field_tva is written here -- montant_ttc is read but never
+  // set, so this can never trip figli_compta_ledger_node_presave()'s
+  // sum(répartition) == montant_ht check either way. skip_validation is
+  // set anyway (a) for consistency with every other bulk-migration
+  // script in this module and (b) because it also now suppresses the
+  // TTC auto-computation added in the same presave function, which
+  // would otherwise silently overwrite the untouched historical TTC the
+  // moment this script calls save().
+  \Drupal::state()->set('figli_compta_ledger.skip_validation', TRUE);
+  foreach ($storage->loadMultiple($nids) as $node) {
+    if (!$node->hasField('field_montant_ht') || $node->get('field_montant_ht')->isEmpty()
+      || !$node->hasField('field_montant_ttc') || $node->get('field_montant_ttc')->isEmpty()) {
+      $skipped++;
+      continue;
+    }
+    $ht = (float) $node->get('field_montant_ht')->value;
+    $ttc = (float) $node->get('field_montant_ttc')->value;
+    if (abs($ht) < 0.0001) {
+      $skipped++;
+      continue;
+    }
+    $rate = round(($ttc / $ht - 1) * 100, 4);
+    $node->set('field_tva', $rate);
+    $node->save();
+    $filled++;
+  }
+  \Drupal::state()->delete('figli_compta_ledger.skip_validation');
+
+  return "TVA calculée pour $filled lignes, $skipped laissées vides (pas de Montant TTC renseigné ou Montant HT nul).";
+}
diff --git a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.libraries.yml b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.libraries.yml
index b66eab1..e26db2f 100644
--- a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.libraries.yml
+++ b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.libraries.yml
@@ -36,11 +36,14 @@ dashboard_compte:
     - figli_compta_ledger/vue
 
 ledger_form:
+  js:
+    js/ledger-form.js: {}
   css:
     theme:
       css/ledger-form.css: {}
   dependencies:
     - core/drupal
+    - core/once
 
 admin_chrome:
   css:
diff --git a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.module b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.module
index ff6dbae..f5cc7ea 100644
--- a/web/modules/custom/figli_compta_ledger/figli_compta_ledger.module
+++ b/web/modules/custom/figli_compta_ledger/figli_compta_ledger.module
@@ -106,6 +106,17 @@ function figli_compta_ledger_form_alter(&$form, FormStateInterface $form_state,
     $form['status']['#access'] = FALSE;
   }
 
+  // Montant TTC is computed from Montant HT + TVA (see
+  // figli_compta_ledger_node_presave()) -- readonly rather than #disabled
+  // so js/ledger-form.js can still show a live preview as the associate
+  // types, and so the field still visually reads as "there but not for
+  // you to edit" instead of vanishing. The actual value saved is always
+  // recomputed server-side regardless of what this input holds.
+  if (isset($form['field_montant_ttc']['widget'][0]['value'])) {
+    $form['field_montant_ttc']['widget'][0]['value']['#attributes']['readonly'] = 'readonly';
+    $form['field_montant_ttc']['widget'][0]['value']['#description'] = t('Calculé automatiquement à partir du montant HT et de la TVA.');
+  }
+
   $form['#attached']['library'][] = 'figli_compta_ledger/ledger_form';
 
   $request = \Drupal::request();
@@ -200,8 +211,9 @@ function figli_compta_ledger_node_form_ajax_submit(array $form, FormStateInterfa
  * source spreadsheets' raw data, including known répartition mismatches --
  * those get surfaced as visible inconsistencies in the dashboard instead of
  * being silently fixed. Set the 'figli_compta_ledger.skip_validation' state
- * flag around such a bulk import to bypass this check; new lines entered by
- * associates through the form are never exempted.
+ * flag around such a bulk import to bypass this check *and* the Montant TTC
+ * auto-computation below; new lines entered by associates through the form
+ * are never exempted from either.
  */
 function figli_compta_ledger_node_presave(NodeInterface $node) {
   if ($node->bundle() !== 'ligne_comptable') {
@@ -221,7 +233,7 @@ function figli_compta_ledger_node_presave(NodeInterface $node) {
   // silently mislabeling every edit with its predecessor's save time.
   $node->setRevisionCreationTime(\Drupal::time()->getRequestTime());
 
-  if (!$node->hasField('field_montant_ht') || !$node->hasField('field_repartition')) {
+  if (!$node->hasField('field_montant_ht')) {
     return;
   }
   if (\Drupal::state()->get('figli_compta_ledger.skip_validation', FALSE)) {
@@ -229,6 +241,38 @@ function figli_compta_ledger_node_presave(NodeInterface $node) {
   }
 
   $montant_ht = (float) $node->get('field_montant_ht')->value;
+  $tva = $node->hasField('field_tva') && !$node->get('field_tva')->isEmpty() ? (float) $node->get('field_tva')->value : 0.0;
+
+  // Montant TTC is no longer entered by hand -- it's a pure function of
+  // Montant HT and TVA (see figli_compta_ledger_update_8004()). Only
+  // recompute it when HT or TVA actually changed, though, rather than on
+  // every save unconditionally: the TVA backfilled onto pre-migration
+  // lines (figli_compta_ledger_update_8005()) is a best-effort
+  // reconstruction and doesn't reproduce every historical Montant TTC to
+  // the exact centime, so blindly recomputing on an unrelated edit
+  // (fixing a typo in Libellé, say) would silently nudge an untouched
+  // historical value -- exactly what this module's "never correct
+  // historical data" rule exists to prevent. A genuinely new/changed
+  // HT or TVA has no such history to protect.
+  if ($node->hasField('field_montant_ttc')) {
+    $ht_or_tva_changed = $node->isNew();
+    if (!$ht_or_tva_changed && isset($node->original)) {
+      $original = $node->original;
+      $orig_ht = $original->hasField('field_montant_ht') && !$original->get('field_montant_ht')->isEmpty()
+        ? (float) $original->get('field_montant_ht')->value : NULL;
+      $orig_tva = $original->hasField('field_tva') && !$original->get('field_tva')->isEmpty()
+        ? (float) $original->get('field_tva')->value : 0.0;
+      $ht_or_tva_changed = $orig_ht !== $montant_ht || abs($orig_tva - $tva) > 0.00005;
+    }
+    if ($ht_or_tva_changed) {
+      $node->set('field_montant_ttc', round($montant_ht * (1 + $tva / 100), 2));
+    }
+  }
+
+  if (!$node->hasField('field_repartition')) {
+    return;
+  }
+
   $somme = 0.0;
   foreach ($node->get('field_repartition')->referencedEntities() as $paragraph) {
     if ($paragraph->hasField('field_montant') && !$paragraph->get('field_montant')->isEmpty()) {
diff --git a/web/modules/custom/figli_compta_ledger/js/ledger-form.js b/web/modules/custom/figli_compta_ledger/js/ledger-form.js
new file mode 100644
index 0000000..ccb8906
--- /dev/null
+++ b/web/modules/custom/figli_compta_ledger/js/ledger-form.js
@@ -0,0 +1,60 @@
+/**
+ * @file
+ * Live preview of Montant TTC = Montant HT * (1 + TVA/100) on the ligne
+ * comptable add/edit form -- purely cosmetic, so the associate sees the
+ * computed amount before saving instead of a blank/stale readonly field.
+ * The authoritative computation is server-side, in
+ * figli_compta_ledger_node_presave() (figli_compta_ledger.module) --
+ * whatever ends up in this input on submit is discarded and recomputed
+ * there regardless of whether this script even ran.
+ */
+(function (Drupal, once) {
+  'use strict';
+
+  Drupal.behaviors.figliLedgerTvaCalc = {
+    attach: function (context) {
+      // once()'s selector match runs through context.querySelectorAll(),
+      // which -- like any querySelectorAll -- never matches the context
+      // node itself, only its descendants. The AJAX dialog opening this
+      // form passes the <form class="figli-ledger-form"> element itself
+      // as context, so a selector of '.figli-ledger-form' silently
+      // matched nothing and this behavior never ran. Selecting the HT
+      // input directly sidesteps that: it's always a genuine descendant
+      // of whatever context gets passed (document on a full page load,
+      // the form itself from the dialog, or anything in between).
+      once('figli-ledger-tva-calc', '[name="field_montant_ht[0][value]"]', context).forEach(function (ht) {
+        var form = ht.closest('form');
+        var tva = form.querySelector('[name="field_tva[0][value]"]');
+        var ttc = form.querySelector('[name="field_montant_ttc[0][value]"]');
+        if (!tva || !ttc) {
+          return;
+        }
+
+        function recompute() {
+          var htValue = parseFloat(ht.value);
+          if (isNaN(htValue)) {
+            return;
+          }
+          var tvaValue = parseFloat(tva.value);
+          if (isNaN(tvaValue)) {
+            tvaValue = 0;
+          }
+          ttc.value = (htValue * (1 + tvaValue / 100)).toFixed(2);
+        }
+
+        // Deliberately not called once up front on an existing (edit)
+        // line: the stored Montant TTC may predate this computed-field
+        // architecture and not be perfectly reproducible from HT/TVA to
+        // the centime (see figli_compta_ledger_update_8005()'s
+        // docblock) -- showing a recomputed number the instant the
+        // modal opens, before the associate has touched anything, would
+        // misrepresent what's actually saved. Only recompute once HT or
+        // TVA are actually edited, matching the same "only overwrite
+        // Montant TTC when HT/TVA actually changed" guard in
+        // figli_compta_ledger_node_presave().
+        ht.addEventListener('input', recompute);
+        tva.addEventListener('input', recompute);
+      });
+    }
+  };
+})(Drupal, once);