diff --git a/rental_fees/models/rental_fees_computation.py b/rental_fees/models/rental_fees_computation.py
index e0369b649..339493c18 100644
--- a/rental_fees/models/rental_fees_computation.py
+++ b/rental_fees/models/rental_fees_computation.py
@@ -599,23 +599,44 @@ def _add_compensation(
         )
 
     def _add_fees_periods(self, device, periods):
-        for period in periods:
-            def_line = period["fees_def_line"]
+        "Insert computation details, further detailing the period in forecast mode"
+
+        def insert_detail(fees, contract, from_date, to_date, def_line, is_forecast):
             self.env["rental_fees.computation.detail"].sudo().create(
                 {
                     "fees_computation_id": self.id,
-                    "fees": period["fees"],
                     "fees_type": "fees",
                     "lot_id": device.id,
-                    "contract_id": period["contract"].id,
-                    "from_date": period["from_date"],
-                    "to_date": period["to_date"] - one_day,  # dates included in the DB
+                    "contract_id": contract.id,
+                    "from_date": from_date,
+                    "to_date": to_date - one_day,
                     "fees_definition_id": def_line.fees_definition_id.id,
                     "fees_definition_line_id": def_line.id,
-                    "is_forecast": period["is_forecast"],
+                    "is_forecast": is_forecast,
                 }
             )
 
+        for period in periods:
+            if self.has_forecast:
+                for from_date, to_date, amount in period["monthly_fees"]:
+                    insert_detail(
+                        amount,
+                        period["contract"],
+                        from_date,
+                        to_date,
+                        period["fees_def_line"],
+                        period["is_forecast"],
+                    )
+            else:
+                insert_detail(
+                    period["fees"],
+                    period["contract"],
+                    period["from_date"],
+                    period["to_date"],
+                    period["fees_def_line"],
+                    period["is_forecast"],
+                )
+
     @job(default_channel="root")
     def _run(self):
         self.ensure_one()
@@ -647,7 +668,9 @@ def _run_for_fees_def(self, fees_def):
 
             device_fees = 0.0
             for period in periods:
-                period["fees"] = period["fees_def_line"].compute_fees(period)
+                monthly_fees = period["fees_def_line"].compute_monthly_fees(period)
+                period["monthly_fees"] = monthly_fees
+                period["fees"] = sum(amount for _ds, _de, amount in monthly_fees)
                 device_fees += period["fees"]
 
             if no_rental_limit:
diff --git a/rental_fees/models/rental_fees_definition.py b/rental_fees/models/rental_fees_definition.py
index 8a913f5a4..c06f73d2b 100644
--- a/rental_fees/models/rental_fees_definition.py
+++ b/rental_fees/models/rental_fees_definition.py
@@ -5,6 +5,15 @@
 from odoo.tools import misc
 
 
+def month_intervals(period):
+    one_month = relativedelta(months=1)
+    from_date = period["from_date"]
+    while from_date < period["to_date"]:
+        to_date = from_date + one_month
+        yield from_date, to_date
+        from_date = to_date
+
+
 class RentalFeesDefinition(models.Model):
     _name = "rental_fees.definition"
     _description = (
@@ -283,24 +292,23 @@ def compute_end_date(self, origin_date):
             )
 
     @api.multi
-    def compute_fees(self, period):
-        """Simplist computation of the fees based on the invoiced amounts in
-        the period.
+    def compute_monthly_fees(self, period):
+        """Return a (from_date, to_date, amount) list of monthly fees for the period
 
         When contract periodicity is bigger than a month, the fees may
         be 0 for a while then paid in one big go.
+
+        When the period is a forecast, we use the contract forecast table,
+        otherwise the invoice table is used.
         """
 
         self.ensure_one()
 
+        result = []
+
         if self.fees_type == "fix":
-            if self.monthly_fees == 0.0:
-                return 0.0
-            else:
-                return (
-                    self.monthly_fees
-                    * relativedelta(period["to_date"], period["from_date"]).months
-                )
+            for from_date, to_date in month_intervals(period):
+                result.append((from_date, to_date, self.monthly_fees))
 
         elif self.fees_type == "proportional":
             if period["is_forecast"]:
@@ -311,7 +319,7 @@ def compute_fees(self, period):
                         ("date_invoice", "<", period["to_date"]),
                     ]
                 )
-                return sum(p.price_subtotal * self.monthly_fees for p in forecasts)
+                date_amounts = [(p.date_invoice, p.price_subtotal) for p in forecasts]
 
             else:
                 _pt = self.fees_definition_id.product_template_id
@@ -319,7 +327,7 @@ def compute_fees(self, period):
                     "contract_line_id.sale_order_line_id"
                     ".product_id.product_tmpl_id.storable_product_id"
                 )
-                invoice_lines = self.env["account.invoice.line"].search(
+                ilines = self.env["account.invoice.line"].search(
                     [
                         ("contract_line_id.contract_id", "=", period["contract"].id),
                         ("date_invoice", ">=", period["from_date"]),
@@ -327,11 +335,18 @@ def compute_fees(self, period):
                         (_path_to_storable, "=", _pt.id),
                     ],
                 )
+                date_amounts = [
+                    (i.date_invoice, i.price_total - i.price_tax) for i in ilines
+                ]
 
-                return (
-                    sum(i.price_total - i.price_tax for i in invoice_lines)
-                    * self.monthly_fees
-                )
+            for from_date, to_date in month_intervals(period):
+                total = 0.0
+                for date, amount in date_amounts:
+                    if from_date <= date < to_date:
+                        total += amount
+                result.append((from_date, to_date, total * self.monthly_fees))
+
+        return result
 
     def format_fees_amount(self):
         if self.fees_type == "proportional":