diff --git a/fusion_centralize_billing/models/charge.py b/fusion_centralize_billing/models/charge.py
index a194f38d..6f614800 100644
--- a/fusion_centralize_billing/models/charge.py
+++ b/fusion_centralize_billing/models/charge.py
@@ -1,7 +1,9 @@
 # -*- coding: utf-8 -*-
 # Copyright 2026 Nexa Systems Inc.
 # License OPL-1
-from odoo import fields, models
+import math
+
+from odoo import api, fields, models
 
 
 class FusionBillingCharge(models.Model):
@@ -51,3 +53,21 @@ class FusionBillingCharge(models.Model):
         "res.currency", default=lambda self: self.env.company.currency_id,
     )
     active = fields.Boolean(default=True)
+
+    def _compute_billable(self, total_quantity):
+        """Return (overage_units, amount) for total period usage under this charge.
+
+        - overage_units = usage above included_quota (never negative)
+        - 'standard'/'package'/'volume': priced per `unit_batch` block, partial block rounds up.
+          (graduated tiers are out of scope for the core; treated as 'standard'.)
+        """
+        self.ensure_one()
+        overage = max(0.0, (total_quantity or 0.0) - (self.included_quota or 0.0))
+        batch = self.unit_batch or 1.0
+        if self.charge_model == 'package':
+            # whole packages over the RAW quantity (quota ignored for package counting)
+            blocks = math.ceil((total_quantity or 0.0) / batch) if total_quantity else 0
+            return overage, round(blocks * (self.price_per_unit or 0.0), 2)
+        # standard / volume / graduated-fallback: price the overage in (rounded-up) batches
+        blocks = math.ceil(overage / batch) if overage > 0 else 0
+        return overage, round(blocks * (self.price_per_unit or 0.0), 2)
diff --git a/fusion_centralize_billing/tests/__init__.py b/fusion_centralize_billing/tests/__init__.py
index 7399d626..23d9aa9c 100644
--- a/fusion_centralize_billing/tests/__init__.py
+++ b/fusion_centralize_billing/tests/__init__.py
@@ -1 +1,2 @@
 from . import test_identity
+from . import test_charge
diff --git a/fusion_centralize_billing/tests/test_charge.py b/fusion_centralize_billing/tests/test_charge.py
new file mode 100644
index 00000000..ae33f141
--- /dev/null
+++ b/fusion_centralize_billing/tests/test_charge.py
@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+from odoo.tests.common import TransactionCase, tagged
+
+
+@tagged('post_install', '-at_install')
+class TestChargeMath(TransactionCase):
+
+    def setUp(self):
+        super().setUp()
+        self.metric = self.env['fusion.billing.metric'].sudo().create(
+            {'name': 'API Calls', 'code': 'api_calls', 'aggregation': 'sum'})
+
+    def _charge(self, **kw):
+        vals = {
+            'name': 'Maps overage', 'plan_code': 'maps-business',
+            'metric_id': self.metric.id, 'charge_model': 'standard',
+            'included_quota': 5_000_000.0, 'price_per_unit': 0.10, 'unit_batch': 1000.0,
+        }
+        vals.update(kw)
+        return self.env['fusion.billing.charge'].sudo().create(vals)
+
+    def test_under_quota_is_free(self):
+        charge = self._charge()
+        overage_units, amount = charge._compute_billable(4_000_000.0)
+        self.assertEqual(overage_units, 0.0)
+        self.assertEqual(amount, 0.0)
+
+    def test_standard_overage_per_1k(self):
+        charge = self._charge()
+        # 6,000,000 used - 5,000,000 quota = 1,000,000 overage = 1000 batches * $0.10
+        overage_units, amount = charge._compute_billable(6_000_000.0)
+        self.assertEqual(overage_units, 1_000_000.0)
+        self.assertAlmostEqual(amount, 100.0, places=2)
+
+    def test_partial_batch_rounds_up(self):
+        charge = self._charge(included_quota=0.0)
+        # 1,500 units, batch 1000 -> 2 batches -> $0.20
+        _, amount = charge._compute_billable(1_500.0)
+        self.assertAlmostEqual(amount, 0.20, places=2)
+
+    def test_package_model_charges_whole_packages(self):
+        charge = self._charge(charge_model='package', included_quota=0.0, unit_batch=1000.0, price_per_unit=2.0)
+        # 2,001 units -> 3 packages -> $6.00
+        _, amount = charge._compute_billable(2_001.0)
+        self.assertAlmostEqual(amount, 6.0, places=2)