diff --git a/api/source/index.ts b/api/source/index.ts
index 50a9bc390..1a84d45a4 100644
--- a/api/source/index.ts
+++ b/api/source/index.ts
@@ -13,6 +13,7 @@ import { redisCacheMiddleware } from './redis-cache.js'
 import { docRoutes } from './route/doc.js'
 import { openapiRoutes } from './route/openapi.js'
 import Sentry, { requestHandler, tracingMiddleWare } from './sentry.js'
+import { getUnitKey } from './units.js'
 
 type State = Koa.DefaultState
 type Context = Koa.DefaultContext
@@ -42,7 +43,7 @@ app.use(cors())
 
 router.use('/api/v1', docRoutes(), openapiRoutes(openapi))
 
-const apiRoutes = publicodesAPI(new Engine(rules))
+const apiRoutes = publicodesAPI(new Engine(rules, { getUnitKey }))
 
 router.use(
 	'/api/v1',
@@ -65,4 +66,4 @@ const server = app.listen(port, function () {
 	console.log('listening on port:', port)
 })
 
-export { server, app }
+export { app, server }
diff --git a/api/source/units.ts b/api/source/units.ts
new file mode 100644
index 000000000..e99c75380
--- /dev/null
+++ b/api/source/units.ts
@@ -0,0 +1,25 @@
+// units from site/source/locales/units.yaml:
+const units = Object.entries({
+	heure_plural: 'heures',
+	jour_plural: 'jours',
+	'jour ouvré_plural': 'jours ouvrés',
+	semaine_plural: 'semaine',
+	trimestre_plural: 'trimestres',
+	'trimestre validé_plural': 'trimestres validés',
+	an_plural: 'ans',
+	employé_plural: 'employés',
+	point_plural: 'points',
+	mois_plural: 'mois',
+	manifestation_plural: 'manifestations',
+	'titre-restaurant_plural': 'titres-restaurant',
+	part_plural: 'parts',
+	enfant_plural: 'enfants',
+})
+
+export const getUnitKey = (unit: string): string => {
+	const key = units
+		.find(([, trans]) => trans === unit)?.[0]
+		.replace(/_plural$/, '')
+
+	return key || unit
+}