diff --git a/src/tax.ts b/src/tax.ts
new file mode 100644
index 0000000..bb1b61f
--- /dev/null
+++ b/src/tax.ts
@@ -0,0 +1,10 @@
+import { totalCents, type CartItem, type Discount } from './cart'
+
+export function totalWithTaxCents(
+  items: CartItem[],
+  taxRatePercent: number,
+  discount?: Discount,
+): number {
+  const base = totalCents(items, discount)
+  return base + base * (taxRatePercent / 100)
+}