diff --git a/src/cart.js b/src/cart.js
index 0acba56..70a2850 100644
--- a/src/cart.js
+++ b/src/cart.js
@@ -8,6 +8,10 @@ export function applyDiscount(total, percent) {
   return Math.round(total - (total * percent) / 100)
 }
 
+export function tax(cents, ratePercent) {
+  return Math.round((cents * ratePercent) / 100)
+}
+
 export function formatPrice(cents) {
   return `$${(cents / 100).toFixed(2)}`
 }
diff --git a/test/cart.test.js b/test/cart.test.js
index 5b4743b..c8a6ad5 100644
--- a/test/cart.test.js
+++ b/test/cart.test.js
@@ -1,6 +1,6 @@
 import test from 'node:test'
 import assert from 'node:assert/strict'
-import { subtotal, applyDiscount, formatPrice, isValidCoupon, shipping, total } from '../src/cart.js'
+import { subtotal, applyDiscount, tax, formatPrice, isValidCoupon, shipping, total } from '../src/cart.js'
 
 test('subtotal multiplies price by quantity', () => {
   assert.equal(subtotal([{ price: 250, qty: 2 }, { price: 100, qty: 3 }]), 800)
@@ -18,6 +18,15 @@ test('applyDiscount of zero percent changes nothing', () => {
   assert.equal(applyDiscount(1000, 0), 1000)
 })
 
+test('tax takes a percentage rate', () => {
+  assert.equal(tax(1000, 10), 100)
+  assert.equal(tax(999, 10), 100)
+})
+
+test('tax of zero rate is zero', () => {
+  assert.equal(tax(1000, 0), 0)
+})
+
 test('formatPrice always shows two decimals', () => {
   assert.equal(formatPrice(1250), '$12.50')
   assert.equal(formatPrice(700), '$7.00')