diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..085764a 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- discount(items, percent) takes a percentage off the subtotal before tax and returns the rounded total.
 
 ## 1.2.0
 - total() rounds through roundCents().
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..cd515c5 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -8,3 +8,10 @@ export function subtotal(items) {
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
+
+/** Returns the total after taking `percent` off the subtotal, before tax. */
+export function discount(items, percent) {
+  const validItems = items.filter((item) => item.qty > 0);
+  const discounted = subtotal(validItems) * (1 - percent / 100);
+  return roundCents(discounted * (1 + TAX_RATE));
+}
diff --git a/tests/lib/cart.test.js b/tests/lib/cart.test.js
index 7a9eab3..1074901 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,8 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { subtotal, total, discount } from '../../src/lib/cart.js';
+import { TAX_RATE } from '../../src/generated/rates.js';
+import { roundCents } from '../../src/lib/money.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -11,3 +13,34 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('discount takes the percentage off before tax', () => {
+  assert.equal(discount(items, 10), roundCents(22.5 * (1 + TAX_RATE)));
+});
+
+test('discount of zero percent equals the total', () => {
+  assert.equal(discount(items, 0), total(items));
+});
+
+test('discount of an empty cart is zero', () => {
+  assert.equal(discount([], 10), 0);
+});
+
+test('discount ignores items with zero or negative quantity', () => {
+  const cart = [...items, { price: 100, qty: 0 }, { price: 100, qty: -1 }];
+  assert.equal(discount(cart, 10), discount(items, 10));
+});
+
+test('discount of 100 percent is zero', () => {
+  assert.equal(discount(items, 100), 0);
+});
+
+test('discount rounds once at the end on a half-cent boundary', () => {
+  const cart = [{ price: 1.005, qty: 1 }];
+  assert.equal(discount(cart, 0), roundCents(1.005 * (1 + TAX_RATE)));
+});
+
+test('discount handles large numbers', () => {
+  const cart = [{ price: 1000000, qty: 1000 }];
+  assert.equal(discount(cart, 10), roundCents(900000000 * (1 + TAX_RATE)));
+});