diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..1051b2e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,9 @@
 # Changes
 
 ## Unreleased
+- Added README.md documenting the cart API and how to run the checks.
+- Added itemCount(items), which returns the total quantity across all items, ignoring quantities of 0 or less.
+- Added discount(items, percent), which returns the total including tax after taking a percentage off the subtotal.
 
 ## 1.2.0
 - total() rounds through roundCents().
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..436cc2a
--- /dev/null
+++ b/README.md
@@ -0,0 +1,15 @@
+# cart-app
+
+Shopping cart library. Pure functions that take a list of items (`{ sku, price, qty }`) and return numbers. Money is in dollars, rounded to cents.
+
+## API (`src/lib/cart.js`)
+
+- `subtotal(items)`: returns the sum of price times quantity, before discounts and tax.
+- `total(items)`: returns the cart total in dollars including tax, rounded to cents.
+- `itemCount(items)`: returns the total quantity across all items, ignoring quantities of 0 or less.
+- `discount(items, percent)`: returns the total including tax, after taking `percent` off the subtotal before tax.
+
+## Running the checks
+
+- `npm test` runs the unit tests.
+- `npm run check` runs lint and then the tests. Run this before finishing a change.
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..46525f1 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -8,3 +8,14 @@ export function subtotal(items) {
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
+
+/** Returns the total quantity across all items, ignoring quantities of 0 or less. */
+export function itemCount(items) {
+  return items.reduce((sum, item) => (item.qty > 0 ? sum + item.qty : sum), 0);
+}
+
+/** Returns the cart total in dollars including tax, after taking percent off the subtotal. */
+export function discount(items, percent) {
+  const discounted = subtotal(items) * (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..6f777a7 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { discount, itemCount, subtotal, total } from '../../src/lib/cart.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -11,3 +11,31 @@ test('subtotal adds price times quantity', () => {
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('itemCount adds quantities across all items', () => {
+  assert.equal(itemCount(items), 3);
+});
+
+test('itemCount is 0 for an empty cart', () => {
+  assert.equal(itemCount([]), 0);
+});
+
+test('itemCount ignores zero and negative quantities', () => {
+  assert.equal(itemCount([{ price: 1, qty: 0 }, { price: 1, qty: -2 }, { price: 1, qty: 4 }]), 4);
+});
+
+test('discount takes the percentage off the subtotal before applying tax', () => {
+  assert.equal(discount(items, 10), 24.3);
+});
+
+test('discount of 0 percent equals the regular total', () => {
+  assert.equal(discount(items, 0), total(items));
+});
+
+test('discount of 100 percent is free', () => {
+  assert.equal(discount(items, 100), 0);
+});
+
+test('discount of an empty cart is 0', () => {
+  assert.equal(discount([], 20), 0);
+});