diff --git a/CHANGES.md b/CHANGES.md
index 9c38282..3f23923 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Add discount(items, percent): total with the percentage taken off the subtotal before tax.
 
 ## 1.2.0
 - total() rounds through roundCents().
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..7d5e535 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -8,3 +8,8 @@ export function subtotal(items) {
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
+
+export function discount(items, percent) {
+  const discounted = subtotal(items) * (1 - percent / 100);
+  return roundCents(discounted * (1 + TAX_RATE));
+}