diff --git a/src/fx.js b/src/fx.js
index dae9d0c..a542517 100644
--- a/src/fx.js
+++ b/src/fx.js
@@ -1 +1,26 @@
-// Currency conversion. (Not implemented yet.)
+// Currency conversion.
+import { SUPPORTED_CURRENCIES } from './config.js';
+import { cached } from './lib/ttl-cache.js';
+import { roundCents } from './lib/money.js';
+import { fetchRate } from './providers/fx-provider.js';
+
+const RATE_TTL_MS = 10 * 60 * 1000;
+
+const getRate = cached(fetchRate, { ttlMs: RATE_TTL_MS });
+
+export async function convert(amountCents, from, to) {
+  if (!Number.isInteger(amountCents)) {
+    throw new Error(`invalid amount: ${amountCents}`);
+  }
+  for (const currency of [from, to]) {
+    if (!SUPPORTED_CURRENCIES.includes(currency)) {
+      throw new Error(`unsupported currency: ${currency}`);
+    }
+  }
+  if (from === to) return amountCents;
+  // The provider only quotes USD pairs, so cross rates go through USD.
+  const rate = from === 'USD' || to === 'USD'
+    ? await getRate(from, to)
+    : (await getRate(from, 'USD')) * (await getRate('USD', to));
+  return roundCents(amountCents * rate);
+}
diff --git a/src/index.js b/src/index.js
index c7c695a..707ca81 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,4 +1,5 @@
 export { categoryName } from './categories.js';
 export { shippingCents } from './shipping.js';
 export { vatRate } from './tax.js';
+export { convert } from './fx.js';
 export { roundCents, formatCents } from './lib/money.js';
diff --git a/tests/fx.test.js b/tests/fx.test.js
new file mode 100644
index 0000000..570e385
--- /dev/null
+++ b/tests/fx.test.js
@@ -0,0 +1,34 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { convert } from '../src/fx.js';
+import { fetchCount } from '../src/providers/fx-provider.js';
+
+test('convert applies the rate and rounds to whole cents', async () => {
+  assert.equal(await convert(1000, 'USD', 'EUR'), 920);
+  assert.equal(await convert(1001, 'USD', 'GBP'), 791);
+  assert.equal(await convert(1000, 'USD', 'JPY'), 151200);
+});
+
+test('convert uses the inverted pair', async () => {
+  assert.equal(await convert(920, 'EUR', 'USD'), 1000);
+});
+
+test('convert to the same currency returns the same amount', async () => {
+  assert.equal(await convert(1234, 'EUR', 'EUR'), 1234);
+});
+
+test('convert caches rates between calls', async () => {
+  const before = fetchCount();
+  await convert(500, 'GBP', 'USD');
+  await convert(900, 'GBP', 'USD');
+  assert.equal(fetchCount() - before, 1);
+});
+
+test('convert crosses non-USD pairs via USD', async () => {
+  assert.equal(await convert(1000, 'EUR', 'GBP'), 859);
+});
+
+test('convert rejects bad input', async () => {
+  await assert.rejects(() => convert(100, 'USD', 'XYZ'), /XYZ/);
+  await assert.rejects(() => convert(10.5, 'USD', 'EUR'), /10.5/);
+});