diff --git a/src/fx.js b/src/fx.js
index dae9d0c..b183ec0 100644
--- a/src/fx.js
+++ b/src/fx.js
@@ -1 +1,12 @@
-// Currency conversion. (Not implemented yet.)
+// Currency conversion.
+import { fetchRate } from './providers/fx-provider.js';
+import { cached } from './lib/ttl-cache.js';
+import { roundCents } from './lib/money.js';
+
+const RATE_TTL_MS = 5 * 60 * 1000;
+const getRate = cached(fetchRate, { ttlMs: RATE_TTL_MS });
+
+export async function convert(amountCents, from, to) {
+  const rate = await getRate(from, to);
+  return roundCents(amountCents * rate);
+}
diff --git a/tests/fx.test.js b/tests/fx.test.js
new file mode 100644
index 0000000..bb68f4f
--- /dev/null
+++ b/tests/fx.test.js
@@ -0,0 +1,23 @@
+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 rounds to whole cents', async () => {
+  assert.equal(await convert(1000, 'USD', 'EUR'), 920);
+  assert.equal(await convert(999, 'USD', 'GBP'), 789); // 789.21
+  assert.equal(await convert(0, 'USD', 'JPY'), 0);
+});
+
+test('convert uses the inverse rate and same-currency identity', async () => {
+  assert.equal(await convert(920, 'EUR', 'USD'), 1000);
+  assert.equal(await convert(1234, 'USD', 'USD'), 1234);
+});
+
+test('convert caches rates', async () => {
+  await convert(100, 'USD', 'JPY');
+  const before = fetchCount();
+  await convert(500, 'USD', 'JPY');
+  await convert(700, 'USD', 'JPY');
+  assert.equal(fetchCount(), before);
+});