diff --git a/src/lib/formatUsd.test.ts b/src/lib/formatUsd.test.ts
new file mode 100644
index 00000000..20a1fea9
--- /dev/null
+++ b/src/lib/formatUsd.test.ts
@@ -0,0 +1,56 @@
+import { describe, expect, it } from 'vitest';
+import { formatUsd } from './utils';
+
+describe('formatUsd()', () => {
+  it('formats a typical whole-dollar amount', () => {
+    expect(formatUsd(100)).toBe('$100.00');
+  });
+
+  it('formats a typical amount with cents', () => {
+    expect(formatUsd(1234.56)).toBe('$1,234.56');
+  });
+
+  it('formats zero as $0.00', () => {
+    expect(formatUsd(0)).toBe('$0.00');
+  });
+
+  it('formats a small fractional amount', () => {
+    expect(formatUsd(0.99)).toBe('$0.99');
+  });
+
+  it('formats a large amount with comma grouping', () => {
+    expect(formatUsd(1000000)).toBe('$1,000,000.00');
+  });
+
+  it('formats a negative number with a minus sign', () => {
+    const result = formatUsd(-50.25);
+    expect(result).toContain('50.25');
+  });
+
+  it('formats negative zero as $0.00', () => {
+    expect(formatUsd(-0)).toBe('$0.00');
+  });
+
+  it('returns NaN-containing string for NaN input', () => {
+    const result = formatUsd(NaN);
+    expect(result).toContain('NaN');
+  });
+
+  it('returns NaN-containing string for undefined coerced to number', () => {
+    const result = formatUsd(undefined as unknown as number);
+    expect(result).toContain('NaN');
+  });
+
+  it('rounds to two decimal places', () => {
+    expect(formatUsd(19.999)).toBe('$20.00');
+  });
+
+  it('handles very small positive amount', () => {
+    expect(formatUsd(0.01)).toBe('$0.01');
+  });
+
+  it('handles amount with more than two decimal places', () => {
+    const result = formatUsd(1.005);
+    expect(['$1.00', '$1.01']).toContain(result);
+  });
+});