diff --git a/src/lib/server/recipes/scaler.ts b/src/lib/server/recipes/scaler.ts
new file mode 100644
index 0000000..c3390cd
--- /dev/null
+++ b/src/lib/server/recipes/scaler.ts
@@ -0,0 +1,16 @@
+import type { Ingredient } from '$lib/types';
+
+export function roundQuantity(q: number): number {
+  if (q === 0) return 0;
+  if (q < 1) return Math.round(q * 100) / 100;
+  if (q < 10) return Math.round(q * 10) / 10;
+  return Math.round(q);
+}
+
+export function scaleIngredients(ings: Ingredient[], factor: number): Ingredient[] {
+  if (factor <= 0) throw new Error('factor must be positive');
+  return ings.map((i) => ({
+    ...i,
+    quantity: i.quantity === null ? null : roundQuantity(i.quantity * factor)
+  }));
+}
diff --git a/tests/unit/scaler.test.ts b/tests/unit/scaler.test.ts
new file mode 100644
index 0000000..ec77cd6
--- /dev/null
+++ b/tests/unit/scaler.test.ts
@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { scaleIngredients, roundQuantity } from '../../src/lib/server/recipes/scaler';
+import type { Ingredient } from '../../src/lib/types';
+
+const mk = (q: number | null, unit: string | null, name: string): Ingredient => ({
+  position: 0,
+  quantity: q,
+  unit,
+  name,
+  note: null,
+  raw_text: ''
+});
+
+describe('roundQuantity', () => {
+  it.each([
+    [0.333333, 0.33],
+    [12.345, 12],
+    [1.55, 1.6],
+    [100.49, 100],
+    [0.5, 0.5]
+  ] as const)('rounds %f to %f', (input, expected) => {
+    expect(roundQuantity(input)).toBe(expected);
+  });
+});
+
+describe('scaleIngredients', () => {
+  it('scales parsed quantities', () => {
+    const scaled = scaleIngredients([mk(200, 'g', 'Mehl'), mk(3, null, 'Eier')], 2);
+    expect(scaled[0].quantity).toBe(400);
+    expect(scaled[1].quantity).toBe(6);
+  });
+
+  it('leaves null quantities alone', () => {
+    const scaled = scaleIngredients([mk(null, null, 'etwas Salz')], 2);
+    expect(scaled[0].quantity).toBeNull();
+    expect(scaled[0].name).toBe('etwas Salz');
+  });
+
+  it('rounds sensibly', () => {
+    const scaled = scaleIngredients([mk(100, 'g', 'Butter')], 1 / 3);
+    expect(scaled[0].quantity).toBe(33);
+  });
+});