Scope and assumptions
New amount = old amount × (new servings ÷ old servings). The scale factor as a percent: 2× = 200%, 0.5× = 50%. Illustrative.
Scaling a recipe looks like adding half again for six people instead of four. It isn't. Every ingredient is tied to the serving count by the same ratio, and the moment you apply that ratio to one ingredient and eyeball another, the balance the recipe was built on disappears.
The arithmetic is one multiplication, applied everywhere. The judgement is about what the multiplication doesn't cover.
One factor, applied to everything
New amount = old amount × (new servings ÷ old servings)
The ratio in the brackets is the scale factor. Written as a percentage, it's just that factor times 100:
| Target batch | Factor | As a percentage |
|---|---|---|
| Half | 0.5 | 50% |
| Same | 1.0 | 100% |
| Double | 2.0 | 200% |
| Triple | 3.0 | 300% |
A base recipe, three ways
Two cups flour, one cup sugar, half a teaspoon salt — for four servings:
| Ingredient | ×0.5 (2 servings) | ×2 (8 servings) | ×3 (12 servings) |
|---|---|---|---|
| Flour (2 c) | 1 c | 4 c | 6 c |
| Sugar (1 c) | ½ c | 2 c | 3 c |
| Salt (½ tsp) | ¼ tsp | 1 tsp | 1½ tsp |
Every row moves by the same factor. That's what keeps the ratios intact — and it's the step people skip when they scale the flour and leave the salt alone, producing something technically edible and definitely bland.
Double is 200%, not "add 100%"
Both statements describe the same cake, but they say different things. "I doubled it" means the new batch is 200% of the original. "I added 100%" describes the increase, not the total. Say which one you mean, especially when writing the note for someone else.
The distinction matters more with repeated scaling: doubling twice is ×4, not ×3. Percent changes compound, they don't add.
What the multiplication doesn't handle
Four things sit outside the ratio:
- Eggs. One and a half eggs is not a thing you can crack. Weigh and beat them — a large egg is roughly 50 g — and use a fraction of the mixture.
- Pan geometry. Doubling the batter in the same tin doubles the depth, which changes how heat reaches the centre. Split into two tins to keep the depth and most of the timing.
- Oven temperature. It stays where it is. Larger volumes need more time, not more heat; raising the temperature browns the outside before the middle sets.
- Seasoning and salt. These don't scale perfectly linearly for taste. Scale the recipe amount, then adjust at the end — the chemistry needs the full ratio, the palate may not.
Where the scaling goes wrong
Applying the factor to only some ingredients; treating a double batch as "+100%" and confusing the total with the increase; forgetting the ×100 when stating the factor as a percentage; rounding tiny amounts carelessly so a scaled pinch becomes nothing; scaling the oven temperature along with the quantities; and doubling the batch in one pan without adjusting time.
FAQ
How do I scale an egg in a recipe? Weigh and beat it, then use a proportion — one large egg is about 50 g. For 1.5×, use 75 g of beaten egg rather than one and a half shells.
Does cooking time double when I double a recipe? No. Time depends on thickness and pan size, not total volume. A doubled batch in the same pan needs longer, but rarely twice as long.
Why does doubling change the pan I need? Doubling the volume increases the depth, and deeper batter bakes unevenly. Splitting into two pans keeps the depth — and often the timing — the same.
How does baker's percentage work? Flour is fixed at 100%, and every other ingredient is expressed relative to it. Scaling then means multiplying every percentage by the same factor, with flour still at 100%.
Sources: Khan Academy, Scaling; cooking guides.
Disclaimer: Illustrative. Cooking is imprecise; adjust to taste. Informational only.
Sources and references
- Khan Academy — Scalinghttps://www.khanacademy.org/math