Rule of Three Calculator guide
How to Scale a Recipe for More or Fewer People
To scale a recipe, set the original servings against each ingredient amount, then the new servings against x. For a recipe serving 6 that uses 450ml of stock, scaling to 10 servings gives x = (450 × 10) ÷ 6 = 750ml. Apply the same multiplier to every ingredient — but treat salt, spices, leavening and cooking time separately.
The basic move
A recipe serves 6 and calls for 450ml of stock. You are cooking for 10. How much stock?
This is a direct proportion — more people, more stock — so lay it out and apply the rule of three:
- 6 servings → 450 ml
- 10 servings → x ml
Doing it once, then reusing it
x = (450 × 10) ÷ 6 = 750 ml.
Here is the part worth knowing: you only need to do the division once. Work out the scaling factor — new servings divided by old, so 10 ÷ 6 = 1.667 — and then every other ingredient is a single multiplication by that number.
200g of rice becomes 333g. Three eggs become five. One tablespoon of oil becomes 1.67, which you would round to a generous 1½.
Doing it that way is faster and, more importantly, keeps the proportions between ingredients intact, which is what actually matters for the dish working.
The ingredients that do not scale
This is where recipe scaling stops being pure arithmetic, and it is the part most calculators will not tell you.
- Salt and strong spices — scale to about 75–80% of what the maths says, then taste. Seasoning intensity does not rise in a straight line with volume, and an over-salted large batch is hard to rescue.
- Baking powder and bicarbonate of soda — scale, but cautiously. Too much leavening makes a cake rise fast and then collapse.
- Alcohol, vanilla and other strong extracts — scale to roughly 75%, for the same reason as spices.
- Cooking time — do not scale this at all. A double batch of stew does not take twice as long; it takes somewhat longer to heat through and then the same time to cook.
- Pan size — a doubled cake batter in the same tin will not cook through. Increase the surface area, not the depth.
Working with awkward numbers
Scaling three eggs by 1.667 gives five, which is fine. Scaling two eggs by 1.5 gives three, also fine. Scaling one egg by 1.5 gives half an egg, which is not fine in the same way.
The practical fix is to beat one egg, weigh it, and use the fraction you need — a large egg is around 50g without its shell, so half is 25g. It sounds fussy but it takes twenty seconds and it matters in baking, where the ratio of egg to flour genuinely changes the result.
For anything savoury, round to a whole egg and stop worrying about it.
Scaling down
Everything above works identically in reverse — a recipe for 8 taken down to 3 uses a factor of 0.375.
The one extra thing to watch when scaling down is that some quantities become impractically small. A quarter of a tin of tomatoes, or an eighth of a teaspoon of a spice, is a measurement you cannot really make. In those cases round to something you can actually measure and accept the small imprecision.
