Recipe scaler: cook any recipe for a different number of people

Paste the ingredients, set the two numbers, and the amounts are rewritten the way a cook writes them: halves and thirds stay halves and thirds, grams and millilitres stay decimals. Lines with nothing to multiply, like “salt to taste”, are left exactly as they were. Nothing is sent anywhere; the arithmetic happens in your browser.

The parts of a recipe that do not scale

Multiplying the ingredient list is the easy half. These are the places where the maths quietly stops being right, and they are the reason a doubled recipe sometimes comes out wrong even though every amount was correct.

The pan does not double

Twice the batter in the same tin is twice as deep, and depth changes everything: the edges set long before the middle does. Doubling a cake usually means two tins rather than one deeper one. Going the other way, a halved recipe in a full-size roasting tray spreads thin and burns.

Time does not scale, temperature sometimes does

A doubled tray of vegetables needs longer than the recipe says, because a crowded pan steams instead of roasting. Two trays on two shelves at the original time works better than one heaped tray for twice as long. For deeper bakes, dropping the oven about 15 °C and extending the time is usually closer than keeping the temperature and hoping.

Salt, spice and yeast are not linear

Salt roughly follows the food, but chilli, garlic, strong spices and anything smoked climb faster than you expect. Scale them by about three-quarters of the factor and correct at the end, which you can do, unlike unsalting a stew. Yeast is the opposite case: doubling the dough does not need double the yeast, because the rise is driven by time and temperature more than by quantity.

Eggs come in whole numbers

Three eggs scaled by 1.5 is 4.5 eggs. Beat five and hold back a couple of tablespoons, or round to the nearest whole egg and adjust the liquid a little. For a small bake, half an egg is about 25 g of beaten egg.

Reduction does not care about your numbers

A sauce reduced “until it coats the back of a spoon” is finished by texture, not by clock. Double the liquid in a wider pan and it will get there faster than you expect; in the same narrow pan, far slower. Ignore the stated time and watch the pan.

A shortcut worth knowing

Scaling by weight beats scaling by volume every time. “Two cups” of flour is anywhere between 240 and 300 grams depending on how it was scooped, and that error doubles when the recipe does. Weight has no such ambiguity, which is why bakers work in grams.

If your recipe is written in cups and you own a scale, weigh the flour once, write the number down, and every future scaling of that recipe becomes exact.