Every home cook eventually faces the same Sunday-morning math: the recipe serves four, and there are nine people coming. Scaling is ratio arithmetic — but doing it well takes three rules the arithmetic alone doesn't teach.
Rule 1: Ingredients scale, behavior doesn't
Multiply every ingredient by target ÷ original and you are 90% done. The remaining 10% is cooking behavior: a doubled casserole in the same-depth pan needs maybe 25–50% more time, not double. A tripled batch of cookies on four trays browns the same per tray. Ask yourself "what actually changes in the pan?" and adjust time and temperature from there.
Rule 2: Not every number deserves multiplying
- Season to taste: scale spices and salt to ~75–80% of the computed amount, then finish at the stove. Flavors concentrate in larger batches.
- Leaveners: round baking powder and soda down. Baking is close enough to chemistry that overshooting tastes metallic.
- Alcohol: scale generously in braises, conservatively in sauces — the raw bite concentrates.
- Cook time and pan count: never scale. Recompute both from pan depth instead.
Rule 3: Round like a cook, not a spreadsheet
Nothing ruins a scaled recipe faster than "0.83 eggs." Round to the marks on your tools: measuring spoons live in eighths and thirds (⅛, ¼, ⅓, ½, ⅔, ¾), cups in quarters and thirds, and anything above 10 units rounds to halves. For awkward eggs, beat and weigh — a large egg is ~50 g out of the shell, so half an egg is 25 g of beaten egg. Our recipe scaler applies these rounding rules automatically so the math never leaves you holding a pipette.
Worked example: chicken and rice for 9
A four-serving bake calls for 1½ cups rice, 3 cups stock, and 1 lb chicken. Nine ÷ four = 2.25. Rice: 3⅜ cups → round to 3⅓ (a cook's rounding). Stock: 6¾ cups. Chicken: 2¼ lb. Cook time goes from 35 to ~50 minutes at the same temperature, because the batch went into a deeper dish — verified with a thermometer, not the clock.
When not to scale at all
Delicate bakes with precise structure — soufflés, macarons, sponge cakes — tolerate scaling poorly; make two full batches instead. And for anything bread-like, yeast populations don't scale linearly either: scale flour by the ratio but knead and proof by touch. For everything else — stews, sheet-pan dinners, casseroles, quick breads — the ratio plus these three rules is all you need.