How to write the lines
One ingredient per line, quantity first, the way a recipe already prints it. 2 cups all-purpose flour, 1 1/2 tsp salt, 8 oz cream cheese, 3 eggs, 1 onion. Mixed fractions, plain fractions and decimals all read. Leading dashes and bullets are stripped, so you can paste straight from a recipe page. A line with no number at all — "salt to taste", "a handful of parsley" — cannot be scaled and gets listed back to you untouched.
If a line has no unit, the number is treated as a count of whole things. "1 onion" doubled is 2 onions, and halved it rounds up to 1 rather than telling you to produce half an onion.
What the rounding does
| Kind of amount | What happens | Why |
|---|---|---|
| Cups, tablespoons, teaspoons, sticks | Snapped to the nearest 1/8 | 1/8 tsp is the smallest mark on a normal measuring set. 0.83 cup is not a thing you can measure; 7/8 cup is. |
| Eggs, cloves, cans, slices, whole vegetables | Rounded up | You cannot buy 1.4 cans. Up, never down, so nothing comes out short. |
| Grams, ounces, pounds, millilitres | Tiered: to 10 above 100, to 1 above 10, to 0.1 below | 225 g of beef becoming 220 g changes nothing you can taste. Carrying two decimal places pretends to a precision the scale does not have. |
Two amounts get flagged rather than rounded. Teaspoons that come out at 3 or more are also shown in tablespoons, and tablespoons at 4 or more in cups, because 6 tsp is easier to measure as 2 tbsp and 8 tbsp is easier as 1/2 cup.
What does not scale linearly
Three things break when you multiply them by the same number as everything else.
Salt and spice. Doubling a braise does not double the surface area or the evaporation, so a doubled quantity of salt lands saltier than the original. Going up, hold back roughly 20% and correct at the end. Going down, the straight scaling is close enough. Baking is the exception: salt in a dough controls fermentation and gluten as much as flavour, so use the full scaled amount there.
Pan size and cooking time. Double the batter and you have not doubled the baking time; you have changed the depth, which changes it in a way no multiplier predicts. Keep the depth the same and use two pans, or accept that a deeper pan needs a lower temperature and considerably longer. For stovetop work, a doubled batch in the same skillet steams instead of browning — cook it in two rounds.
Eggs. Because they round up, a 1.5x scaling of a three-egg cake gives you five eggs where the honest answer is 4.5. In a custard or a cake where eggs are the structure, weigh them instead: a large US egg out of the shell is about 50 g, so 4.5 eggs is 225 g of beaten egg, which you can measure exactly.
Scaling down has its own floor
Below about a third of the original, some recipes stop working regardless of arithmetic. A quarter batch of bread dough is too small to knead properly. A single-serving cake in a full-size pan bakes to a cracker. Braises need enough liquid depth to stay submerged. When the target servings are very small, it is often better to make the smallest workable batch and eat it twice than to scale the recipe down to something the equipment cannot handle.
Questions people ask
Why does it round eggs up instead of telling me to use half an egg?
Because rounding up is recoverable and rounding down is not. An extra half-egg of moisture in a batch of muffins is a difference you will not notice; being half an egg short of the binding in a meatball mix is a difference you will. If the eggs matter structurally — custard, choux, sponge — crack the eggs into a bowl, beat them, and weigh out what you need. A large US egg out of the shell is about 50 g, of which roughly 30 g is white and 20 g yolk.
Can I scale by a ratio instead of by servings?
Yes, indirectly. The tool only ever uses the ratio of the two serving numbers, so to double a recipe put 1 and 2, to halve it put 2 and 1, to make one and a half times put 2 and 3. The words on the fields say servings because that is what people are usually holding, but nothing in the arithmetic cares what the two numbers mean as long as they are in the right proportion.
It read my line wrong. What happened?
The parser wants a number at the start of the line and, optionally, a recognised unit word right after it. Lines like "flour, 2 cups" or "a couple of onions" will not read. Ranges are also a problem: "2 to 3 cloves garlic" takes the 2 and treats "to" as part of the name. Rewrite it as a single number. If a unit is unrecognised it is treated as part of the ingredient name and the number is still scaled correctly, which is usually the outcome you wanted anyway.
Should I scale the baking soda and baking powder?
Scale them, but check the result against the flour. The working range for baking powder is roughly 1 to 1.25 tsp per cup of flour, and for baking soda about 1/4 tsp per cup of flour plus enough acid to react with it. Straight scaling keeps those ratios intact, so it is normally fine. Where it goes wrong is when the eighth-fraction snapping pushes a small amount off — 3/8 tsp rounding around at low quantities is a real proportional change. Below about 1 tsp, weigh the leavening or use the smallest measuring spoons rather than the snapped fraction.