Multiplying is the easy half
Four servings to seven is a factor of 1.75, and that is one line of arithmetic. Every recipe scaler online does it. Run it honestly and here is what comes out the other side:
- 1.75 eggs
- 0.4375 tsp of baking powder
- 437.5 g of flour
- 1.75 tins of chopped tomatoes
Every one of those is correct and none of them is a thing anybody can do. You have whole eggs, a spoon set with a quarter and a half in it, a scale that reads to the gram, and a tin opener. Printing the exact number is how a tool tells you it has never been used to cook anything.
What the rounding actually does
Four rules, each matched to the thing being measured rather than to a number of decimal places.
Counts go whole, and never to nothing. Eggs, onions, rashers, tins. A halved recipe needing 0.4 of an egg needs one egg, and the tool says it adjusted rather than leaving you to notice.
Spoons go to the quarters that exist. A measuring spoon set is a quarter, a half and one, with an eighth on the better ones. There is no three eighths spoon, so rounding 0.4375 to three eighths would hand you a number and leave the actual decision exactly where it was. Below an eighth of a teaspoon it is a pinch and the tool says pinch.
Weights step by size. Nearest 25 g over a kilo, 5 g over a hundred, 1 g over twenty, half a gram below that. Nobody is chasing a tenth of a gram of flour and no domestic scale would show it if they were.
Anything it cannot read comes back untouched. Headings, "to taste", a line written in a way the parser has never seen. It is marked as left alone and it is still there, because the alternative is worse than useless: a list quietly missing an ingredient is a list you trust and should not.
Seasoning to taste does not double
Salt scales with what your mouth wants, not with the mass in the pan, and a doubled recipe often wants noticeably less than double, particularly with stock, cured meat or cheese in it. So a line reading "salt and pepper to taste" passes straight through, which is the correct answer and is also the answer nobody bothers to code.
Salt given as a measured quantity in a bake is a different case and does scale, because there it is doing structural work: it slows yeast, it strengthens gluten, and in a sweet recipe it is balancing the sugar rather than seasoning the dish.
Three things that do not scale, and one that pretends to
Cooking time. Heat moves from the outside in, so a bigger mass is not more of the same thing, it is a different problem. Twice the cake mixture in one tin is a burnt edge and a wet middle. Twice the stew in a wider pan is frequently the same time or less.
Oven temperature. A bigger, deeper bake usually wants slightly cooler and longer, so the middle catches up before the outside gives up. That is a judgement about the specific thing you are baking and no multiplier knows it.
Tin size. Double the mixture needs roughly double the volume, and volume does not follow the number on the tin: going from a 20 cm round to a 23 cm round buys 32 percent more capacity, not 15. Depth multiplies straight through on top of that.
And the one that pretends: pan surface. Anything you are browning is limited by how much of it touches the hot metal, not by the size of the pan. Triple a stew and the meat needs browning in batches even though the recipe said to do it in one, because crowded meat steams in its own liquid at any temperature.
Down is harder than up
Most people assume scaling down is the safe direction and it is the one that catches them out more often. A quarter of a sauce recipe in a full sized pan has a large surface and very little liquid, so it reduces far faster than the recipe expects and can catch before the flavours have done anything. Use the smallest pan that will hold it.
Small batches of baking have the opposite problem: the mixture is a thin layer, it heats through almost immediately, and the timing in the recipe is generous by a wide margin. Start checking at about two thirds of the stated time and expect to be surprised.
Common questions
How do I scale a recipe up or down?
Divide the servings you want by the servings the recipe makes, then multiply every quantity by that number. Four servings to six is 1.5. The multiplying is the easy half and it is where most calculators stop. The half that decides whether the answer is usable is what you do with 1.75 eggs and 0.4375 of a teaspoon, because a kitchen cannot measure either.
What do I do about half an egg?
Round to the nearest whole one, which is what this tool does, and never round down to none. If the recipe is small enough that half an egg genuinely matters, beat one in a cup and use about half of it by weight: a large egg out of its shell is roughly 50 g, so 25 g is a fair half. For most things, a whole extra egg makes a slightly richer result and nothing worse.
Can I just multiply the cooking time as well?
No, and this is the one that ruins things. Time does not scale with quantity, because heat travels from the outside in and a bigger mass is not simply more of the same. Twice the cake mixture in the same tin is not twice the bake, it is a burnt outside and a raw middle. Twice the stew in a wider pan is often the same time or less. Quantities scale linearly; the bake does not.
Do I need a bigger tin if I double a cake?
Yes, and it is the change people skip. A tin holds a volume, so doubling the mixture needs roughly double the volume, and going up in diameter buys you area as the square of the radius rather than in proportion. A 20 cm round tin to a 23 cm one is 32 percent more capacity, not 15. That calculation is a page of its own here, because doing it wrong gives you either a flat cake or one that climbs out of the tin.
Why does it round to quarter teaspoons?
Because that is what a measuring spoon set contains: a quarter, a half and one, with an eighth on the better ones. Rounding 0.4375 tsp to three eighths would be arithmetic dressed as a measurement, since you still have to choose between two spoons you actually own. Anything below an eighth of a teaspoon is called a pinch, because that is what it is.
Does salt scale with the rest of the recipe?
Not reliably, and the tool leaves "to taste" alone for that reason. Seasoning scales with what your mouth wants rather than with the mass in the pan, and a doubled recipe frequently needs less than double the salt, particularly in anything with stock or cured meat in it. Where a recipe gives salt as a measured quantity in a bake it is doing a job beyond flavour, so that one does scale.
Can I paste a recipe straight off a website?
That is what the box is for. It reads plain numbers, decimals, fractions typed as 1/2 and pasted as the glyph, mixed numbers written either way, ranges like 2-3 cloves, and pack lines like 1 x 400g tin. Anything it cannot read with confidence comes back exactly as you wrote it and is marked as untouched, which is deliberate: a scaler that quietly drops a line it did not understand has taken an ingredient out of your dinner.
How far can you scale a recipe before it stops working?
Further down than up, and the limits are physical rather than arithmetic. Scaling down runs into the pan: a quarter recipe of a sauce in a full sized pan reduces far too fast and can catch. Scaling up runs into the same problem in reverse, plus crowding, which is the big one for anything browning. Meat that is crowded steams instead of browning however hot the pan is, so a tripled stew usually needs the browning done in batches even though the recipe says one.
Does baking scale as well as cooking?
Less well, because baking is a set of ratios rather than a list of amounts. Flour, fat, sugar, liquid and raising agent are in proportion to one another, so rounding two of them in different directions changes the recipe rather than just the size of it. That is why the rounding here steps by size instead of to a fixed number of decimals, and why doubling a delicate cake is more reliably done as two batches in two tins than one big one.