What a portion scaler actually does
A recipe portion scaler takes a recipe written for one number of people and works out the amount of each ingredient you need for a different number, whether that's two portions for a quiet Tuesday or eight because half the extended family are staying. Most recipes default to serving four, and most households aren't four people every night, so the gap between what's written on the page and what you actually need is one you'd otherwise fill by guesswork. The scaler does that arithmetic for you and turns it into a shopping-list-ready quantity for each ingredient.
The maths behind the scaling
The tool works out a scaling factor by dividing the number of portions you want by the number the recipe was written for, then multiplies every ingredient quantity by that factor. A recipe for 4 stretched to 6 gets everything multiplied by 1.5; scaled down to 2, everything is halved. That part is just arithmetic, and it works reliably for anything measured by weight or volume, so 400g of tinned tomatoes becomes 600g and 200ml of stock becomes 300ml.
Where it gets less tidy is with ingredients that only come in whole units. Halving a recipe that calls for 3 eggs leaves you with 1.5 eggs, which isn't something you can measure out at the worktop. The scaler will show you the number, but the decision about whether to round to 1, round to 2, or beat one egg and use half of it is yours to make.
Where scaling stops being pure arithmetic
Some parts of a recipe respond to a change in quantity in ways a multiplication sum doesn't capture. This is the bit that's more judgement than arithmetic, and it's worth knowing about before treating every figure on the scaled list as final.
- Seasoning: salt, chilli and strong spices rarely scale in a straight line, so doubling a curry doesn't always mean doubling the chilli. Add roughly what the maths suggests, then taste and adjust.
- Baking: raising agents such as baking powder and bicarbonate of soda don't behave proportionally once a recipe gets much bigger or smaller, so scaled baking recipes can turn out denser or flatter than the original.
- Cooking time: a bigger batch of stew takes longer to come to temperature and reduce, but not in proportion to the increase in ingredients. A pan of chilli for 8 won't need double the time a pan for 4 needed, though it will usually need some extra.
- Pan and tin size: scaling a recipe up often means the mixture no longer fits the pan the method assumes, which changes how quickly and how evenly it cooks.
Scaled quantities are a starting point
Once you've got a scaled ingredient list, it's worth reading it through before you shop. Round awkward quantities to something you can actually buy: if 350g of mince scales to 525g, decide whether you're buying a 500g pack and adjusting slightly or a 600g pack and having a little extra. Check the method for any step that mentions pan size or cooking time and read it against the quantity you're now working with, since those are the two areas most likely to need a small adjustment on the day.
Fitting scaled portions into your week
Scaling a recipe up is one of the more straightforward ways to make sure there are leftovers: cook for 6 when there are 3 of you, and two of tonight's portions are already tomorrow's lunch. If you do that kind of forward cooking regularly, the batch cooking session planner is built around it. Once you know how many portions a scaled recipe produces, the weekly meal budget planner can help you work out what that means for the week's shop and the cost per portion, though prices vary by retailer and region so treat any total as a starting point.
What the scaler can't tell you
The tool works from the quantities you enter and a straightforward multiplication, so its answer is only as good as the recipe and numbers you put in. It doesn't know your oven runs hot, that your pan is smaller than the one the recipe was tested in, or that the chilli in your cupboard is stronger than average, so it can give you a number that's off, and it's worth treating the output as a solid starting point rather than a guaranteed result. Simple scaling tends to hold up well for soups, stews and curries, and it's shakier for baking, where the chemistry is less forgiving of a rough multiplication.
Try the recipe portion scaler next time a recipe is written for a different number of people than the number actually sitting down to eat.