Not Everything in a Recipe Scales Linearly
Doubling a recipe by doubling every number works for some ingredients and fails badly for others. The arithmetic is trivial; knowing where it stops applying is the part worth reading.
| Ingredient | Scales linearly? | What to do instead |
|---|---|---|
| Flour, sugar, liquid, butter | Yes | Multiply directly. |
| Salt and spices | Roughly | Start at 75% of the multiplied amount and adjust at the end. |
| Yeast | No | Increase by about half the multiplier. Doubling yeast over-proofs. |
| Baking powder and soda | No | Use around 75–80% of the multiplied amount in large batches. |
| Cooking time | No | Judge by internal temperature and colour, not the clock. |
| Pan size | By area, not by count | See below, this is where most halved cakes fail. |
Leavening is the one that surprises people. Yeast and chemical raisers work on the surface area and gas retention of the dough, not on its mass, so doubling them in a doubled batch produces a coarse crumb and a yeasty taste. Scaling them at roughly three-quarters of the multiplier is the usual correction.
Pan Size Is an Area Problem
Halving a cake recipe and baking it in a pan half the diameter is the classic mistake, because area scales with the square of the radius rather than with the radius itself.
A 20 cm round pan has an area of about 314 cm². A 25 cm pan has about 491 cm², more than half again, not a quarter more. So a recipe written for the 20 cm pan needs roughly 1.56 times the quantities to fill the 25 cm pan to the same depth, and using a straight 1.25 multiplier gives a thin, dry cake that bakes too fast.
The working rule is to scale by the ratio of the areas. For round pans that is the ratio of the squared radii; for rectangular ones, simply length times width.
Time and Temperature Do Not Follow the Multiplier
A doubled batch does not take twice as long, and it rarely takes the same time either. What changes is the depth of the food and the load on the oven.
- Same depth, wider pan: time is close to unchanged.
- Deeper pan: noticeably longer, and often better at a slightly lower temperature so the outside does not overcook before the centre sets.
- Two trays instead of one: longer overall, because the oven has more mass to heat and airflow is restricted. Rotating the trays partway matters more than the extra minutes.
For anything where doneness is a food-safety question rather than a preference, use a thermometer. Poultry at 74°C, most baked goods around 93–99°C in the centre. A recipe's stated time was written for one pan in one oven and was never a measurement of your kitchen.
Fractions, Eggs and Awkward Numbers
Scaling by anything other than a whole number produces quantities no measuring spoon has. Some conversions worth keeping to hand: three teaspoons make a tablespoon, so two-thirds of a tablespoon is two teaspoons. Sixteen tablespoons make a US cup, so three-quarters of a cup is twelve tablespoons.
Eggs are the genuinely awkward case, because a third of an egg is not something you can measure by cracking. Beat the egg, then weigh or measure it, a large egg is roughly 50 g out of the shell, so a third is about 17 g. For a single egg in a recipe scaled by 1.5, using one egg plus a tablespoon of the beaten second one is closer than rounding to two.
Where a recipe gives weights rather than volumes, scaling is both easier and more accurate, which is why baking recipes increasingly do. A kitchen scale removes the entire class of problem that cup measurements create.