The formula
amount of substitute = amount missing × conversion factor
Where it comes from
A substitution has two parts and only one of them is a number. The factor says how much to use: butter to oil goes at eighty per cent because butter is sixteen per cent water and oil is not. The adjustment says what else has to change, and without it the swap fails: baking powder to bicarbonate only works if the batter has an acid in it, because bicarbonate needs something to activate it and baking powder already carries that inside.
How to work it out by hand
- Pick the ingredient you are out of and the amount the recipe asks for
- Multiply by the factor of the substitute you are going to use
- Apply the adjustment: less liquid, an added acid, or whatever it calls for
- Expect the texture to differ: hardly any substitution is identical
What is worth knowing
Three swaps people get wrong. Honey for sugar is not one to one: honey is sweeter and carries water, so it goes at three quarters and the liquid comes down. Egg for flaxseed binds but does not lift, so it works in cookies and not in an airy sponge. And cream replaced by evaporated milk cooks fine but will not whip, because there is not enough fat to trap air. In general a substitution changes the texture even when the flavour survives, and the more central the ingredient, the more it shows.