Baking tin size converter
Work out what a different tin does to a recipe. It is an area problem, not a width problem, which is why one inch is more than a quarter. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
One inch is a quarter more
An 8-inch round tin and a 9-inch round tin sound almost the same. They are not. Area goes with the square of the radius, so the ratio is 9² to 8² — 81 to 64, which is 26.6% more. A recipe for the 8-inch spread into the 9-inch comes out thin, dry and overcooked, with nothing about the numbers 8 and 9 to warn you.
Going the other way is worse: an 8-inch recipe in a 7-inch tin is 30% too much mixture for the space, so it either overflows or stays raw in the middle while the top browns.
Depth is the other half, and it is the half that decides the time
Heat arrives from the outside. A batter spread thin cooks quickly; the same batter piled deep takes much longer, and the outside is finished long before the middle is. So when you change tins, the time changes even though the recipe has not. Shallower means check early; deeper means turn the oven down a little and give it longer.
A square tin holds more than a round one of the same size
A 9-inch square is 81 square inches. A 9-inch round is 63.6. The square holds 27.3% more — the round tin is the square with its corners cut off, and a circle is π/4 of the square it fits inside, which is 78.5%. The corners are the other 21.5%.
This is where the old rule of thumb comes from: a square tin takes a recipe written for a round one an inch bigger. An 8-inch square is 64 square inches and a 9-inch round is 63.6 — within one per cent of each other. The rule of thumb is right, and that is why.
What it will not do
- It will not tell you the new cooking time. Time depends on depth, and depth depends on how much mixture you actually pour in — which is your decision, not the tin's.
- It will not account for tins that slope. Most round cake tins are straight-sided; loaf tins and some springforms are not, and a sloped tin holds less than its top measurement suggests.
- It will not help with anything where the shape is the point. A bundt, a loaf and a tray bake are not interchangeable at the same volume.