Cake Tin Converter

Why area and not diameter

The instinct is to compare the numbers printed on the tins, and it is consistently wrong. A 23 cm tin is 15 per cent wider than a 20 cm tin, but it is 32 per cent bigger across the base, because area goes up with the square of the radius. Pour a 20 cm recipe into it and you get a cake two-thirds the depth you expected, done ten minutes early, with a dry edge and not enough middle.

Depth is what actually governs baking. Heat has to travel from the tin wall and the oven air into the centre of the batter, and the distance it has to cover is the depth, not the width. That is why a shallower cake in a wider tin bakes faster at the same temperature, and why a deep loaf needs a lower oven for longer.

The two questions this answers

Scale the recipe. You want the same cake, just in the tin you own. Multiply everything by the area ratio. This is the right answer for a birthday cake that has to be a particular size, and it is what the calculator gives you by default.

Use the batter as it is. You cannot be bothered scaling, or you have no more eggs. Pour it in and adjust the time. Perfectly reasonable for a family cake, as long as you know which direction the time moves — shallower is faster, deeper is slower, and deeper also wants ten degrees off the oven.

Australian tins, American recipes

Australian tins are sold in centimetres and Australian recipes are written for a 20 cm round. American recipes are written in inches for a 9 inch round — 23 cm — or a 9 × 13 inch sheet, which is roughly 33 × 23 cm and has no standard Australian equivalent at all. Our nearest common tin is the 30 × 20 cm lamington tin, about 20 per cent smaller.

British recipes use centimetres like ours but often specify a deep tin, meaning 7 to 8 cm rather than the 4 to 5 cm of a standard Australian sandwich tin. If a fruit cake recipe calls for a deep 20 cm tin and you use a sandwich tin, the batter will go over the side long before it sets. That is a capacity problem rather than an area one, and the fix is to bake the surplus separately.

Scaling the awkward ingredients

Flour, sugar, butter and liquid scale cleanly — multiply and weigh. If you are working from a recipe in cups, convert it to grams with the cups to grams converter first, because scaling 1.32 cups of flour is not a thing you can measure and 297 g is.

• Eggs come in whole units. Round to the nearest one, or beat an egg and use half by weight. A large Australian egg is about 50 g out of the shell.
• Raising agents scale slightly less than proportionally once you go past about double. Multiply baking powder and bicarb by roughly 0.9 of the factor for large scale-ups, or the crumb goes coarse and the cake domes and sinks.
• Salt, spices and vanilla can be scaled straight, but taste as you go on a big multiplier — they can get assertive faster than the rest of the recipe does.
• Baking time stays close to the original when you scale to keep the same depth. It is the depth that changes the time, not the total quantity.

Filling and testing

Fill a butter cake tin about two-thirds, and a whisked sponge or anything heavily raised no more than half. Level the top before it goes in — a scaled recipe often makes just enough extra to matter. Bake in the centre of the oven, and if you are using a fan-forced setting, remember the 20°C rule: a recipe written at 180°C wants 160°C with the fan on, and a sponge would rather have the fan off altogether.

Test with a skewer in the centre, not halfway out. A deep cake in a small tin can read done at the edge and be raw in the middle for another fifteen minutes. If the top is browning too fast, a loose sheet of foil buys you that time without stopping the cook. A stand mixer makes the scaled-up batches easier; the maths is the same either way.