Cake Tin Sizes and Recipe Scaling

Scale by the area of the tin's base, not by its width. A 23 cm round tin holds roughly a third more than a 20 cm round tin, even though it is only 3 cm wider, because area grows with the square of the diameter.

Enter the tin the recipe wants and the tin you own: Cake Tin Size Converter

The area of a round tin is π times the radius squared. Going from 20 cm to 23 cm across raises the area from about 314 cm² to about 415 cm², a 32% increase. Judging by the diameter alone, you would guess 15% and end up with a thin, dry, quickly overbaked cake.

Square tins hold more than round tins of the same nominal size, which is where the familiar rule of thumb comes from: a square tin is roughly equivalent to a round tin about 2 cm wider. The converter does the actual arithmetic rather than the rule of thumb.

Two tins with the same base area but different depths do not bake the same. A deeper batter takes longer, needs a slightly lower temperature to avoid a domed, cracked top, and may need covering part way through. A shallow tray of the same volume bakes much faster.

Fill tins about two thirds full. Fuller than that and the mixture climbs out; much less and the cake bakes pale and flat.

1. Note the tin the recipe was written for, including its shape and depth.

2. Work out the scaling factor between that tin and yours with the Cake Tin Converter.

3. Multiply the ingredients by that factor: Recipe Serving Scaler. Round eggs to whole ones where you sensibly can.

4. Check the batter quantity looks right for the tin rather than trusting the multiplier blindly: Cake Batter Quantity.

5. Adjust the time. A deeper cake needs longer at a slightly lower temperature; a shallower one needs less. Start checking at 75% of the original time.

Traybakes are simply area arithmetic on a rectangle, which makes them the easiest conversion of all, and the easiest to get wrong by eye, because a 20 x 30 cm tin holds twice as much as a 20 x 15 cm one.

For rectangular tins and brownie trays: Traybake and Tin Size Converter

Servings depend on the size of the finished cake and how it is cut, celebration slices are much smaller than afternoon-tea slices.

For portions from a finished cake: Cake Serving Calculator

Scaling by diameter. The single most common cause of a dry, flat cake.

Keeping the original baking time. The time belonged to the original depth.

Ignoring the fan. If the recipe gives a conventional temperature, convert it: Fan Oven Converter.

Overfilling. Two thirds full, and use any spare batter for a few cupcakes.