Adapting a sponge recipe to a smaller tin

[sunglassesonthetable]

I regularly cook a Vic Sponge using a Nigella recipe. It's made in 21cm pans and takes 25 mins to cook.

How would I adapt it for 15 cm tin ? I could work out the amount by trial and error I think but how long would I leave in the oven? Is there a way to work this out?

Any ideas clever people?

How many eggs in the recipe? Realistically you can't use eg 2/3 of an egg so if there are 3 then use 2 and reduce all other quantities by a third.

15cm is probably only about half the volume of 21cm (cba to work it out) but you've got space to expand upwards.

Same temperature, check it after 20 mins and take out of the oven when it's golden and shrinking away from the sides.

If it were me I'd reduce each ingredient by a quarter and reduce the cooking time to 19 or 20 minutes. After that time check with a cake tester/cocktail stick and you'll know if it needs a couple of extra minutes or not.

21 = 10.5cm radius. 10.5x10.5 = 110.25

15 = 7.5cm radius... 56.25

56.25/110.25 is approx 1/2 so the cake would be half the size. I'd probably cut to 17 minutes to start with and test them then.

DoodlePug makes a good point about the number of eggs!

Augend23, that would be for a square tin. Most recipes assume round tins.
So 21/2 = 10.5 radius * 3.14 gives about 33cm sq
15/2 = 7.5 * 3.14 gives about 23.5 cm sq , which is about about 71%

DoodlePugs point about the eggs is important though. If it is 4 eggs in a 21cm tin, 3 eggs and 75% of the rest of the ingredients should work out very nicely volume wise.

Nope...

Area = pi X r X r

So for the 21cm that's 3.14 X 10.5 X 10.5

For the 15 it's 3.14 X 7.5 X 7.5

So the pi's cancel and the ratio of the sizes is the 10.5 squared to the 7.5 squared.

If they were square tins you'd need to use the full length of the sides.

Or you could make the usual amount, fill the smaller tin and distribute the rest into cupcake cases. Or have apple purée in another tin and add the left over sponge mixture on top for Eve's pudding.

Sponge mixture sings while cooking and falls silent once it's done. Or use a toothpick.

Make your normal sponge mix, put less in and for less time. Then if it goes wrong you have mixture left to try again.

Use 2 eggs. Weigh them and use the same weight of sr flour, sugar and butter.

Thank you ( in no particular order ) @sproutsandparsnips @TerrifiedandWorried @FinallyHere @Augend23 @MangoBiscuit @hernameis @DoodlePug

There's lots of cake food for thought there. Going to be listening for the sponge singing.

Off to have a bash . Very helpful .

Augend23, you are, of course, correct. I didn't square it, that'll teach me for trying to do maths while lacking sleep! Sorry.