Thought you’d never use that geometry you learned in school?
Well now’s your chance!
Being able to increase or decrease recipes makes life a lot easier if you don’t have a certain pan or are cooking for more (or less!) people.
The secret is to keep the surface area the same size, so the depth of the mixture in the pan is about the same. Of course you don’t have to be so fussy about it, but I really enjoy a little bit of kitchen math from time to time.
EXAMPLE:
Converting Loaf Pan to 20cm (8in) Round Pan
1. calculate the surface area of the loaf pan
For example say the recipe calls for a loaf tin 24x12cm (9x5in).
The surface area just equals length x width (breadth) = 24 x 12cm (9 x 5in) = 288cm2 (45in2)
2. calculate the surface area of the round pan
For example say we want to use a round tin that is 20cm (8in) diameter (across)
The surface area equals pi (3.14) x radius squared = 3.14 x half the diameter x half the diameter = 3.14 x 10 x 10cm (3.14 x 4 x 4in) = 314cm2 (50in2)
3. Work out the difference between the two.
= 288 / 314cm2 (45 / 50in2) = 0.9
So they’re very similar (the closer the number is to 1. the more similar the pans).
4. Increase or decrease the ingredients accordingly.
For this example I’d keep the ingredients the same and expect the baking time to be slightly shorter in the round pan because it is slightly larger so the cake won’t be as deep.
EXAMPLE:
Converting a 20cm (8in) Round Pan to a 23cm (9in) Round Pan
1. calculate the surface area of the 20cm (8in) pan
The surface area equals pi (3.14) x radius squared = 3.14 x half the diameter x half the diameter = 3.14 x 10 x 10cm (3.14 x 4 x 4in) = 314cm2 (50in2)
2. calculate the surface area of the 23cm (9in) Round Pan
For example say we want to use a round tin that is 20cm (8in) diameter (across)
The surface area equals pi (3.14) x radius squared = 3.14 x half the diameter x half the diameter = 3.14 x 11.5 x 11.5cm (3.14 x 4.5 x 4.5in) = 415cm2 (63.5in2)
3. Work out the difference between the two.
23cm (9in) Pan / 20cm (8in) Pan
= 415 / 314cm2 (63.5 / 50in2) = 1.3
So the larger pan is about 30% larger. Or 1/3rd larger.
4. Increase or decrease the ingredients accordingly.
So for our example we’d increase each ingredient by 1/3rd (multiply by 1.3)
Note:
For things like eggs, that are tricky to split, I usually just round up or down to the nearest whole egg. But if you’re a bit nervous, just crack and egg and lightly mix it. Then use half or whatever you need and save the rest to add to an omelet or scrambled eggs.
Too easy!
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