Cookaholics Bulletin Board

Say you want to scale a recipe to make a larger batch. I find recipes for 9x13" pans, but want to bake 18x24" trays. So:
I multiply 9x13 and get 117.
I do the same for 18x24 and get 432.
Then I divide. The scaling factor, as I call it, is 3.7.
I multiply each ingredient by 3.7 to get the amount I need.
You can build a chart of scaling factors for the sizes you use the most.

Excellent! Thanks Becky!

That is pretty useful Becky. I assume it would work down as well e.g. 9" X 13" to a 7" X 10" would give you a 0.598 multiplier.

Would it work for round pans? If so what would you multiply. I get the math, just not sure what number I'd use to start.

I always had to start with volume before I got LC that did the scaling for me. If total volume was 6 cups. and the original had 1 cup milk then I'd do 1 is to 6 as X is to 9=1.5. It is nice not have to know the volume.

My math teacher is laughing somewhere right now, thinking "Told you this would come in handy"

Awesome! Now, how do you do round pans? I'd like to make a small pi.


(I slay me.)

Well, pi day is coming up...

You slay me too.

Amy

The calculation is based on area (and assumes you are making the recipe to the same depth regardless). The area of a round pan (say... a pie pan) is pi * r^2 or pi * r * r. Now normally pans are given in diameter, so more usefully it is pi * d * d /4. We can use 3.14 for pi, and dividing by 4, we get 0.785, so our final calculation is 0.785 * d * d. So, for a 9" round pan, your number would be:

0.785 * 9 * 9

or 63.585

This is Becky's scaling factor. So, if you recipe was designed for 9x13 (117 scaling factor), and you want to make a 9" round pan instead, use 63.585/117 or 0.54 scaling. So basically you would use half the ingredients.

If that is way too long to read, then here is the short version:

The scaling factor for round pans is 0.785 * d * d where d is the pan diameter.

I was a math major, so I love this stuff, and should have been able to figure it out, but it has been a while since I used my brain that way. I was trying to a void having to the diameter bit. I also caught that pans had to be the same height. Thanks Paul.

Sooo...who is going to put together the generic Excel spreadsheet?

I vote for Jim.