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How do I Calculate How Much Soap to Make to Fit My New Mold?

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How do I Calculate How Much Soap to Make to Fit My New Mold? George Doyle / Getty Images
Question: How do I Calculate How Much Soap to Make to Fit My New Mold?
Shelly from Cincinnati recently asked, "My family is in love with these new greek yogurts that come in short round plastic containers. I think they would make interesting round soap bars, but I don't know how much soap to make to fill them. (I've saved 10 of them.)"
Answer: Great idea Shelly - you get yummy yogurt and soap molds at the same time!

I've seen those containers. They're short and round, but taper a little from the top to the base, so you're not going to be able to use the standard methods of calculating the volume of a soap mold. You're going to need to use the "water method" - which calculates the total volume of the mold using water.

I happened to have one of those containers handy, and I filled it just to the mark where it flares out. It held 5.2 ounces of water. If you multiply that by 1.8 (the cubic inches of an ounce of water) you get 9.36.

Then (and this is all covered in my methods of calculating the volume of a soap mold article) you multiply the cubic inches by .40 to get the total amount of soap oils for the recipe that will fill that container.

9.36 X .40 = 3.74 ounces of soap oils

Multiply that by 10...and you get 37.4 ounces of soap making oils to make enough soap to fill 10 of those containers. (To allow for a little bit of waste, I'd round that up to 38.)

Then, divide the 38 ounces of soap oils into the percentages in your recipe. For example, if your recipe was:

  • 35% olive oil
  • 30% coconut oil
  • 30% beef tallow
  • 5% castor oil
Your recipe would be:
  • 13.3 ounces of olive oil (38 X .35)
  • 11.4 ounces of coconut oil (38 X .30)
  • 11.4 ounces of beef tallow (38 X .30)
  • 1.9 ounces of castor oil (38 X .05)
That's pretty much the procedure for sizing a recipe to fit any mold. The water method is great for intricate single cavity molds that can't be measured like standard log molds.

Send me some pictures of your yogurt container soaps!

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