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Soapy Math: Finding Mold Volume and Converting Recipes to Fit

I recently had someone ask me how I knew how much soap batter would fit into a particular mold.  And then how I knew how much of each oil to use when making my soap recipes.  

For this exact mold, I was able to refer to the Bramble Berry website, where it says that the mold holds 50 ounces total.  But other times or with other molds, this may not be the case.  So let's look at some of the soapy math!

Don't worry, it's really not as bad as you think!  Here's the general formula that I have found works for mold volume:

Length x width x height x 0.4

And that's it!  Let's look at a specific example.  For the mold I used above, the inner dimensions are 10" x 3 5/8" x  2.25".  Multiply those numbers together, then multiply it by 0.4.  You should get this:

10 x 3.625 x 2.25 . 0.4 = 32.625

32.625 ounces.  But that's not the 50 ounces that it says on the website!  Absolutely correct.  What this number actually means is this is the total weight of the oils that will fit into this mold.  You can then plug this total oil weight into your recipe and get some specific numbers.  So let's do that.

First we need a soap recipe.  Let's stay simple and use something very basic, like this one:

Olive oil - 50%
Palm oil - 30%
Coconut oil - 20%

You notice that I used percentages.  This will allow our recipe to be scaled up or down as necessary to fit any size mold!  

Only a little more math to go.  Multiply each percent of oil by the total oil weight to get the actual amount of each oil needed.

Olive oil - 50% x 32.625 = 16.3125
Palm oil - 30% x 32.625 = 9.7875
Coconut oil - 20% x 32.625 = 6.525

Most lye calculators will do this step for you, if you plug in the percentages.  We'll need a lye calculator anyway, to help determine the amount of lye and water needed to complete our recipe and fill our mold.  


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