Using the inside dimensions of your mold, multiply the width by the length, then multiply that number by the height or thickness you want your soap to be. To get the amount of oils for your recipe, multiply that number by 0.4
Example: width: 9″ length: 12″ height: 1.25″
9 x 12 x 1.25 = 135 x 0.4 = 54 ounces of oils
Plug that number into a lye calculator (I like SoapCalc), along with the oils & percentages, and you will have your recipe! For the water amount, I use twice the amount of lye.
If you would prefer grams simply multiply the ounce measure by 28.35. For the example above: 54oz x 28.35g/oz = 1530.87g Or rounded properly: 1531g of oils.
For a round mold, such as a PVC pipe, the volume can also be calculated by using this formula:
Pi x radius x radius (again) x height. Pi = 3.14, radius is half the diameter (width across the circle). To get the amount of oils, multiply that number by 0.4
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