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Soap Making Basics Part 2 How to Calculate the Volume of a Soap Mold

Posted by Chris Greten on

Yesterday we talked about how to turn your recipes into percentages so you can determine how much oil you need for your mold. If you missed yesterday's article  click here to read it.

To determine the amount of oil you need in a recipe to fill a mold requires a little math but don't be scared it is not that hard to do. Let's start with the easiest one to figure out first! If you are using a square or rectangle mold you first need to figure out the total volume which is Length x Height X width. The mold that I love and use the most is  Regular Silicone Loaf Soap Mold    I use this mold for most of my soaps and it is the mold pictured above. The mold makes a 40 - 44 ounce loaf of soap. It measures 3.5 x 8 x 2.5 so multiplying these numbers together I get 70 for the total volume. To figure out how many ounces of oil I need I take the total volume which is 70 and multiply that by .40 which is 28. So now we know we need 28 ounces total of the oils to fill my mold. (I like my soaps to be extra-large and overflow the top so I round up to 30 and that usually gives me a nice loaf with a nice top like the photo below.)

In yesterday’s article we learned that we need our oils in the recipe to be:

Olive oil = 40%
Coconut Oil = 25%
Palm Oil = 25%
Sweet Almond Oil = 10%


Now to figure out how much Lye and water you need to put these numbers into the soap calculator. I like to use Brambleberry's Lye calculator.  In the calculator, you want to choose soap and choose percentages and put 28 ounces in the box that says what will your total ounces weigh. Find your ingredients in the list below and enter the percentage beside each ingredient. You do not need to enter the percent sign or a decimal point. Finally choose 5% superfat that is a nice safe number for a beginner. Superfat is the amount of lye that is discounted in the final product. If you start out with 5% it will give you a nice lather and your bar will harden nicely. Hit submit and it will generate a recipe with the amount of each oil as well as lye and water.

If your mold is round or a tube, like PVC or a Pringles can, the formula is calculated different. You multiply pi (3.14) x the radius squared and multiply that number by the height of the mold. You measure the radius (1/2 the total width of the tube, squared) so let’s say your PVC is 6” across so your calculation would be 3 x 3. Let’s say the height is 4. So your formula is 3 x 3 x 3.14 x 4(width squared x pi x height) 3 x 3 x 3.14 x 4 = 13.14 That gives you the volume and you multiply that number x .40 to determine your ounces of oil.
13.14 x .4 = 5.256 ounces oil needed

Finally, if you have an odd shaped mold you can use water to determine the volume. Fill the mold with water and then pour the water into a measuring cup. Multiply the number of ounces of water by 1.8. If the mold hold 16 ounces of water 16 x 1.8 = 28.8 Now multiply the answer by .40 and that will tell you how many ounces of oil you will need. 28.8 x .40 = 11.52 ounces of oil needed.

I hope that this has been helpful to you. This was very useful to me when I first started making soap. Let me know if helped you!



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