Volume of my box mold 3

We know how to calculate volume for the box molds, so far – so good. Do you know we get mold questions all the time? First I want to tell you that I am not a fan of dividers in soap molds. I think the bars become irregular and misshapen. When cutting soap I really strive for consistency. This does not mean your bars must be the same size, it just means that if you are thinking of molds that have dividers then I ask that you consider this concept before placing an order for a mold with dividers. If consistent sized bars are on your plus list then dividers are not for you. If irregular bars really make you happy then dividers could be very beneficial to you.


Let’s cover cold process soap first. You need to know these things about cold process soap:

  • One pound of fat will make 20 to 22 ounces of finished soap.
  • A mold declaring it is a one pound mold will only hold one pound of finished soap, it is not the starting point of how much fat to use.
  • Your location (relative humidity) as well as any additives you add to the soap (oatmeal, clay, extra water or milk, lavender buds, etc) will determine the final weight of the soap which is why we do not list a specific weight, we list a range of possibilities.
  • One pound of fat will make approximately 44 cubic inches of soap. This could also be written that 450 grams of fat will make approximately 721 cubic centimeters of finished soap.
  • Your mileage will vary, testing and weighing is the ONLY way for you to know exactly what your recipe and location can produce.
  • To figure the 44 cubic inches of soap I am making these assumptions: 1) the amount of liquid you are using to mix with your lye totals 6 fl oz, no more and no less; 2) I am figuring a small amount of possible additives such as a Tablespoon of ground oatmeal, not 1/2 of a cup; 3) You are not whipping air into the soap, just nice and easy.

    UPDATE! Our Measurement Calculator will help you figure the volume of a rectangular prism. By clicking here the Measurement Calculator link will open in a new tab or window. You can enter your box mold measurements and get an answer. Proof your work and know your calculator entries are correct.

    So, now what? Once we know how many cubic inches (or centimeters) are in your mold, we can work to find out the starting weight of fats for making soap.

    Your mold: Length x Width x Height = Volume of Retangular prism soap mold.
    Using cubic inches? Volume of your mold divided (written as /) 44 = weight of fats in POUNDS needed to fill mold to desired depth
    Using cubic centimeters? Volume of your mold / 721 = weight of fats in POUNDS needed to fill mold to desired depth
    To convert from pounds of fat to grams of fat you must do this: pounds * 454 = grams

    You might wonder why these have a reference of one pound of fats – it is ONLY because I find one pound of fats is easy to weigh and it is a good test-sized batch. One kilogram of fats is too big for a test batch for me. Another thought, I do not think of each oil using a lesser amount than 1/2 ounce. No tenths of an ounce here! If at all possible I formulate with whole ounces in mind. What does this mean?

  • OK to use: 1 ounce, 2 ounces, 3 ounces, 4 ounces, etc. These are whole ounces.
  • NOT OK to use: 5.1 ounces, 5.2 ounces, 5.3 ounces, etc. These are using 1/10th of an ounce. This takes too much time and is not beneficial to the finished soap. Keep it simple!
  • Tomorrow I will share how to figure volume of a cylinder. This sounds tough, doesn’t it? It isn’t. Calculating volume of cylinders is easier than making ice!

    Anyone ready to share their math in the comments? Tell us your mold size, the cubic inches of your mold, then the amount of finished soap your mold holds, and finally the amount of fat you need to make the soap to fit your mold.


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    About Tina

    I started thesage.com in 1996 with the help of my husband. Now I get to help people make all kinds of soaps and bath and body products. I think my favorite things to make are lip balms and lotions/creams. Of course I get most of the soap technical support questions because that is my strong knowledge area. Glad this blog is here!

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    3 thoughts on “Volume of my box mold

    • Cee

      Using the measurement calculator:
      Mold Size:
      3.75″ wide X 2.5″ deep X 13″ long = 122″ cubic inches (rounded up)
      122″ divided by 44″ = 2.75 lbs of soap

      To figure the amount of fats needed I use a factor of 1.37
      122 cu in = 68 fl oz (rounded up)
      divide 68 fl oz by 1.37 = 50 fl oz of fat

      Since the only way to convert ‘fl oz’ to ‘oz net wt’ is to weigh the actual fats used, I would then weigh each fat into an 8 cup Pyrex measuring cup until it reaches the 50 fl oz mark. Correct?

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      • Tina Post author

        Do you wish to pour 2.5 inches deep? Or do you wish to pour 2.25 inches deep?

        For the soap mold you have the 3.75 x 2.5 x 13 = 121.88 cubic inches (ci). Rounding to 122 is fine.

        122 ci of mold space / 44 (cubic inches from 1 lb of fat) = 2.78 lbs of fat needed to make the mold size you have just described.

        Don’t enter fluid ounces here, we aren’t using that as a unit of measure. 1 lb of fats makes approximately 44 cubic inches of soap.

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      • Tina Post author

        One more thing. 2.78 lbs of fat converted to ounces looks like this:

        2.78 (lbs) x 16 (number of ounces in a pound) = 44.48

        In this instance I would round to 44.5 ounces of total fat for the batch I want to make.

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