Soap Mold Calculator: How Much Oil Do I Need?
Measure the inside of your mold and how deep you will pour. The calculator turns that volume into the total weight of oils your recipe should use, so the batch fills the mold without leftovers.
The formula
Worked example. a 12 × 3.5 × 2.5 in loaf mold holds 105 cubic inches, so it takes about 42 oz of oils.
How to use it
- Measure the inside length and width of the mold, or the inside diameter of a round tube mold, and decide how deep you will pour.
- Choose your units. Metric results are in grams, imperial results in ounces.
- Enter how many molds you are filling. Use the adjustment if your molds usually come out a little short or overfilled.
- Enter the total oil weight in the soap lye calculator, divided between your oils by your recipe percentages.
Questions
Where does the 0.40 factor come from?
It is a long-standing rule of thumb among soapmakers: once lye and water are added, 0.40 oz of oils fills about one cubic inch of mold. Some makers prefer 0.38 or 0.42 for their own recipes; the 95% and 105% presets cover that range.
Does the water amount change the result?
Slightly. The factor assumes a typical amount of water. A recipe with a large water discount makes a denser batter, so the same mold needs a little more oil. Fill the mold with water once and compare if you need precision.
How do I turn the total into a recipe?
Write your recipe as percentages, for example 50% olive, 30% coconut and 20% shea. Multiply each percentage by the total oil weight and enter those weights in the soap lye calculator.