Whether you use a traditional soap mold, or some other container (you just have to make sure it won't leak, won't react with the ly and is flexible enough to allow you to unmold the soap) - to calculate just how much soap you need to make to fill your soap mold you just:
- Calculate the volume of the mold in cubic inches (or cm)
- Multiplying the volume by .40 to find the amount of oils in the recipe
- Enter that amount of oils into a lye calculator to get the final recipe
Then, prepare that recipe as you normally would, and it should come very close to perfectly filling your soap mold, whatever its size or shape.
1. Standard Square or Rectangular Molds
For square or rectangular molds, you multiply the length by the width by the height. (L X W X H) If you're not going to fill the mold to the top, just calculate to the height that you want the soap. (For example, if your mold is 4 inches deep, but you're only going to pour 2 inches of soap into it, just use "2" in the calculation.
So, let's say we have a log mold that is 15 inches long, 2.5 inches wide and 3.5 inches tall - and we want to fill the mold to the top. We multiply 15 X 2.5 X 3.5 = 131.25.
Then, we take the total volume and multiply it by .40. 131.25 X .40 = 52.5 ounces of oils.
Then using the percentages in your recipe, calculate each of the individual oils to come up with your final recipe sized perfectly for that mold.
2. Round or Tube Molds
It's not as hard as it sounds. The radius is 1/2 of the total width of the tube. For a 3" tube, the radius is going to be 1.5". "Squared" = the number times itself - so 1.5 X 1.5 or 2.25.
Pi is generally calculated at 3.14.
So...for a 3" PVC tube mold that is 6" high, the calculation is:
3.14 X 1.5 X 1.5 X 6
Pi X radius X radius X height
That calculation will give you the volume, which you then multiply by .40 to get the amount of oils needed.
3.14 X 1.5 X 1.5 X 6 X .40 = 16.96 ounces of oils.
Then using the percentages in your recipe, calculate the individual oils.
3. Water Method - for Odd/Irregular Shaped Molds
If your mold is shaped irregularly, or is otherwise hard to get an accurate measure, you can use the "water" method. (Actually, you can use the water method for all shapes of molds.)
Fill the mold with water and then pour that water into a measuring cup. Multiply the number of ounces of water by 1.8 to get the total cubic inches of the mold.
For example, if your mold holds 12 ounces of water - 12 X 1.8 = 21.6 cubic inches.
21.6 cubic inches (in the mold) X .40 = 8.64 ounces of oils in that recipe.
Enter the 8.64 ounces of oils (best to round up to 9 or at least to 8.6) - allocated by the percentages of each oil in your recipe, and you'll have your final recipe.
When all else fails...or if you just want to double check your math, there are a few online calculators that will help you calculate the recipe for your particular soap mold:
Summer Bee Meadow's Lye Calcuator and Recipe Resizer - you enter your basic recipe first, and then resize it to fit the mold size you desire.
Soapmaker Software - a complete software program for calculating recipes, resizing for different molds, calculating costs of ingredients and batches of soap and more.