Whether you use a traditional soap mold or some other container, you can calculate just how much soap you need to make to fill that particular mold.
There are a number of great online Lye/Soap Calculators to assist you in making sure that your soap batch is going to fit your mold. Click on the links below to check out these calculators to see which is your preferred option.
LyeCalc (my favourite and the one I typically use when creating a new recipe)
Most online lye/soap recipe calculators scale your recipes easily and give you the recipe in both measurements and percentages. Easy peasie!
Calculate it Yourself
If your mold is square or rectangular you can calculate the volume of the mold in cubic inches (L x W x H).
For instance, if it is 6 inches high, 3 inches wide, and 10 inches long - you would calculate 6 x 3 x 10 = 180 cubic inches. This is the calculation for filling the mold to the top, but if you decide to fill it 4 inches high instead of 6 inches high, all you need to do is adjust your numbers to represent the height change.
Converting Inches to Ounces
To convert cubic inches to ounces, you will then need to multiply the total (180 cubic inches) by 0.554 (specifically 0.554113) as one cubic inch equals 0.554113 fluid ounces. We multiply the volume by 0.554 to convert from inches to ounces.
180 cubic inches x 0.554 = 99.72 ounces
Converting Volume to Weight
Soap makers generally calculate soap recipes by weight and not volume, therefore we need to calculate the weight of the soap, or the weight of the oils (oil can account for at least 70%+ in the recipe).
Water weighs 1:1 but oils do not. A volumetric ounce of oil does not actually weigh one ounce. Calculations for weight usually lean more towards the weight of oils per ounce being approximately 0.9ounces.
Multiply the volume of the mold by 0.9 to find the total amount of oils in the recipe.
99.72 oz x 0.9 = 89.748 oz (this is the total soap volume that will fill your mold)
Alternatively, you can easily enter your mold size into an online calculator such as LyeCalc
Once you have your mold size (or you can calculate this also in LyeCalc), enter the percentage of your chosen butters/oils into the calculator to get the final recipe quantities - this will allow you to fill your mold without any excess soap batter.
Prepare the recipe (see post on Creating Cold Process Soap) as you normally would using the quantities from the lye calculator, and the soap should come close to perfectly filling the soap mold, whatever its size
What about Circles or Tubes?
For circles (cake pan molds) or tube molds (PVC pipe), you need to multiply PI by the radius of the circle squared and multiply that by the height of the mold. For instance, PI is 3.14 (specifically 3.14159) - therefore you would calculate PI (3.14 x the radius x the radius x the length of the tube, or the depth of the cake pan).
Okay, so you have a 4-inch tube, and it is 10 inches long
PI = 3.14
Radius = 2 inches
length of the tube = 10 inches
To find the volume of your container your calculation is:
3.14 x 2 x 2 x 10 (which equals - 125.6 cubic inches)
place this calculation into an online lye calculator along with the percentages of oils desired for your recipe and it will give you a mold filling recipe outcome.
Let me know what you think
Has this helped you to be more confident in calculating your soap amount, or using the online Lye Calculator?