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Calculating Mold Volume


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Someone asked for this to be posted as a sticky for future reference.


One of the most challenging tasks when making cold process soap is knowing how much you need to fill the mold. Typically CP makers use Oils as a base for the formulation.


For a typical mold, you only need to calculate the volume of the container and multiply by 0.4 for typical lye solutions.


Example: Log Mold Tall Skinny mold by Nurture.

Length: 11

Width 2.5

Depth of pour (desired bar height) 3.5


the volume of a recangular log is L x W x H (11 x 2.5 x 3.5) = 96.25

Multiply that total by 0.4 to find the weight of oils (in ounces) 96.25 * 0.4 = 38.5 oz of oils. THis is for "full water". Adjust the calculations up/down to approximate your solution needs to fill that mold. 

FOr instance, I use a 40% lye solution (much less water), so instead of 0.4, my calculation is 0.42 which results in a nice round 40 ounces of oils per log.


Example 2: PVC Pipe Mold (Cylinder) that is 3" across and 12" tall

The formula to find the volume of a cylinder is V=(pi)x(R^2)xH

Volume = pi x Radius squared x height. (the area of a circle times the height of the cylinder)

Volume = 3.14 x 1.5" radius (1/2 of diameter) squared x 12 inches high

Volume = 3.14 x 2.25 x 12

Volume = 84.78

Volume x 0.4 = amount of oils 

84.78 x 0.4 =  33.9 ounces of oil in that batch to fill the mold with typical water.


In my case with a 40% lye solution I'd need 84.78 * .42 or 35.6 ounces of oils.


This formula works for anything you can calculate the volume for.


Example 3: Slab mold  12 x 12 x 1' thick bars

12 x 12 x 1 = 144

144 x .4 = 57.6 ounces of oils


So volume times 0.4 for full water.


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  • 1 year later...
2 hours ago, Jlp said:


I do not understand how the 0.4 factor comes into play? Where did it come from? Please shed more light on that number and how it was determined. Thank you. 

It is tied to the lye solution concentration. The water to lye % you use may change this number slightly.

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