Calculate the Soap Recipe Size to Fit a Mold

Mold volume in cubic centimeters x ~ 0.75 = Weight of oils in grams

OR

Mold volume in cubic inches x ~ 0.4 = Weight of oils in fluid ounces

To calculate volume:

For a rectangular prism (brick) shape, multiply the length x width x depth of the soap batter pour..

For a cylinder, multiply Pi x Radius squared x Depth of the soap batter pour.

For odd shapes, pour water to the mark and weigh the water. The water weight in grams is equal to the volume in cubic centimeters.

Explanation:

Density of water is = 1 g/cm3. One gram of water is equal to one cubic centimeter. 1000 g (water) = 1000 cm3.

Density of lye is = 2.13 g/cm3.

Densities of oils and fats depend on the oil type and are less than the density of water.

For example, in this simple castile soap recipe,

Olive oil 1000 g 1000 g / 0.92 g/cm3 = 1087 cm3
Water: 257.40 g 257.40 g / 1 g/ = 257.40 cm3
NaOH 128.70 g 128.70 g / 2.13 g/cm3 = 60.42 cm3
Total volume = 1404.82 cm3

The coefficient = the proportion of oils by volume (olive oil in this case) to the total volume = 1087 cm3 / 1404.82 cm3 = 0.77

The prepared batter will expand at the beginning because of the exothermic reaction. It will contract when the saponification is complete.

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