Calculate Soap Recipe Volume 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/cm^{3}. One gram of water is equal to one cubic centimeter. 1000 g (water) = 1000 cm^{3}.
Density of lye is = 2.13 g/cm^{3}.
Densities of oils and fats depend on the oil type, temperature, 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/cm^{3} 
= 1087 cm^{3} 
Water: 
257.40 g 
257.40 g / 1 g/ 
= 257.40 cm^{3} 
NaOH: 
128.70 g 
128.70 g / 2.13 g/cm^{3} 
= 60.42 cm^{3} 


Total volume: 
= 1404.82 cm^{3} 
The coefficient = the proportion of oils by volume (olive oil in this case) to the total volume = 1087 cm^{3} / 1404.82 cm^{3} = 0.77
The prepared batter will expand at the beginning because of the exothermic reaction. It will contract when the saponification is complete.