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Calculating the size of a soap mold

calculations soap mold

Hi There! Today I am going to give you 3 ways to figure out the size of your soap mold today. 

Method #1: Sometimes when I need to know the size recipe I can fit into a mold I cheat and look it up on the site I purchased it from. But for those times that I either can't remember where I got a mold from, if it's been discontinued or even if it is a homemade mold like a milk carton or pringles can for instance there is a calculation for figuring this out. AND it's quite simple :)

Method #2: The below formulas will give an answer in ounces but this can easily be converted to grams if that it your preferred working unit.

For square and rectangular molds: 

Length x Width x Height = X

X x 0.4 = Oz of oils needed (this number can be used in the soap calculator.)

If using centimeters your multiple is 0.7 instead of 0.4


If you need this in pounds take the ounces needed and divide by 16.

Example: 12” x 4” x 3” = 144”

144” x 0.4 = 57.6 oz always round down to prevent over-pouring so 57 oz of oil will fit into this mold.

In pounds this would be 57.6 oz / 16 oz = 3.6 pounds

For cylinder molds:

3.14 x radius squared x height of pour = X

Radius is the distance from the center of the circle to the edge. The  diameter is the distance across the entire circle. So a 3" round PVC pipe has a radius if 1.5"



X x 0.4 = Ounces of oils needed

Example: To figure out for a 3" round by 12" height of a cylinder mold:

3.14 x 1.5" x 1.5" x 12" = 84.78"

84.78" x 0.4= 33.91 ounces of oils (round down to 33 oz).


Method #3: As a third option if you are unsure of your math skills or just to confirm that you did your math correctly you can also place your mold on your scale and tare to "0" then fill with water to the height you want your soap to be. Soap is more dense than water so your batch may be a little shorter than you anticipate but it will get you pretty close without going over. This method will get you the full size of your batch, water, oils and lye.

I hope these tips and tricks help you in your soap making and calculating endeavors. Please feel free to ask questions!

As always,

Your Soapsmith

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