Got a new mold? Don't know how much soap batter you should make to fill it? Use the rules of thumb described below to estimate how much total fat to use when designing a recipe to fill a specific mold with soap.
These rules of thumb are based on making a basic soap -- water, fat, and alkali (lye) -- with no other additives, including fragrance.
If your recipe also includes fragrance, salt, oatmeal, or other bulky ingredients, you may end up with more soap batter than you need when using these rules of thumb. It is a good idea to prepare a small extra mold just in case you have some extra batter.
Getting started
Decide what units you want to use. If you use weight ounces for your soap recipes, measure your mold in inches. If you use grams, measure your mold in centimeters.
Measure the mold
For a rectangular mold, measure the inside width and inside length of the mold.
For a cylindrical mold, measure the inside diameter (the largest distance across the circle.)
For either a rectangular or cylindrical mold, decide how deep you want the soap in the mold. The soap batter could go all the way to the top of the mold or the level could be a bit lower. Measure that depth.
For an irregular mold that cannot be easily measured, pour water into the mold to the desired depth and measure the water.
If you are using inches and ounces, weigh this water in weight ounces (more accurate) or measure the water volume in fluid ounces (less accurate).
If you are using centimeters and grams, weigh this water in grams (more accurate) or measure the water volume in milliliters (less accurate).
The 0.40 Rule for inches and ounces
Mold volume, rectangular, cubic inches = Length X Width X Depth
Mold volume, cylindrical, cubic inches = 3.14 X Diameter X Diameter X Depth / 4
Mold volume, irregular, cubic inches = Water weight (weight oz) X 1.8
OR
Mold volume, irregular, cubic inches = Water volume (fluid oz) X 1.8
Weight of fat, ounces = 0.40 X Mold volume
This "weight of fat" is the estimated amount of soapmaking fats that will give you enough soap batter to fill the mold.
Use an online soap recipe calculator to create a recipe using this total weight of fat.
If you find the 0.40 Rule gives you too much or too little batter for the mold, adjust the 0.40 factor up or down to refine the answers this rule provides. Many people use a slightly smaller factor -- 0.36 to 0.38 -- to reduce the batter volume by 5% to 10%. This allows extra room for fragrance and other additives.
The 0.70 Rule for centimeters and grams
Mold volume, rectangular, cubic centimeters = Length X Width X Depth
Mold volume, cylindrical, cubic centimeters = 3.14 X Diameter X Diameter X Depth / 4
Mold volume, irregular, cubic centimeters = Water weight (grams)
OR
Mold volume, irregular, cubic centimeters = Water volume (milliliters)
Weight of fat, grams = 0.70 X Mold volume
This "weight of fat" is the estimated amount of soapmaking fats that will give you enough soap batter to fill the mold.
Use an online soap recipe calculator to create a recipe using this total weight of fat.
If you find the 0.70 Rule gives you too much or too little batter for the mold, adjust the 0.70 factor up or down to refine the answers this rule provides. Many people use a slightly smaller factor -- 0.63 to 0.67 -- to reduce the batter volume by 5% to 10%. This allows extra room for fragrance and other additives.
Online soap recipe resizer
An alternative to using the 0.40 Rule or 0.70 Rule is to use the recipe resizing feature included in some online soap recipe calculators. The calcs at Bramble Berry and SummerBeeMeadow are examples of calcs that provide resizers.
You must know the volume of your mold before using a resizer, so measure your mold and calculate its volume as explained in the sections above. The online calcs handle recipe resizing in different ways, so you will have to see which one works best for you.
More discussion about the 0.40 and 0.70 Rules
https://www.soapmakingforum.com/threads/how-to-figure-how-much-oil-for-mold.2909/
https://www.soapmakingforum.com/threads/using-40-to-calculate-my-soap-mold-oils.58581/
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Extra credit 1. Derive the 0.40 Rule for Inches and Ounces
People repeatedly ask how the 0.40 Rule came to be. I've read some pretty wild explanations. Here's a reasonable derivation of this rule of thumb --
Assumptions and supporting info
Classic "trinity" recipe of 1/3 coconut oil, 1/3 olive oil, and 1/3 palm or lard.
Assume a superfat of 5%.
Assume a "full water" lye concentration for this recipe of 28%.
Pick a total weight of 10 ounces of fats for the purposes of calculation. In other words, my "trinity" recipe is 3.333 oz of coconut, 3.333 oz of olive, and 3.333 oz of lard. (Any other numbers can be used, as long as the weights of the fats are all equal.)
Enter this "trinity" recipe into an online soap recipe calculator such as Soapcalc or calculate the recipe by hand.
What is the weight of NaOH? Soapcalc NaOH = 1.46 wt oz
What is the weight of water? Soapcalc water = 3.74 wt oz
What is the weight of lye solution? Lye = NaOH + water = 1.46 + 3.74 = 5.20 wt oz
Specific gravity of 28% lye solution is 1.310 (Dow Chemical Co.)
Specific gravity of typical soaping fats is about 0.92 (various sources)
Conversion from fluid ounces to cubic inches: 1 cu in = 0.554 fl oz
Calculations
What is the volume of oils?
Weight of oils = 10 wt oz
Volume = Weight / Specific gravity
Volume of oils = 10 wt oz / 0.92 = 10.87 fl oz
What is the volume of the 28% lye solution?
Weight of lye solution (from the previous section) = 5.20 wt oz
Volume = Weight / Specific gravity
Volume of lye solution = 5.20 wt oz / 1.310 = 3.97 fl oz
What is the total volume of soap batter based on 10 ounces (wt) of oils?
Total volume of soap batter = Oil volume + Lye solution volume
Total volume of soap batter, fl oz = 10.87 + 3.97 = 14.84 fl oz
Convert previous answer from volume in fluid ounces to volume in cubic inches --
Total volume of soap batter, cu in = Fluid ounces / (0.554 Fl oz/Cu in)
Total volume of soap batter, cu in = 14.84 / 0.554 = 26.79 cu in
Results
A basic "full water" soap recipe using 10 wt oz of oil will fill a mold that has a volume of 26.79 cu in. So how much volume of soap batter will 1 weight ounce of oil create?
Soap batter volume per 1 wt oz of oil = 26.79 cu in batter / 10 wt oz oil = 2.68 cu in / wt oz
For any given weight of oil, what volume of mold is needed?
Mold volume, cu in = 2.68 X (Total oil, wt oz)
For a given mold volume, what weight of oil is needed?
Total oil, wt oz = (Mold volume, cu in) / 2.68
Convert this division problem into the equivalent multiplication problem, so it looks more like the usual 0.40 Rule --
Total oil, wt oz = 0.37 X (Mold volume, cu in)
Conclusions
The 0.37 factor I calculated is slightly smaller than the 0.40 factor you normally see for this rule of thumb.
One reason for the difference is the original person who created the original "0.40 Rule" may have used slightly different assumptions than I did.
Or perhaps the original person preferred to have extra soap batter rather than not enough -- the 0.40 factor increases the batter volume by about 8% compared with using a 0.37 factor.
Or perhaps the original person thought "0.40" would be easier to remember than "0.37" so the number got changed for ease of remembering.
Regardless, this rule is an estimate only. Feel free to tweak the 0.40 Rule as you see fit.
Extra credit 2. Derive the 0.70 Rule for Centimeters and Grams
When converted to grams, the "Trinity" recipe described above uses 94.5 g each of coconut, olive, and lard.
Total fat weight = 3 X 94.5 = 283.50 g
Superfat = 5%
"Full water" lye concentration = 28%
From Soapcalc --
Weight of NaOH = 41.29 g
Weight of water = 106.16 g
Weight of lye solution = NaOH + water = 41.29 + 106.16 = 147.45 g
Specific gravity of 28% lye solution is 1.310 (Dow Chemical Co.)
Specific gravity of typical soaping fats is about 0.92 (various sources)
Conversion from milliliters (mL) to cubic centimeters (cu cm): 1 mL = 1 cu cm
Volume of oils = 283.50 / 0.92 = 308.15 mL
Volume of lye solution = 147.45 / 1.31 = 112.56 mL
Total volume of soap batter, fl oz = 308.15 + 112.56 = 420.71 mL = 420.71 cu cm
A basic "full water" soap recipe using 283.50 g of oil will fill a mold that has a volume of 420.71 cu cm. So how much volume of soap batter will 1 gram of oil create?
Soap batter volume per 1 g of oil = 420.71 cu cm batter / 283.50 g oil = 1.48 cu cm / g
For any given weight of oil, what volume of mold is needed?
Mold volume, cu cm = 1.48 X (Total oil, g)
For a given mold volume, what weight of oil is needed?
Total oil, g = (Mold volume, cu cm) / 1.48
Convert this division problem into the equivalent multiplication problem, so it looks more like the usual 0.70 Rule --
Total oil, g = 0.68 X (Mold volume, cu cm)
The 0.68 factor I calculated is slightly smaller than the 0.70 factor you normally see for this rule of thumb. This rule is an estimate only. Feel free to tweak this factor as you see fit.
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