Soap mould calculator
Measure the inside of the mould to the height you will pour, and this works out the batch that fills it and the oils that make that batch. It uses your recipe’s lye and water, which the old “cubic inches × 0.4” rule assumes for you.
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The rule of thumb
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How it was worked out
Calculated at soapformulator.com, every value sourced. Check the figures against your own scale.
The formula, and where it comes from
volume, loaf = length × width × pour height volume, cylinder = π × (diameter ÷ 2)² × pour height batch (g) = volume (mL) × batter density × fill oils (g) = batch ÷ (1 + lye/oils + water/oils + extras/oils) the old rule (oz) ≈ volume (in³) × 0.4
- Volume of a rectangular prism V = l × w × h; of a cylinder V = π × (d ÷ 2)² × h. 1 in³ = 16.387064 mL.
- Empirical default. Fresh cold-process soap batter has a density close to water, ≈ 0.95–1.0 g/mL; the community rule "in³ × 0.4 = oz of oils" implies about 0.69 g/mL of oils, which is ≈ 1.05 g/mL of batter at 38 % water and 5 % superfat. Calibrate by weighing a filled mold once. — Density varies with whipping, fragrance, temperature and how full the mold is poured. Treat the default as a starting point and calibrate.
Measuring the mould
Measure the inside, not the outside, and use the height you will pour to rather than the brim. A liner or baking paper takes a little of the volume. For a mould with sloping sides or rounded corners the volume setting is more accurate: fill the mould with water to the pour line, weigh the water, and count each gram as a millilitre.
Cylinders and tubes
For a round mould or a length of pipe, enter the inside diameter and the pour height, which for a tube is its length. The volume is π × (diameter ÷ 2)² × height, so an error in the diameter counts twice: measure across the middle, not at the rim. A tube 7.5 cm across and 25 cm long holds 1104 mL, which takes about 778 g of oils in a typical bar.
Cavity moulds
A tray of cavities is sold by the volume of one cavity. Enter that and the number of cavities. If the listing gives no volume, fill one cavity with water and weigh it.
Batter density
The batch a mould holds is its volume times the density of the batter. Fresh cold-process batter weighs close to water, about 0.95 to 1.0 g per mL, so 1.0 is the default. Whipped soap, or batter full of air, is lighter. To calibrate, weigh one full mould of your usual recipe and divide by its volume.
Where “cubic inches × 0.4” comes from
The rule turns cubic inches of mould into ounces of oils. It quietly assumes one recipe, with about 38 % water and 5 % superfat, and batter a little heavier than water. For a loaf 25 × 9 × 6 cm it says 934 g of oils. At the default density, a typical bar fills that loaf with 889 g of oils when the water is 38 % of the oils, and with 951 g at a 33 % lye solution: the less water a recipe carries, the more oil fills the same mould.
Common questions
How much soap fills a 25 × 9 × 6 cm loaf mould?
The loaf holds 1350 mL, which at the default density is 1350 g of batter. For a typical bar at 5 % superfat with a 33 % lye solution, that is 951 g of oils, 132 g of sodium hydroxide and 266 g of water.
How do I work out a mould’s volume?
Length × width × pour height for a loaf or slab; π × (diameter ÷ 2)² × pour height for a cylinder; the volume of one cavity times the number of cavities for a tray. Or fill the mould with water to the pour line and weigh it: a gram of water is a millilitre.
Why is the batch heavier than the oils?
The lye and the water are part of the batter too. A typical bar at 5 % superfat with a 33 % lye solution weighs 1.42 times its oils, so a mould is sized by the whole batch and the oils are worked back from it.
How full should I fill the mould?
Most makers pour a little short of the brim, so the top can be textured and nothing spills when the mould is moved. Enter the height you will pour to, or set the fill below 100 %.
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