Soap Mould Volume Calculator
Moulds are measured in volume, recipes in weight, and batter density is what joins them — soap batter runs a little lighter than water, 0.9–1.0 g/mL, and 0.95 is the default here. A 1,200 mL mould therefore holds about 1,140 g of batter. Reading the oil back out of that takes one assumption: the lye is a fixed share of the oil weight (13.5% by default, a realistic figure for common blends) and the water is a multiple of the lye (2× by default). The batter is then 1 + 0.135 + 0.27 = 1.405 times the oil, so 1,140 ÷ 1.405 = 811 g of oil, 109.5 g of lye and 219 g of water. The true lye share is set by your oil blend, so once the blend is fixed, put its lye ÷ oil figure from the multi-oil lye calculator into that field. Measure the mould with water, and expect a silicone mould to take about 5% more than the arithmetic, because the walls bow out under the batter.
Your numbers
Batter total
1,140g
Oil weight
811g
Lye needed
109.5g
Water
219g
Formula
Batter total = Mould volume × Batter density, Oil weight = Batter total ÷ (1 + Lye as share of oil ÷ 100 × (1 + Water : lye ratio))
Do not fill to the rim. Batter rises as it heats, and a batch that gels over the top sets in a puddle around the mould — leave 1–1.5 cm of headroom. Pour too little instead and the bars come out shallow and crumble when cut. Density moves with the recipe: hard oils such as coconut make it heavier, and whipped batter is far lighter. Once you have poured a mould, note its batter weight divided by its volume and use that — your own figure beats any default.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Mould volume (mL)What the mould holds; measure it with water. | 1,200 | 0 and up |
| Batter density (g/cm³)Grams per cm³ of batter; from a poured mould it is batter weight ÷ mould volume. | 0.95 | 0.6 ~ 1.3 |
| Lye as share of oil (%)Lye as a percentage of the oil weight — lye ÷ oil once the blend is fixed. | 13.5 | 0 ~ 25 |
| Water : lye ratio (×)Water per part lye; 2:1 is the common choice. | 2 | 0 ~ 4 |
Step by step
Quick reference table
Results when only Mould volume (mL) changes and everything else stays put.
| Mould volume (mL) | Batter total (g) | Oil weight (g) | Lye needed (g) |
|---|---|---|---|
| 600 | 570 | 406 | 54.8 |
| 900 | 855 | 609 | 82.2 |
| 1,200 | 1,140 | 811 | 109.5 |
| 1,800 | 1,710 | 1,217 | 164.3 |
| 2,400 | 2,280 | 1,623 | 219.1 |
What each result means
| Result | At default values |
|---|---|
| Batter total (g)Oils, water and lye together. | 1,140 |
| Oil weight (g)Total oils in the recipe, excluding water and lye. | 811 |
| Lye needed (g)Lye needed; too little leaves it soft, too much leaves it caustic. | 109.5 |
| Water (g)Water to dissolve in; lye goes into water, never the reverse. | 219 |
Common mistakes
Do not fill to the rim. Batter rises as it heats, and a batch that gels over the top sets in a puddle around the mould — leave 1–1.5 cm of headroom. Pour too little instead and the bars come out shallow and crumble when cut. Density moves with the recipe: hard oils such as coconut make it heavier, and whipped batter is far lighter. Once you have poured a mould, note its batter weight divided by its volume and use that — your own figure beats any default.
Glossary
- Mould volume
- What the mould holds; measure it with water.
- Batter density
- Grams per cm³ of batter; from a poured mould it is batter weight ÷ mould volume.
- Lye as share of oil
- Lye as a percentage of the oil weight — lye ÷ oil once the blend is fixed.
- Water : lye ratio
- Water per part lye; 2:1 is the common choice.
- Batter total
- Oils, water and lye together.
- Oil weight
- Total oils in the recipe, excluding water and lye.
- Lye needed
- Lye needed; too little leaves it soft, too much leaves it caustic.
- Water
- Water to dissolve in; lye goes into water, never the reverse.
Frequently asked questions
Q. How is Soap Mould Volume Calculator calculated?
Batter total = Mould volume × Batter density, Oil weight = Batter total ÷ (1 + Lye as share of oil ÷ 100 × (1 + Water : lye ratio)) — Moulds are measured in volume, recipes in weight, and batter density is what joins them — soap batter runs a little lighter than water, 0.9–1.0 g/mL, and 0.95 is the default here. A 1,200 mL mould therefore holds about 1,140 g of batter. Reading the oil back out of that takes one assumption: the lye is a fixed share of the oil weight (13.5% by default, a realistic figure for common blends) and the water is a multiple of the lye (2× by default). The batter is then 1 + 0.135 + 0.27 = 1.405 times the oil, so 1,140 ÷ 1.405 = 811 g of oil, 109.5 g of lye and 219 g of water. The true lye share is set by your oil blend, so once the blend is fixed, put its lye ÷ oil figure from the multi-oil lye calculator into that field. Measure the mould with water, and expect a silicone mould to take about 5% more than the arithmetic, because the walls bow out under the batter.
Q. Can you walk through an example?
With Mould volume 1,200mL, Batter density 0.95g/cm³, Lye as share of oil 13.5%, Water : lye ratio 2×, the answer is Batter total 1,140g.
Q. What do I need to enter?
Enter Mould volume, Batter density, Lye as share of oil, Water : lye ratio. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Mould volume (mL) moves the answer to Mould volume (mL) 600 → Batter total (g) 570 and Mould volume (mL) 2,400 → Batter total (g) 2,280. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Do not fill to the rim. Batter rises as it heats, and a batch that gels over the top sets in a puddle around the mould — leave 1–1.5 cm of headroom. Pour too little instead and the bars come out shallow and crumble when cut. Density moves with the recipe: hard oils such as coconut make it heavier, and whipped batter is far lighter. Once you have poured a mould, note its batter weight divided by its volume and use that — your own figure beats any default.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.