Soap Lye Calculator (Sodium Hydroxide for Cold Process Soap)
Grams of sodium hydroxide and water for a batch of cold-process soap from the weight and saponification value of each oil, the superfat and the water ratio.
Results
The dashed line marks the value you entered.
What this tool does
Every fat needs a fixed amount of lye to turn into soap, given by its saponification value: grams of NaOH per gram of oil. Multiply, add up, then take off the superfat, the share of oil left unreacted so the bar stays gentle, usually 5 to 8 %. Saponification values differ a little between suppliers and sources, so use the figure printed for your oil and cross-check with a second calculator. Lye burns skin and eyes: add it to the water, never the other way round, and wear gloves and goggles.
Formula
NaOH = Σ (oil weight × SAP value) × (1 − superfat) · KOH = NaOH × 56.106 ÷ 39.997 · water = oils × water ratio
Variables
| Symbol | Meaning | Unit |
|---|---|---|
o1 | Weight of oil or fat 1 | g |
s1 | NaOH saponification value of oil 1 | g/g |
o2 | Weight of oil or fat 2 | g |
s2 | NaOH saponification value of oil 2 | g/g |
o3 | Weight of oil or fat 3 | g |
s3 | NaOH saponification value of oil 3 | g/g |
sf | Superfat | % |
wr | Water as a percentage of the oils | % |
NA | Sodium hydroxide (lye) needed | g |
WA | Water | g |
LS | Lye concentration of the solution | % |
KO | If you use potassium hydroxide instead | g |
TO | Total weight of oils | g |
Worked example
- Weight of oil or fat 1500 g
- NaOH saponification value of oil 10.134 g/g
- Weight of oil or fat 2300 g
- NaOH saponification value of oil 20.183 g/g
- Weight of oil or fat 3200 g
- NaOH saponification value of oil 30.141 g/g
- Superfat5 %
- Water as a percentage of the oils33 %
- Sodium hydroxide (lye) needed142.6 g
- Water330 g
- Lye concentration of the solution30.2 %
- If you use potassium hydroxide instead200.0 g
- Total weight of oils1000 g
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.