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Soap Making Lye Calculator
Calculate the lye and water for a single-oil soap batch from its SAP value and your target superfat. Press Calculate.
About the Soap Making Lye Calculator
Saponification values (SAP) tell you how much alkali a gram of oil needs. They are usually quoted as milligrams of potassium hydroxide per gram of oil. For sodium hydroxide, divide the KOH SAP by 1.403. A 5% superfat leaves 5% of the oil unsaponified, so you multiply the calculated lye by 0.95.
This calculator handles one oil at a time. Real recipes blend oils, so calculate each oil separately and add the lye, or use a full recipe calculator. Always run the numbers twice, use a scale accurate to 0.1 g, and follow lye safety: gloves, eye protection, ventilation and never add water to lye.
How the calculation works
- NaOH (g) = oil (g) × SAP_NaOH ÷ 1,000 × (1 − superfat).
- SAP_NaOH = SAP_KOH ÷ 1.403.
- KOH (g) = oil (g) × SAP_KOH ÷ 1,000 × (1 − superfat).
- Water (g) = lye (g) × water:lye ratio.
Reference values
| Oil | SAP (mg KOH/g) | NaOH per 1,000 g at 5% |
|---|---|---|
| Olive | 190 | 128.7 g |
| Coconut | 257 | 174.0 g |
| Palm | 200 | 135.4 g |
| Castor | 180 | 121.9 g |
| Shea butter | 128 | 86.7 g |
| Sunflower | 189 | 128.0 g |
| Canola | 179 | 121.2 g |
Worked example: olive and coconut oil
For 1,000 g of olive oil (SAP 190 mg KOH/g) at 5% superfat, NaOH = 1,000 × 190 ÷ 1,000 ÷ 1.403 × 0.95 = 128.7 g. For coconut oil (SAP 257), the same calculation gives 174.0 g. A 2:1 water-to-lye ratio uses 257 g or 348 g of water respectively.
Real recipes blend oils, so calculate each oil separately and add the lye. Always run the numbers twice, use a scale accurate to 0.1 g, and follow lye safety: gloves, eye protection, ventilation, and never add water to lye. A recipe that looks slightly off is worth rechecking before any oil and lye meet.