How to use the soap lye calculator
- Choose NaOH for bar soap or KOH for liquid soap.
- Add each oil or fat and its weight. If your supplier gives a SAP value, type it in the “Own SAP” box.
- Set the superfat (5% is a safe default for bars) and the lye purity (90% for most KOH).
- Choose a water method: lye concentration (30–33% is typical), water:lye ratio, or water as % of oils.
- Optionally add fragrance as a % of oils. Weigh everything on a digital scale.
Lye calculation formula
Lye (0% superfat) = Σ (oil weight × NaOH SAP value) NaOH needed = Lye × (1 − superfat %) KOH needed = NaOH needed × 1.4027 ÷ KOH purity Water (at concentration C) = Lye ÷ C − Lye
Worked example
700 g olive oil and 300 g coconut oil at 5% superfat, with a 2:1 water:lye ratio:
- Olive: 700 × 0.135 = 94.5 g; coconut: 300 × 0.183 = 54.9 g; total 149.4 g
- 5% superfat: 149.4 × 0.95 = 141.9 g NaOH
- Water: 141.9 × 2 = 283.9 g
- Batch weight ≈ 1,000 + 141.9 + 283.9 = 1,426 g
Common NaOH SAP values
| Oil | NaOH SAP | Oil | NaOH SAP |
|---|---|---|---|
| Olive | 0.135 | Shea butter | 0.128 |
| Coconut (76°) | 0.183 | Cocoa butter | 0.137 |
| Palm | 0.141 | Sweet almond | 0.139 |
| Castor | 0.129 | Avocado | 0.138 |
| Lard | 0.138 | Beef tallow | 0.140 |
| Sunflower | 0.136 | Babassu | 0.179 |
For KOH, multiply the NaOH value by 1.4027 (before allowing for purity).
Making liquid soap with KOH
Liquid soap uses potassium hydroxide, which makes a soft paste instead of a hard bar. Most liquid soap makers use a 0–3% superfat (extra oil can make diluted soap cloudy) and enter the KOH purity from the label, usually 90%. The paste is then diluted with water to the thickness you like. Because KOH is heavier per mole than NaOH, a liquid soap recipe needs about 1.4 times more lye by weight than the same oils made into bar soap.
Lye safety
Sodium and potassium hydroxide are caustic. Wear splash goggles, gloves and long sleeves. Always add lye to water — never water to lye — in a heat-proof container, in a ventilated space, away from children and pets. Keep vinegar away from skin burns; rinse with plenty of cool water and seek medical help for eye contact.
Tips for better soap
- Balance hard and soft oils. Coconut, palm and butters make a hard, bubbly bar; olive and liquid oils make it gentle. Many recipes keep coconut at 15–30% of oils.
- Castor oil at 5–10% boosts lather; more makes bars sticky.
- Cure bars for 4–6 weeks so water evaporates and the bar becomes milder and harder.
- Test pH with strips after curing if you want reassurance; a well-made bar is typically pH 9–10.