Calculate lye for any soap recipe
Every cold-process or hot-process soap recipe lives or dies by its lye amount. Too much lye leaves a harsh, caustic bar; too little leaves it soft and greasy. This calculator uses each oil’s saponification value to compute the exact NaOH or KOH needed for your blend, then applies your superfat and water percentages.
How it works
Each oil has a published SAP value — the grams of lye needed to saponify one gram of that oil. The tool sums the lye contribution of every oil in your recipe:
lye (NaOH) = Σ (oil weight × oil SAP value)If you are making liquid soap with potassium hydroxide, the figure is scaled by the KOH-to-NaOH molar-mass ratio (about 1.403), because KOH is heavier per molecule. Superfat then reduces the lye so a chosen fraction of oils stays unsaponified for a gentler bar:
lye used = lye × (1 − superfat %)
water = total oils × water %The tool also reports the resulting lye concentration so you can match it to your preferred working method.
Common oil SAP values (NaOH)
| Oil / Butter | Approx. NaOH SAP value (per gram) |
|---|---|
| Olive oil | 0.134 |
| Coconut oil (76°) | 0.178 |
| Palm oil | 0.141 |
| Sunflower oil | 0.134 |
| Shea butter | 0.128 |
| Castor oil | 0.128 |
| Cocoa butter | 0.137 |
SAP values are averages derived from typical fatty-acid compositions. Values can vary slightly between suppliers and growing regions — this is why verifying against a second lye calculator is always recommended.
Worked example
A 1,000 g recipe of 500 g olive, 300 g coconut, 200 g palm, NaOH, 5% superfat, water at 38%:
- Olive: 500 × 0.134 = 67.0 g NaOH
- Coconut: 300 × 0.178 = 53.4 g NaOH
- Palm: 200 × 0.141 = 28.2 g NaOH
- Full-strength total = 148.6 g NaOH
- After 5% superfat reduction = 148.6 × 0.95 = ~141.2 g NaOH
- Water = 1,000 × 0.38 = 380 g
This recipe produces a balanced bar: coconut for lather and hardness, olive for conditioning and longevity, palm for hardness and a stable bar. The 5% superfat leaves a small reserve of unsaponified oils to condition the skin and provide insurance against a trace of excess lye.
NaOH vs KOH — which to use
- NaOH (sodium hydroxide): Makes solid bar soap. More commonly available as “lye” or “caustic soda.” Higher potency per gram than KOH.
- KOH (potassium hydroxide): Makes liquid or soft soap, shaving soap, and cream soap. Requires roughly 1.4× the weight of NaOH to saponify the same amount of oils. Often sold as “potassium hydroxide” or “caustic potash.”
The tool converts automatically based on your selection, applying the molar-mass ratio for KOH.
Safety — read before mixing
Lye is highly caustic and will cause serious burns on skin and eyes. Always follow these precautions:
- Add lye to water, never water to lye. The reverse causes a violent exothermic reaction and can splash boiling lye.
- Wear nitrile gloves and splash-proof goggles. Lye vapour is also irritating — work in a ventilated space or outdoors.
- Keep white vinegar nearby to neutralise spills (though flushing with water is also effective for skin contact).
- Weigh lye on a precise digital scale — a 5% error in lye weight can mean the difference between a gentle bar and a caustic one.
- Always verify against a second trusted lye calculator before you mix. Use this tool as a planning aid, not a production instruction.