Soap Lye Calculator

Soap making is one of the few crafts with a genuinely exact calculation at its centre. Every fat has a saponification value — the grams of sodium hydroxide it takes to convert one gram of that fat into soap. Those values are published and reliable, which is why a lye calculator can be trusted in a way that most craft rules of thumb cannot.

Add up the lye each oil demands, then deliberately use slightly less than the full amount. That shortfall is the superfat: the percentage of oils left unsaponified, which is what makes a bar feel conditioning rather than stripping. Five percent is the standard, and it also covers the small variations between batches of natural oil.

Water does not appear in the chemistry at all — it is only there to dissolve the lye so it can meet the oils evenly. That is why you can discount it. Less water means a faster trace and a shorter cure; more water means a longer working time.

1. Step 1 Lye each oil needs for each oil: grams × its SAP value Olive is 0.135, coconut 0.183, castor 0.129. Swapping oils changes this figure.
2. Step 2 Apply the superfat lye = Σ(grams × SAP) × (1 − superfat%) A 5% superfat means you use 95% of the theoretically required lye.
3. Step 3 Convert if you are using KOH KOH = NaOH_amount × 1.403 ÷ purity Potassium hydroxide is heavier per molecule and usually only 90% pure.
4. Step 4 Work out the water water = oils × water% or, by strength: water = lye ÷ concentration% − lye
5. Step 5 Total batch weight batch = oils + lye + water Expect to lose 5–10% of this to evaporation during cure.

Example: 500 g olive, 300 g coconut and 200 g palm. That is 500 × 0.135 + 300 × 0.183 + 200 × 0.142 = 150.9 g of NaOH at full saponification. At 5% superfat you use 143.4 g, with 330 g of water at the 33% default.

• Saponification values are mid-range published figures. Natural oils genuinely vary by one to two percent between harvests and suppliers, which is one of the reasons a superfat above zero is standard practice.
• Superfat is applied as a straight reduction in lye. It does not control which oils remain unsaponified — saponification is not selective, so you cannot "superfat with shea" by adding it at trace.
• KOH conversion uses the standard 1.403 factor and divides by your stated purity. Most potassium hydroxide sold to soap makers is 90%; using 100% by mistake produces a lye-heavy batch.
• Water is not part of the chemistry and its amount is a working choice, not a correctness one. Anything from a 25% to a 40% lye concentration makes soap; it changes trace speed, cure time and how prone the batch is to gel.
• Fatty-acid quality figures — hardness, cleansing, conditioning — are deliberately not shown. They require a full fatty-acid profile per oil, and an approximated version would be worse than none.
• Batch weight is the wet weight at pour. Cold-process soap loses roughly 5–10% to evaporation over a four to six week cure.
• Additives at trace — fragrance, clays, oats, colourants — are not included in the totals. Fragrance is typically 3–5% of oil weight and does not affect the lye calculation.
• All weights are grams, in both regions. Saponification values are defined per gram of oil, and working in grams is universal practice among soap makers regardless of country.
• This calculator assumes accurate scales and pure lye. Drain cleaner is not a substitute for soap-grade sodium hydroxide: it contains additives that will ruin a batch and can react unpredictably.