Saponification Value Calculator — Soap Lye Amount
Select an oil, enter its weight and your desired superfat percentage. The calculator shows the saponification value (mg KOH per gram of oil) and the exact grams of NaOH (for bar soap) or KOH (for liquid soap) you need, plus water and batch weight.
Oil / fat
g
%
Soap type
Grams of lye for complete saponification minus your superfat allowance
- 1
Full NaOH for complete saponification
500 g × 0.134 = 67 g - 2
Superfat reduction factor
1 − 5 ÷ 100 = 0.95 - 3
NaOH needed (with superfat)
67 × 0.95 = 63.65
How does this calculator work?
Multiply oil weight by its NaOH SAP value (e.g., 0.134 for olive oil), then reduce by your superfat fraction to get grams of NaOH for bar soap. For liquid soap, multiply by 1.403 for KOH. Add water at 33 % of oil weight. The saponification value (mg KOH/g) equals NaOH SAP × 1 402.8.
Formula
How this is calculated
The saponification value (SV) is a chemistry constant: the number of milligrams of potassium hydroxide (KOH) required to completely saponify one gram of a fat or oil. Each oil has a characteristic SV determined by its fatty-acid composition and the average molecular weight of its triglycerides. This calculator stores experimentally established NaOH SAP values (g NaOH per g oil) for 15 common oils and derives the KOH equivalents from the molar mass ratio (56.11 / 40.00 = 1.4028).
For soap making, the lye amount is the oil weight multiplied by the NaOH (or KOH) SAP value, then reduced by the superfat percentage. Superfat is the fraction of oil deliberately left unsaponified — it stays in the finished soap and contributes to a richer skin feel. A superfat of 5–8 % is typical for leave-on bar soap; liquid soaps often use 2–3 %. Water is set at 33 % of oil weight, which is a widely used default; some recipes use 28–38 %.
The SAP values listed here are consensus figures from established soap-making references (e.g., Kevin Dunn, 'Scientific Soapmaking', 2010) and are accurate for the typical fatty-acid profiles of these oils. Unusual or blended oils may deviate; always cross-check with a dedicated lye calculator if you are scaling up production.
Frequently asked questions
The saponification value (SV) is the mg of KOH needed to completely convert 1 g of fat into soap. It depends on the average molecular weight of the oil's triglycerides: shorter or more saturated fatty-acid chains give a higher SV (e.g., coconut oil at ~268 mg/g) while longer unsaturated chains give a lower SV (e.g., olive oil at ~188 mg/g).
NaOH (caustic soda) produces hard bar soap; KOH (potassium hydroxide) produces softer, water-soluble soap used in liquid and shaving soaps. The conversion factor between them is MW_KOH / MW_NaOH = 56.11 / 40.00 ≈ 1.403, so you need about 40 % more KOH by weight for the same oil.
Yes — a 5 % superfat is a standard, well-tested default. It means 5 % of the oil remains unsaponified and stays in the bar, contributing to moisturising feel and a small safety buffer against excess lye. Very high superfats (>10 %) can lead to rancidity; very low superfats (<2 %) risk a lye-heavy soap that irritates skin.
Also known as
TG we-Calculate Editorial Team. (2026). Saponification Value Calculator — Soap Lye Amount [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/saponification-value-calculator
TG we-Calculate Editorial Team. "Saponification Value Calculator — Soap Lye Amount." TG we-Calculate. 2026. https://we-calculate.com/calculator/saponification-value-calculator.
TG we-Calculate Editorial Team, "Saponification Value Calculator — Soap Lye Amount," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/saponification-value-calculator
@misc{wecalculate_saponification_value_calculator, title = {Saponification Value Calculator — Soap Lye Amount}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/saponification-value-calculator}}, year = {2026}, note = {TG we-Calculate} }
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