Soap Lye (Saponification) Calculator
Cold-process soap is made by saponifying oils with sodium hydroxide (lye). The lye needed depends on the oil weight and the oil's saponification value, reduced by a superfat margin so the bar is mild and safe. Enter your oil weight, the NaOH SAP value of your oil, a superfat percentage and a lye concentration, and this tool returns the lye, water and total batch.
Saponification formula
Full lye = oil weight * SAP value
Lye with superfat = full lye * (1 - superfat / 100)
Lye solution = lye / (concentration / 100)
Water = lye solution - lye
Total batch = oil + lye + water
The NaOH SAP value is grams of sodium hydroxide per gram of oil. For a blend of oils, weight-average the SAP values of the components, or run each oil separately and sum the lye.
Worked example
For 500 g of an oil with a NaOH SAP value of 0.134 at 5 percent superfat: full lye = 500 times 0.134 = 67.00 g. With superfat = 67 times 0.95 = 63.65 g NaOH. At 33 percent lye concentration, the solution weighs 63.65 / 0.33 = 192.88 g, so water = 192.88 minus 63.65 = 129.23 g. Total batch = 500 plus 63.65 plus 129.23 = 692.88 g.
Soap lye: frequently asked questions
How is the lye amount for soap calculated?
Each oil has a saponification value: the grams of sodium hydroxide (NaOH) needed to saponify one gram of that oil. Multiply the oil weight by its SAP value to get the lye for full saponification, then reduce it by your superfat percentage to leave some oil unsaponified for a milder bar.
What is superfat and why reduce the lye?
Superfat is the share of oil left unreacted to condition the skin and provide a safety margin against excess lye. A 5 percent superfat means you use 5 percent less lye than full saponification requires, so 95 percent of the calculated NaOH.
Where does the SAP value come from?
The saponification value is a measured chemical property of each fat or oil, published in chemistry references and on lye and oil supplier data sheets. This calculator takes the SAP value as an input so you use the figure for your specific oil rather than a generic number.
Why is soap-making lye safety so important?
Sodium hydroxide is caustic and reacts exothermically with water. Always add lye to water (never water to lye), wear eye and skin protection, work in ventilation, and confirm your amounts. This calculator is a planning aid; verify against your recipe and supplier guidance.
Sources and notes
- The method is standard saponification chemistry: lye equals oil weight times the oil's saponification value, less the superfat margin.
- The saponification value is a measured property of your specific oil; take it from your oil or lye supplier's data sheet. This tool does not assume an oil.
- Lye safety guidance: see your supplier's safety data sheet and follow all protective measures.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.