Soap Lye (SAP) Calculator
Estimate the sodium hydroxide (NaOH) and water for a single-oil cold-process soap batch using saponification values and a superfat.
How it works
Lye (NaOH) = oil weight × SAP value × (1 − superfat%) · Water = lye × (water:lye ratio)
Every fat has a saponification (SAP) value — the grams of lye needed to fully convert one gram of that oil into soap. Multiplying oil weight by the SAP value gives the exact lye, and the superfat percentage deliberately leaves a little oil unreacted so the finished bar is milder and never lye-heavy. Water is not part of the reaction ratio; it simply dissolves the lye, so it is set by a water-to-lye ratio, commonly around 2:1.
Worked example
For 500 g of olive oil (SAP 0.134) at a 5% superfat: 500 × 0.134 × 0.95 = 63.7 g of NaOH. At a 2:1 water ratio that is 63.7 × 2 = 127 g of water.
Frequently asked questions
Is this safe to use for a real batch?
Treat it as an educational single-oil estimate. Before making soap, run your exact recipe through a dedicated soap calculator, weigh everything on a scale, and follow proper lye safety — goggles, gloves, and always adding lye to water, never the reverse.
What does superfat do?
Superfat leaves a small percentage of oils unsaponified so there is no risk of excess lye and the bar feels more moisturising. 5% is a common all-purpose value; higher superfats give softer, more conditioning bars.
Can I mix several oils?
Yes, but each oil has its own SAP value, so you calculate the lye for each oil separately and add them. This calculator covers a single oil — a multi-oil recipe needs a full soap calculator.
Related calculators
Educational estimate for a single oil. Sodium hydroxide is caustic — verify every recipe in a dedicated soap calculator and follow lye-handling safety.