How a lye calculator works: SAP values explained
Why every oil needs a different amount of lye — and why you must never guess.
Don't guess — calculate: the soap lye calculator takes your oil blend and returns the exact NaOH or KOH, water and superfat. Free, and always run your final recipe through a second calculator.
Never make cold-process soap without running your exact recipe through a lye calculator and wearing gloves and eye protection. The numbers below explain the chemistry — they are not a substitute for calculating your specific batch.
Cold-process soap is a chemistry problem: oils + lye + water → soap. The "how much lye" question has one correct answer per recipe, and it comes from the saponification (SAP) value of each oil you use.
What a SAP value is
Every fat and oil has a known SAP value — the amount of lye needed to fully saponify (turn into soap) one gram of that oil. Because each oil's fatty-acid makeup is different, each has its own number. A few examples (grams of NaOH per gram of oil):
| Oil | NaOH SAP (approx.) |
|---|---|
| Coconut oil | 0.178 |
| Olive oil | 0.134 |
| Palm oil | 0.141 |
| Shea butter | 0.128 |
| Castor oil | 0.128 |
SAP values are published in standardized references (AOCS methods, USDA Handbook 8) and are the foundation of Kevin Dunn's Scientific Soapmaking. A calculator multiplies each oil's weight by its SAP value and sums them to get the total lye for the batch.
NaOH vs KOH. Bar (solid) soap uses sodium hydroxide (NaOH). Liquid soap uses potassium hydroxide (KOH), which has its own, higher SAP values. Never substitute one number for the other.
Then you subtract for superfat
You almost never want to saponify 100% of the oils — that risks a harsh, lye-heavy bar. Soapmakers leave a small fraction of oil unsaponified for a gentler bar; this is called superfat (or lye discount). A 5% superfat means you use 95% of the calculated lye:
Lye used = (Σ oil weight × SAP) × (1 − superfat)
Most cold-process recipes run a 5–8% superfat. Higher superfat = more conditioning but softer, shorter-lived bars; lower = harder, more cleansing, but less margin for measuring error.
Gear this guide uses
The lye, oils, and a scale you'll weigh them on once the calculator gives you the numbers.
- Run your recipe, then weigh out sodium hydroxide (NaOH) lye on a 0.1 g digital scale — accuracy here is the whole game.
- Stock the base oils your blend calls for, like coconut oil and olive oil, each with its own SAP value.
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Sources
SAP values vary slightly by source and oil refinement. Always calculate your specific recipe and wear protective equipment.