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How a lye calculator works: SAP values explained
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.
Trace does this math with cited SAP values
Build your oil blend, set your superfat, and Trace returns the exact NaOH (or KOH) weight — using saponification values traced to named sources (Kevin Dunn / AOCS / USDA Handbook 8), with brand-aware oil presets (Bramble Berry, Soaper's Choice, Bulk Apothecary, Essential Depot, WSP). Pay once, no subscription.
Sources
- Kevin M. Dunn, Scientific Soapmaking (the chemistry of cold-process soap)
- AOCS official methods; USDA Agriculture Handbook 8 (fatty-acid / saponification data)
SAP values vary slightly by source and oil refinement. Always calculate your specific recipe and wear protective equipment.