Lye Checker: NaOH Oil by Oil from SAP Values, Superfat and Purity

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Opens on the book's five oils with the SoapCalc SAP values Chapter 3 cites. This page has no SAP table of its own: type the value for each of your oils from your supplier or a lye calculator.

Lye

Superfat you type %

Lye purity % from the spec sheet

Or a true superfat to aim for %, optional

Your oils: grams and each oil's KOH SAP value (mg KOH per g, e.g. 190, or 0.190). Prefilled with SoapCalc's values, as the book cites them; type your supplier's.

Oil 1

SAP, KOH SoapCalc

Oil 2

SAP, KOH SoapCalc

Oil 3

SAP, KOH SoapCalc

Oil 4

SAP, KOH SoapCalc

Oil 5

SAP, KOH SoapCalc

Oil 6

SAP, KOH

Oil 7

SAP, KOH

Oil 8

SAP, KOH

Enter the oils and press Calculate.

The formula

KOH for each oil = grams × SAP (mg KOH per g ÷ 1000)
NaOH = KOH ÷ 1.4025 (56.1 ÷ 40), oil by oil, then added. Never from one average
Lye to weigh = the total × (1 − superfat)
Real NaOH = lye weighed × purity; true superfat = 1 − real NaOH ÷ the lye at 0 %
KOH for liquid soap: no ÷ 1.4025, and divide by the purity, as KOH calculators do

The book's worked example, Chapter 3. The Market Batch's 960 g of oils need 195.792 g of KOH, ÷ 1.4025 = 139.6 g of NaOH at 0 %. At 5 % superfat: 139.6 × 0.95 = 132.62, so weigh 132.6 g. Soap-grade lye is about 98 % pure: 132.6 × 0.98 = 129.95 g of real NaOH, a true superfat of about 6.9 %.

Check it in a lye calculator before you weigh. If this page and your lye calculator differ by more than about a gram, stop and find out why. A 40 % gap usually means a KOH figure was used as an NaOH one. Goggles and gloves, lye into water, never the reverse. Prices are illustrative US supplier prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.