How a lye calculator works
Each oil needs a fixed amount of lye to turn completely into soap — its saponification value (SAP). The calculator multiplies each oil's weight by its SAP, adds them up, converts to sodium hydroxide for bar soap (× 40 ÷ 56.1, because NaOH is the lighter molecule) or divides by purity for potassium hydroxide, and then takes off the superfat: at 5% it uses 95% of that lye, leaving 5% of the oils as free oil in the bar. Water is set from the lye at the concentration you choose. That is the whole thing; the full builder adds fragrance, cost and the soap-quality bars on top of the same numbers.
Olive 35 / coconut 30 / palm 30 / castor 5 at 5% superfat: blend SAP 0.2123 × 1,000 × 0.95 × 0.713 = 143.8 g NaOH; water at 33% = 143.8 × 0.67 ÷ 0.33 = 292.0 g. SoapCalc prints the same figures. Bramble Berry's calculator prints about 148.3 g because it assumes NaOH is 97% pure — not a mistake, a different assumption; the builder lets you set either.
Lye for single oils — the numbers behind the calculator
| Oil | SAP (KOH, g/g) | SAP (NaOH) | Published range (mg KOH/g) | NaOH per kg at 5% SF | KOH (90%) per kg at 0% SF |
|---|---|---|---|---|---|
| Olive oil | 0.190 | 0.1355 | 184–196 | 128.7 g | 211.1 g |
| Coconut oil (76°) | 0.257 | 0.1832 | 250–264 | 174.1 g | 285.6 g |
| Palm oil | 0.199 | 0.1419 | 190–209 | 134.8 g | 221.1 g |
| Lard | 0.198 | 0.1412 | 190–205 | 134.1 g | 220.0 g |
| Beef tallow | 0.200 | 0.1426 | 190–202 | 135.5 g | 222.2 g |
| Shea butter | 0.179 | 0.1276 | 170–190 | 121.2 g | 198.9 g |
| Cocoa butter | 0.194 | 0.1383 | 188–198 | 131.4 g | 215.6 g |
| Castor oil | 0.180 | 0.1283 | 176–187 | 121.9 g | 200.0 g |
| Sweet almond oil | 0.195 | 0.1390 | 190–200 | 132.1 g | 216.7 g |
| Avocado oil | 0.186 | 0.1326 | 177–198 | 126.0 g | 206.7 g |
| Rice bran oil | 0.187 | 0.1333 | 180–195 | 126.7 g | 207.8 g |
| Sunflower oil | 0.189 | 0.1348 | 188–194 | 128.0 g | 210.0 g |
| Palm kernel oil | 0.247 | 0.1761 | 230–254 | 167.3 g | 274.4 g |
| Babassu oil | 0.245 | 0.1747 | 245–256 | 166.0 g | 272.2 g |
Source: SoapCalc oil list, retrieved 2026-08-19; the range column is the spread of values in the soapmaking and lipid literature, which is why superfat exists. Computed by this site's engine.
Why calculators disagree by a gram or two
Three reasons, all visible above. SAP tables differ: an oil's value varies by crop and supplier, and calculators pick different points in the published range — olive is anywhere from 184 to 196 mg KOH/g and most calculators use 190. NaOH purity: SoapCalc (and this site by default) assume 100%; Bramble Berry assumes 97%, which lifts lye about 3%. KOH purity: flake KOH is ~90% pure and any calculator that ignores that under-doses liquid soap by a tenth. None of these differences is large next to a 5% superfat, which is the point of the superfat; all of them are large next to a 0% one.
"Coconut oil" in a calculator means 76° coconut (SAP 0.257); fractionated coconut is 0.325 and would be badly lye-light if you meant the solid stuff. "Olive" and "pomace olive" differ slightly; "palm" and "palm kernel" differ a lot (0.199 vs 0.247). Read the oil name you picked back against the tub on the counter before the lye goes in.
What else you need before you pour
The lye weight is one of four numbers. The mold calculator gives you the oil weight that fits the mold; the concentration converter explains the water choice; the superfat adjuster tells you what you actually made if the scale read off. The recipe builder does all of it on one screen, with a printable sheet — use the button above to carry this recipe across.