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. This is a calculated lye discount, not a measurement of free oil in the finished bar. Water is set from the lye at the concentration you choose. That is the entire calculation; 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. This arithmetic is checked against SoapCalc's formulas and SAP values. Bramble Berry's calculator prints about 148.3 g because its published settings assume NaOH is 97% pure. That's not a mistake, just 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. The two lye columns are each oil on its own at 1,000 g: NaOH at 5% superfat, 90% KOH at 0%.
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 SoapCalc's table uses 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 set to 100% with 90% flakes supplies less active KOH than intended. These differences can consume or exceed a recipe's margin. Match the supplier's purity and the exact oil before comparing results; the superfat guide shows how published SAP ranges affect that margin.
"Coconut oil" in a calculator means 76° coconut (SAP 0.257); fractionated coconut is 0.325. Pick fractionated when the jar on your counter is the solid kind and the calculator asks for about a quarter more lye than the oil can use, which is lye-heavy soap; the opposite mistake leaves the bar soft and greasy. "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.
Which lye do I use, and can I make soap without it?
Choose NaOH for a bar recipe and KOH for a liquid-soap recipe. Their weights are not interchangeable. Use the liquid soap calculator for paste dilution, or the dual-lye calculator when a shave recipe uses both. This quick tool assumes 100% NaOH; open the recipe in the builder to enter another NaOH purity.
Making soap from oils requires an alkali. If you want to avoid handling raw lye, use a ready-made base with the melt and pour calculator; its manufacturer has already made the soap. The process comparison explains the options.
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. Read the handling checklist and weigh by mass, not fluid ounces. For small batches, the scale comparison shows why the last displayed digit matters.