How do you calculate lye for soap?
Multiply each oil's weight by its SAP value, add the results, take off your superfat, and divide by the purity of your lye: that is the lye to weigh. Then set the water from the lye (a lye concentration or a water : lye ratio) or from the oils. Use NaOH SAP values for bar soap and KOH SAP values for liquid soap.
What a SAP value is
An oil's saponification (SAP) value is the amount of alkali one gram of it needs to turn completely into soap. Published sources give it in mg of KOH per gram; calculators turn that into grams of NaOH or KOH per gram of oil. Soap Ledger uses one cited source for each oil and shows it on the Soap Ledger oil reference (sources for every SAP value). For example, Coconut oil needs 0.182854 g of NaOH per gram, and Olive oil needs 0.135448 g.
Worked example: a bar soap with NaOH
1000 g of oils (Olive oil 50%, Coconut oil 25%, Shea butter 20%, Castor oil 5%), 5% superfat, 100% pure sodium hydroxide, and a 33% lye concentration.
| Oil | Weight | SAP NaOH | NaOH needed |
|---|---|---|---|
| Olive oil | 500 g | 0.135448 | 67.72 g |
| Coconut oil | 250 g | 0.182854 | 45.71 g |
| Shea butter | 200 g | 0.129602 | 25.92 g |
| Castor oil | 50 g | 0.129388 | 6.47 g |
| Total | 1000 g | 145.83 g |
- Lye for full saponification: 145.83 g.
- Take off the 5% superfat: 145.83 × (1 − 5 ÷ 100) = 138.54 g.
- Allow for 100% purity: 138.54 ÷ 1 = 138.5 g of NaOH to weigh.
- Water at a 33% lye concentration: 138.54 × (100 ÷ 33 − 1) = 281.3 g (a water : lye ratio of 2.03 : 1, or 28.1% of the oils).
- Raw batter before cure: 1420 g.
Worked example: the same oils for liquid soap with KOH
Liquid soap uses potassium hydroxide, and KOH is often sold at about 90% purity, so the purity step matters more. The same 1000 g of oils with a 3% superfat and 90% pure KOH needs 204.56 g of pure KOH before the superfat, 198.42 g after it, and 220.5 g of KOH as weighed. At a water : lye ratio of 3 : 1 that is 661.4 g of water.
Things that change the answer
- The SAP value you use. Published values for the same oil differ a little, and real oils vary by lot. See why soap calculators give different numbers.
- Superfat. A bigger superfat means less lye. See superfat explained.
- Purity. Use the figure on your supplier's label or certificate. If you divide by 100% when your lye is 90% pure, the bar gets less lye than you planned.
- How you set the water. It doesn't change the lye, only the water. See water as % of oils vs lye concentration vs water : lye ratio.
Every number on this page is worked out by Soap Ledger's calculation engine when the page is built, so it always matches the calculator.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Try it in the lye calculator. Open these recipes and change any number; the lye and water update as you type.
Sources
Written for Soap Ledger in our own words. Numbers in the examples are calculated by Soap Ledger's engine when the page is built. Last updated .