Soap Lye Calculator (Cold Process, NaOH)
Enter the weight of each oil in your recipe, choose a superfat and lye concentration, and get the sodium hydroxide and water you need. All calculations run in your browser.
The formula
Worked example. 1000 g of olive oil (SAP 0.135) at 5% superfat needs 128.25 g of NaOH; at 33% concentration that is 260.4 g of water.
How to use it
- Enter the weight of each oil or butter in your recipe and leave the ones you do not use empty. For an oil that is not listed, use the Other row with the NaOH SAP value from your supplier.
- Set the superfat. 5% is the usual starting point: it leaves a little oil unsaponified so the bar is mild, and it covers small differences between SAP values.
- Set the lye concentration. 33% (about 2 parts water to 1 part lye) is a common default; higher concentrations trace faster and need a shorter cure.
- Weigh the lye and water exactly as shown. Wear goggles and gloves and always add lye to water, never water to lye.
Questions
Where do the SAP values come from?
They are averages from published supplier saponification tables, converted to NaOH (KOH SAP value ÷ 1402.5). Only oils where two independent tables agree are listed. Natural oils vary by a few percent between batches; the superfat absorbs that. If your supplier gives a different NaOH value, enter the oil in the Other row with their number.
Can I use this for liquid soap?
No. Liquid soap uses potassium hydroxide (KOH), which needs different SAP values. This calculator is for bar soap made with sodium hydroxide (NaOH).
Should I double-check the result?
Yes. Run every new recipe through a second lye calculator before you make it, and check that your scale reads correctly. Too much lye makes a bar that can burn skin; too little makes a soft, greasy bar.