Lye Calculator for Cold-Process Soap
Work out how much sodium hydroxide (NaOH) and water a cold-process soap recipe needs. Add each oil by weight, set a superfat, and get the lye, water and total batch weight.
How to calculate lye for soap
- Add each oil in your recipe with its weight in grams.
- Set the superfat percent and the water to lye ratio.
- Read the lye, water and total batch weight, then copy the result.
Examples
1000 g coconut oil at 5% superfat, 2:1 water
coconut 1000 g, superfat 5%, water:lye 2:1
Lye 173.85 g, water 347.7 g, total batch 1521.55 g
Frequently asked questions
Is this lye calculator safe to use as my only check?
No. Always run your exact recipe through a trusted lye calculator before making soap, and wear protection: goggles, gloves and long sleeves. Lye is caustic and a wrong weight can ruin a batch or burn skin. Treat this as a planning aid, not a final authority.
What is superfat and why does it matter?
Superfat is the percent of oils left unsaponified so they stay on your skin as free oil. A 5% superfat means the calculator uses 95% of the lye full saponification would need, which adds a safety margin and a gentler, more conditioning bar.
What water to lye ratio should I use?
A 2:1 ratio (two parts water to one part lye) is a common, beginner-friendly default. Lower water makes a firmer, faster-tracing batch; higher water gives more working time. This tool defaults to 2:1 and lets you change it.
Does this calculate NaOH or KOH?
NaOH, the sodium hydroxide used for solid bar soap. Liquid soap uses potassium hydroxide (KOH), which has different saponification values, so do not use these numbers for a KOH recipe.
What if my oil is not in the list?
Enter a custom SAP value (grams of NaOH per gram of oil) for that oil and it overrides the built-in table. You can find SAP values on supplier sheets or a reference chart.
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