Lye (NaOH) Calculator

Calculate the exact sodium hydroxide and water for any cold-process soap recipe, with a superfat you control.

How it works

Cold-process soap turns oils into soap through a chemical reaction with sodium hydroxide, also called lye. Every oil needs a different amount of lye to fully saponify, a number called its SAP value. The calculator multiplies each oil's weight by its SAP value, adds those amounts together, then subtracts a percentage for superfat, the small amount of oil left unreacted so the finished bar feels conditioning instead of stripping.

Worked example: 500 g olive oil, 300 g coconut oil, and 200 g shea butter, with a 5% superfat. Olive needs 500 × 0.134 = 67 g of lye, coconut needs 300 × 0.178 = 53.4 g, and shea needs 200 × 0.128 = 25.6 g. Add those up and you get 146 g of lye before superfat. Pulling back 5% for superfat brings the final number to 138.7 g of lye. Water comes from a simple 2.3 to 1 water-to-lye ratio, a beginner-safe dilution that gives you time to work before the batter thickens: 138.7 g of lye times 2.3 rounds to 319 g of water.

Why 5% superfat is the beginner default

Superfat is the cushion between using enough lye to react with every drop of oil and leaving a little oil unreacted to keep skin from feeling tight. At 0% superfat you get a harder, more cleansing bar that can feel drying. At 8% the bar is noticeably more conditioning but softer and shorter lived. 5% is the middle ground most recipes and calculators default to, and it doubles as a safety margin: if your scale is slightly off or an oil's real SAP value differs a touch from the published number, you're still unlikely to end up with free, unreacted lye in the finished bar.

FAQ

Why do I need a second calculator to check this one?

Published SAP values are averages. The actual saponification value of an oil shifts a little with the growing season, the supplier, and how it was refined. For most recipes that difference is tiny, but lye isn't something to guess on. Running your numbers through a second calculator, like SoapCalc, and weighing your oils and lye on a scale accurate to 1 g catches typos and gives you a real sanity check before you mix anything.

What if I don't have one of these oils on hand?

Swap in the closest oil you have and expect the lye amount to change, since SAP values differ from oil to oil. Don't substitute a new oil into an existing recipe without rerunning the calculator first. That single skipped step is one of the most common causes of soap that never fully hardens or stays greasy.

Can I use this calculator for hot process soap too?

Yes. The lye and water math is identical for cold process and hot process. The difference between the two methods is how you apply heat after mixing, not how much lye the oils need.

Is it safe to use less water than this calculator gives me?

Experienced soapers sometimes cut the water for a faster trace, but that's a technique to grow into, not a beginner move. Stick with the 2.3 to 1 ratio here until you have a few batches under your belt and understand how your recipe behaves.

For more on the chemistry behind these numbers, read what superfat actually does, the full walkthrough on how to use a lye calculator, and the habits in our lye safety guide for beginners.