Lye Calculator

Build a soap recipe, choose your lye type and water method, and get exact lye, water, and batch weights.

Step-by-step guide

What it's for

Works out how much lye and water your oils need, so your soap comes out safe and the way you want it.

You'll need

The oils you want to use (as percents), and the total weight of oils. Not sure of the weight? The Mold Calculator works it out from your mold.

Fills the tool in with an example so you can see how it works.

Build a soap recipe, choose your lye type and water method, and get exact lye, water, and batch weights.

New to the calculator? Read our step-by-step guide →

1. Type of lye

2. Weight of oils

lb

Not sure how much oil fills your mold? Use the Mold Calculator →

3. Water

4. Additional settings

%
oz

How much fragrance for this batch? Find the safe range in the Fragrance Calculator →

5. Select oils, fats, and waxes

Browse the full oil properties glossary →

ValueOneAll

Oils, Fats and Waxes

6. Recipe oil list

RowOil Name%Weight (lb)
Total 0% 0

Tip: Saved recipes stay in this browser and show up in our Soap Cost Calculator, Ingredient List Maker and Oil Swap Helper, so you can bring them in without typing them again.

Scroll down to view your recipe.

7. Recipe summary

Add oils to the recipe list, with percentages totaling 100%, to see the full recipe summary.

Sodium hydroxide and potassium hydroxide are caustic and cause severe burns. Wear goggles, gloves, and appropriate safety equipment.

Next steps for this recipe

Cost this recipe What each bar costs you and what to charge. Make an ingredient list Your label list, from this recipe. Swap an oil Out of an oil? Find one to use instead. Work out the fragrance How much fragrance oil for this batch?

Good to know

How do you calculate lye for soap making?

Each oil needs a set amount of lye to turn into soap, called its SAP value. The calculator multiplies each oil's weight by its SAP value for the lye you picked, adds them up, then takes off your superfat so a little oil stays free in the bar.

Water comes last, set whichever way you like. It all happens for you, including the extra for 90% pure KOH.

Can I swap NaOH and KOH?

No. The same oils need a different amount of each, so you can't use one in place of the other 1:1. Pick the lye you're actually using and the calculator works out the right amount.

How do I handle lye safely?

Always add lye to water, never water to lye. Adding water to lye can make it heat up fast and boil over.

Wear goggles, gloves and long sleeves whenever you handle lye, and work somewhere with good airflow.

Want more on handling lye safely, plus what to do if it touches your skin? See our Lye Safety FAQ.

Fine print: SAP values are estimates from published data for each oil and can vary a little by supplier or batch. For a brand-new recipe, double-check your numbers before you pour.

See a problem or have an idea? Help us make this better.

These tools are new, and we're still fine-tuning them. If something looks off, like a number that doesn't add up, a missing product or a confusing step, email us at our contact page and we'll fix it. Have an idea, or a tool you'd like us to make? We'd love to hear that too!

These tools are new. Tell us what's off, or what you'd like us to make:
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© 2026 Nurture Handmade. This tool, its data and its wording are ours. Please don't copy, scrape or republish them. See our Terms of Service.

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