Soap & Lye Calculator
This calculator turns your recipe into exact lye and water amounts. Enter your oils, set your superfat, and get numbers ready for the pot. No guesswork, no math errors, just an accurate batch every time.
How to Use This Calculator
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Enter your oils by weight or percentage. Unsure which oils to use? See our guide to the best oils to enter, or check the oil properties chart for a fast reference.
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Choose your superfat. Five percent is standard for cold process soap.
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Get your exact lye and water amounts, calculated instantly.
A Real WSP Recipe, Calculated
Here is what the calculator returns for a classic olive and coconut bar, a reliable formula for new and experienced Makers alike.
Change any oil or percentage and the calculator recalculates the full batch on the spot.
What Superfat (Lye Discount) Means
Superfat is a small lye discount that leaves a portion of oils unsaponified after the reaction. That leftover oil is what keeps a bar gentle and moisturizing instead of drying. Most cold process recipes run a five percent superfat, though facial and baby bars often run higher. See the full breakdown in our guide to what saponification and superfat mean.
Why You Should Never Eyeball Lye
Lye is a strong alkali, and the margin between a safe bar and a caustic one comes down to precise measurement. Too much lye leaves a bar that can burn skin, too little leaves a soft bar that turns rancid before it sells.
Common Lye Mistakes We See
Our team often hears about the same mistakes. Makers measure lye by volume instead of weight, pour water into lye instead of lye into water, or skip the superfat because a past batch turned out fine without it. An accurate scale and a calculator easily solve two of those three main issues.
FAQ
How do you calculate lye for soap?
Multiply each oil's weight by its saponification value, then subtract your superfat percentage. This calculator runs that math automatically. Enter your oils and superfat, and it returns the exact lye weight for a balanced, safe batch.
What superfat percentage should I use?
Most cold process soap makers use five percent, which leaves enough unsaponified oil for a gentle bar without shortening shelf life. Facial bars often run eight to ten percent. Shaving and laundry soap can run closer to zero to three percent.
How much water do I use in soap?
Water amount depends on your lye concentration, typically 25-38%. A 33% concentration is a reliable starting point for most cold process recipes, balancing working time with cure time. This calculator returns the exact water weight for your batch.
Can I use this for hot process too?
Yes. Hot process soap uses the same lye and water calculation as cold process. The difference is technique, not recipe math. Enter your oils and superfat here exactly as you would for a cold process batch.
Keep Learning
New to soap making? Start with our cold process basics guide.
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