Free Soap & Lye Calculator
Work out the exact lye weight for cold-process, hot-process and liquid soap — NaOH or KOH, with superfat and water sized for you. Always by weight, never by volume.
Enter your oils by weight and this free soap and lye calculator gives you the exact sodium hydroxide (NaOH) or potassium hydroxide (KOH) to turn them into soap — the one number you can't afford to get wrong. It works for cold-process and hot-process bar soap and for KOH liquid soap, applies your superfat (lye discount), sizes your water, and predicts how the bar will feel: hardness, cleansing, conditioning, lather and cure time.
Every oil saponifies at its own rate, so the lye is never a figure you can guess — matching it to your exact blend is the whole job of a lye calculator, and getting it right is what keeps a bar safe on skin. Weigh your oils, water and lye on a scale; never measure soap ingredients by volume.
Loading the calculator…
How to use the lye calculator
- Add each oil or butter in your recipe with its weight in grams or ounces.
- Pick your lye — NaOH for a solid bar, KOH for liquid soap — and your superfat (5% is a safe, common starting point).
- Set the water the way you like it: by lye concentration, water-to-lye ratio, or a percentage of your oils.
- Read off the exact lye and water weights, then weigh them out on your scale.
The lye figure updates the moment you change an oil or a weight, so you can tune a recipe and watch the numbers move before you ever open a tub. Beside the result you'll find the golden rule, PPE and first-aid reminders — safety sits next to the number, not buried on another page.
SAP values you can actually check
Every oil in the calculator carries a saponification value drawn from at least two independent references, shown as a documented range rather than a false-precise single number. You can see exactly where each figure comes from on its oil page, along with the date the data was last checked. When suppliers genuinely disagree on an oil, we flag it and tell you to confirm your own batch — we don't quietly pick a number and hope.
Start from an oil or a recipe
Not sure where to begin? Browse the soap making oils to see how each fat behaves in a bar — its lather, hardness and shelf life — or load a tested soap recipe straight into the calculator and scale it to your own batch or mould.
Soap calculator FAQ
- How much lye do I need for my soap?
- It depends entirely on which oils you use and how much of each, because every oil needs a different amount of lye to saponify. Enter your oils by weight above and the calculator gives you the exact NaOH or KOH weight for your recipe.
- What is superfat (lye discount)?
- Superfat is the small share of oils you leave unsaponified so there's no leftover lye in the finished bar — it makes the soap gentler on skin. Around 5% is a common, safe starting point for bar soap. The calculator won't let superfat quietly slip to zero or below without asking you to confirm it.
- Can I use it for liquid soap?
- Yes. Switch the lye type to KOH (potassium hydroxide) for liquid or paste soap; the calculator adjusts the lye and can estimate your dilution water too.
- Why weigh everything instead of using cups or spoons?
- Soap making is chemistry: the lye has to match the oils precisely, and volume measures are far too inconsistent — and lye far too caustic — to be safe. Always weigh your oils, water and lye on a scale.