Soap lye calculator.

Worked example

Recipe: 300 g olive oil, 200 g coconut oil, 50 g castor oil, 5% superfat, NaOH at 100% purity, 33.3% lye concentration. Full-saponification NaOH = 300 x 0.134 + 200 x 0.183 + 50 x 0.128 = 40.2 + 36.6 + 6.4 = 83.2 g. With a 5% superfat, 83.2 x 0.95 = 79.04 g NaOH. Water at a 2:1 water-to-lye ratio = 158.1 g. Total batter before fragrance or colour: 550 + 79.0 + 158.1 = 787.1 g.

How it works

Each oil has a saponification (SAP) value: the grams of NaOH that fully convert one gram of that oil to soap. Multiply each oil's weight by its SAP, add them up, and you have the lye that leaves no free oil. A superfat of 5% uses 95% of that amount on purpose, so a little oil remains in the bar as conditioning. If your lye is less than pure, divide by the purity to weigh more.

For potassium hydroxide the lye weight is the NaOH figure x 1.4028, the ratio of the molar masses (56.106 / 39.997). Water comes from your chosen lye concentration: water = lye x (100 - concentration) / concentration, so 33.3% gives 2 parts water to 1 part lye and 30% gives about 2.3 parts.

Limits. SAP values differ between suppliers and batches, so treat these as typical values. Use the SAP printed by your oil supplier when you have it, make a small test batch first, and test the cured soap before use. This tool does not predict hardness, lather or cleansing, and it does not tell you a recipe is good, only how much lye its oils need.

Lye safety

• Wear splash goggles, long sleeves and chemical-resistant gloves. Lye burns skin and eyes and can cause permanent damage.
• Work in a ventilated space. Always add lye to water, never water to lye, and stir until clear. The solution heats rapidly and releases fumes.
• Use heat-safe stainless steel, HDPE or polypropylene. Never use aluminum, which reacts with lye.
• Keep children and pets out of the room. Label the container. Keep running water nearby: flush skin or eyes with plenty of water for at least 15 minutes and get medical help.
• Weigh lye on a scale that reads to 0.1 g. Never estimate by volume.

Sources and basis (checked 2026-10-05): typical NaOH saponification values as published in soap-making tables such as SoapCalc; molar masses of NaOH (39.997 g/mol) and KOH (56.106 g/mol) from NIH PubChem; hazard information from the CDC NIOSH Pocket Guide to Chemical Hazards entry for sodium hydroxide.

FAQ

How much lye do I need for 500 g of olive oil?

At 5% superfat, 63.65 g of sodium hydroxide (500 x 0.134 x 0.95). At 0% superfat it is 67 g. Use 90 to 95% of the full amount for a bar that is not lye-heavy.

What is the water to lye ratio for cold process soap?

A common choice is 2 parts water to 1 part lye, a 33% lye concentration. Many soapers use 1.5 to 2.5 parts. More water slows trace and lengthens cure time. Less water speeds trace and can overheat.

How do I convert NaOH to KOH?

Multiply the NaOH weight by 1.4028, then divide by the KOH purity because flakes are typically around 90% KOH. This calculator does both when you choose potassium hydroxide.

What superfat should I use?

Five percent is a common starting point for bar soap. Higher superfat leaves more free oil, which can go rancid sooner. Lower is harsher. Liquid soap is often made with 0 to 3%.

Can I use these numbers without a second check?

No. Run a new recipe through a second calculator, compare the lye weights, make a small batch first and test the finished soap. A calculator error or a wrong oil selection can leave soap that irritates skin.