A free soap calculator for NaOH and KOH lye. Combine several oils, set your superfat and lye concentration, and get lye, water, batch weight and oil percentages in seconds.
⚠️ Verify before you make soap
SAP values vary by supplier, batch and refining. The presets here are typical reference values only. Confirm each SAP with your supplier or a trusted reference, and cross-check the result with a second calculator. This tool is not a substitute for safety procedures: lye is caustic and can cause severe burns and blindness. Wear splash goggles, gloves and long sleeves, work with good ventilation, and always add lye to water, never water to lye.
Common range: 25–40% lye concentration (about 1.5:1 to 3:1 water to lye). 33% is roughly 2:1.
Enter a value to convert.
The SAP column above uses grams of your selected lye per gram of oil. Converting NaOH ↔ KOH uses the molar-mass ratio (KOH ≈ 1.403 × NaOH).
Every soap recipe needs the right amount of lye to turn oils into soap. Too little leaves a greasy, soft bar; too much leaves lye-heavy soap that can burn skin. This guide explains how the numbers work so you can check any result, including ours.
A lye-to-oil ratio is the weight of lye needed per weight of oil. It is not one fixed number: each fat has its own ratio, so a recipe's total comes from adding up every oil. The calculator uses:
Lye = Σ (oil weight × SAP) × (1 − superfat %) ÷ lye purity
Worked example: 500 g olive, 300 g coconut, 150 g shea and 50 g castor with NaOH give 147.5 g of lye at 0% superfat. At 5% superfat that is about 140.1 g, and at 33% lye concentration about 284.5 g of water. Do not confuse this with the water-to-lye ratio, which only describes how the lye is dissolved.
The saponification (SAP) value is the amount of lye that fully saponifies one gram of a fat. It is quoted either as grams of NaOH or KOH per gram of oil (for example 0.134 NaOH for olive oil) or as milligrams of KOH per gram (about 188 for olive oil). Published values differ because oils vary with crop, refining and fatty-acid profile, and tables are often rounded or averaged. Use the figure from your supplier's specification sheet when you have one.
Superfat is the percentage of oils deliberately left unsaponified by using less lye. It acts as a safety buffer against small weighing or SAP errors and leaves some conditioning oil in the bar. Many bar recipes use roughly 3–8%. Very low values reduce the buffer; values above about 20% tend to give soft, greasy soap that can go rancid sooner. A 5% superfat means using 95% of the lye needed for full saponification.
Sodium hydroxide (NaOH) makes firm bar soap. Potassium hydroxide (KOH) makes soft paste and liquid soap. KOH needs about 1.4 times as much lye by weight (the ratio of their molar masses), so you can never swap one for the other 1:1. Purity matters too: NaOH is typically sold at about 97–100% and KOH flakes at about 90%, so the weighed KOH must be increased to compensate. Enter your supplier's purity in the settings above.
Lye concentration is lye ÷ (lye + water) × 100. A 33% solution is about 2 parts water to 1 part lye. Common cold-process recipes sit around 25–40%. Lower concentrations (more water) give more working time but a longer cure; higher concentrations trace faster, run hotter and may not dissolve fully. Water does not change how much lye is needed, only how the batch behaves.
Any calculator is only as accurate as the SAP values and quantities entered. Differences between calculators, typing slips, mixed-up units or a wrong lye type are all enough to make an unsafe bar. Before you make soap, check each SAP against your supplier or a trusted reference, run the recipe through a second calculator, re-read the totals against your intent, and consider a small test batch first.
Weigh everything by mass on a digital scale. Choose one that reads to 0.1 g or better (0.01 g is helpful for small batches), tare the container each time, and keep it on a stable, level surface. Check it periodically with a known calibration weight. Do not round the lye down to be "safe": under-weighing is simply a lower superfat, and over-weighing risks lye-heavy soap. Record the weights you actually used.
Multiply each oil's weight by its SAP value, add the results, apply your superfat (multiply by 1 minus the superfat percentage) and divide by lye purity. This calculator does it for every oil in your recipe, then works out the water from your chosen concentration or ratio.
Many bar soap recipes use about 3–8%, but the right figure depends on your oils, intended use and how confident you are in your SAP values and scale. Very low superfat leaves little safety margin; very high superfat can make soft, short-lived bars.
Not directly. KOH needs roughly 1.4 times more lye by weight and is usually less pure, and KOH produces soft or liquid soap rather than a firm bar. Switch the lye type in the calculator so SAP values and purity are updated, and expect a different finished product.
Cold process soapers commonly use 25–40% lye concentration, which is about 1.5:1 to 3:1 water to lye. More water slows trace and lengthens the cure; less water speeds trace and increases heat. It changes handling, not the lye needed.
Each calculator relies on its own SAP data, rounding and defaults for purity or water. Small differences are normal, but large ones signal an error. Compare against a second source and your supplier's SAP values.
No. It only estimates quantities. You are responsible for verifying SAP values and following proper lye handling: eye and skin protection, ventilation, correct mixing order, and safe storage away from children and pets.
Yes if your lye is not 100% pure. Divide the pure lye needed by the purity. Flake KOH is commonly about 90% pure, so you weigh more of it. Check your supplier's label and enter that value.
The lye maths is the same for cold and hot process soap. For liquid soap, choose KOH and a verified purity, and follow a tested dilution method. This tool does not cover mixed NaOH/KOH recipes or paste dilution.