Use the free Soap Lye Calculator below. It runs entirely in your browser, with no sign-up and no download.
Soap is chemistry you can hold
Soap making looks like cooking, but it is chemistry with a deadline. Oils and fats react with a strong alkali, sodium hydroxide for bar soap or potassium hydroxide for liquid soap, and turn into soap and glycerine. The reaction is called saponification, and the amount of lye it needs is fixed by the oils you use. Too little lye and the bar stays greasy and soft. Too much and the bar is harsh, drying and potentially irritating to skin. Getting the lye right is the whole craft.
This calculator does the central arithmetic. Each oil has a saponification value, the grams of sodium hydroxide needed to saponify one gram of that oil. Multiply each oil’s weight by its value, add them up, and you have the lye for a zero superfat batch. Then superfat, the deliberate small excess of oil left unsaponified, trims the lye down and leaves the bar gentler and more conditioning.
Safety is not optional
Lye is caustic. It burns skin on contact, it can blind, and its solution heats up sharply as it dissolves. This calculator is an educational estimator only, and it is not professional advice. Wear chemical splash goggles and gloves, work in a ventilated space, and always add lye to water, never water to lye, because adding water to dry lye can cause a violent eruption of hot caustic liquid. Keep lye locked away from children and pets, label every container, and know where your vinegar and running water are before you start, not after.
If lye touches skin, rinse under running water for a long time and seek medical advice if the exposure is more than trivial. If it reaches eyes, flush continuously and get emergency care. These are not remote warnings. Every experienced soap maker has a story about a splash, and the ones who stayed safe are the ones who were wearing their goggles when it happened.
A worked example
Take a one kilogram batch: 500 grams of olive oil, 300 grams of coconut oil and 200 grams of shea butter, at 5 percent superfat. Olive oil needs 0.135 grams of sodium hydroxide per gram, so 500 grams needs 67.5 grams. Coconut oil at 0.183 needs 54.9 grams for 300 grams. Shea butter at 0.128 needs 25.6 grams for 200 grams. The total at zero superfat is 148 grams, and trimming 5 percent for superfat gives 140.6 grams of sodium hydroxide. With a water to lye ratio of 2 to 1, that is about 281 grams of water.
For liquid soap, the same batch uses potassium hydroxide instead, and potassium hydroxide weighs more per unit of saponifying power. Multiply the sodium hydroxide figure by 1.403 to get the potassium hydroxide figure, which is about 197 grams for this batch. Potassium hydroxide is also usually sold at lower purity than laboratory grade, so many makers adjust for purity as well. When in doubt, run the recipe through a second calculator and compare before you mix.
Superfat, water and curing
Superfat is usually set between 3 and 8 percent for bar soap. Lower superfat gives a harder, longer lasting bar with more cleansing power. Higher superfat gives a gentler bar but one that can go soft or develop orange spots, signs of rancidity, sooner. Coconut oil heavy recipes often use higher superfat to tame their cleansing strength, while pure olive oil castile soap is often made at low superfat and cured for many months.
Water content, set here as a ratio to lye, controls how fast the soap reaches trace and how long it takes to harden. More water means slower trace and a longer cure. Less water means faster trace and a harder bar sooner, but less working time. After moulding, cold process soap needs four to six weeks of curing in air before use, during which the last of the lye completes its reaction and excess water evaporates. Never shortcut the cure on a new recipe.
Frequently asked questions
Can I swap one oil for another without recalculating? No. Every oil has a different saponification value, so swapping oils changes the lye requirement. Recalculate for every recipe change, even a small one.
Why does my soap calculator disagree with yours? Different sources publish slightly different saponification values, and purity assumptions for potassium hydroxide vary. Small differences of a gram or two are normal, but large ones mean one of you has a wrong input.
Is handmade soap lye free when finished? Properly made and fully cured soap contains no free lye. The lye is consumed by the reaction. That is why accurate measurement and full curing matter so much.
Spanish version
Calculadora de Sosa para Jabón – Sosa Cáustica y Potasa con Sobrengrasado y Notas de Seguridad