Soap Lye Calculator
Exact lye and water for any oil blend, with a live read on how the bar will feel — and every step of the chemistry shown, never hidden.
How the soap lye calculator works
Soap is what happens when a fat meets a strong alkali: the alkali splits the fat's triglycerides and turns them into soap, a reaction called saponification. Get the alkali right and you have a clean, gentle bar. Use too much and raw lye stays in the soap, which can burn skin; too little and the bar is soft and greasy. This is a calculation worth doing carefully, and worth understanding rather than trusting blindly — which is why this tool shows every step.
1. From oils to lye: SAP values
Every oil has a known SAP value — the exact weight of alkali needed to saponify one gram of that oil. The calculation multiplies each oil's weight by its SAP value and adds them up:
Total lye = Σ (weight of each oil × its SAP value)
Because sodium hydroxide and potassium hydroxide have different molecular weights, every oil has two SAP values. KOH is heavier, so a liquid-soap recipe needs about 1.40 times as much alkali by weight as the same recipe in bar form.
2. Superfat: the safety margin that also conditions
Using the exact theoretical lye is risky — any small measuring error leaves caustic lye in the bar. So soapmakers deliberately hold a little oil back from the reaction, called superfat or lye discount:
Actual lye = total lye × (1 − superfat% / 100)
Five percent is the standard for a body bar. It leaves free oils that condition the skin and guarantees no raw lye survives. Aggressive recipes — a pure coconut oil bar strips so hard — often run 15 to 20 percent. Zero superfat is a mistake: it leaves no margin for error.
3. KOH purity: the step most calculators skip
Potassium hydroxide is hygroscopic — it pulls moisture from the air — so cosmetic-grade KOH is usually only about 90 percent pure. If a calculator assumes it is pure, it recommends too little active alkali, and the liquid soap comes out cloudy, separated or greasy. The fix is to divide by the purity fraction:
Adjusted KOH = actual lye ÷ purity fraction (so ÷ 0.90 at 90%)
Sodium hydroxide is nearly pure, typically 99–100%, so the adjustment barely matters for bar soap — but this tool still exposes the field, because some suppliers ship NaOH at 97%.
4. Water: three ways to measure it
Water dissolves the lye and carries the reaction. There are three common ways to size it, and this tool offers all three. Percent of oils (default 38%) is the beginner's method — simple and forgiving, giving a fluid batter with plenty of working time. Lye concentration sets how strong the lye solution is, giving experienced makers control over trace and cure. Water-to-lye ratio ties water directly to the alkali, staying consistent whatever oils you pick.
What the quality numbers mean
The real power of a modern calculator is predicting how the bar will feel before you mix anything. Each oil carries a fatty-acid profile, and those acids drive five qualities. The tool adds up the weighted percentages across your blend and scores each one against its industry ideal range.
| Quality | Ideal | Driven by | What it tells you |
|---|---|---|---|
| Hardness | 29–54 | Lauric, myristic, palmitic, stearic | Bar longevity — saturated fats pack into a rigid lattice |
| Cleansing | 12–22 | Lauric, myristic | How strongly it strips oils; high numbers can dry skin |
| Conditioning | 44–69 | Oleic, ricinoleic, linoleic, linolenic | How moisturising it feels — from unsaturated fats |
| Bubbly lather | 14–46 | Lauric, myristic, ricinoleic | Big, fluffy, fast bubbles |
| Creamy lather | 16–48 | Palmitic, stearic, ricinoleic | Dense, stable, lotion-like foam |
Two chemistry notes explain a lot of recipes. Castor oil is roughly 90% ricinoleic acid, which is why a small amount — 5 to 8 percent — lifts both lather scores dramatically; it is a stabiliser, not a base oil. And the high-cleansing oils — coconut, palm kernel, babassu — are the engines of lather but strip hard, so balanced recipes keep them to a minority and lean on conditioning oils like olive and a hardening butter for structure.
Iodine and INS
Two more numbers gauge stability. Iodine measures unsaturation; lower means a harder, longer-lasting bar. Above about 70 the soap tends to be soft and prone to the orange rancidity spots soapmakers dread. INS is an older combined index, and a value near 160 suggests a well-balanced bar.
How to use it
Pick solid or liquid, set your batch weight, then tap a starting recipe or build your own by percentage. Watch the five gauges as you go — green means the blend sits in the ideal range, amber or red means it drifts. When it looks right, the recipe card gives you the exact lye and water, and the show-your-work panel walks through every number so you can check it. Because this is real chemistry on skin, always weigh on a scale and verify a new recipe against a second calculator before making it.
A note on safety and scope: this tool is for planning and learning. Lye is caustic; soapmaking involves real chemical risk. The predicted qualities are guides drawn from fatty-acid averages, not guarantees — cure time, temperature, water content and additives all shape the finished bar. Always follow proper safety practice and never skip verifying a new recipe.
FAQ
How do you calculate lye for soap making?
Multiply each oil's weight by its SAP value for your chosen alkali and add them up — that is the theoretical lye. Reduce it by your superfat percentage to leave some oil unsaponified. For KOH, divide by the purity fraction, usually 0.90.
What is superfat in soap making?
The percentage of oil left unsaponified so no raw lye remains. Five percent suits a body bar; 15–20% offsets a harsh pure-coconut recipe. Never use zero — it leaves no margin for measuring error.
Why does KOH use a purity of 90 percent?
Potassium hydroxide absorbs moisture, so most is about 90% pure. Assuming it is pure recommends too little alkali and gives a cloudy, greasy liquid soap. Divide the required lye by 0.90 — or your supplier's exact figure, sometimes as low as 85%.
What do the soap quality numbers mean?
Predictions from your oils' fatty acids: hardness is longevity, cleansing is how strongly it strips oils, conditioning is how moisturising it feels, and bubbly and creamy describe the lather. Each has an ideal range, but they are guides, not guarantees.
Why is my cold process soap too soft?
Usually too many soft, conditioning oils. Raise the hard ones — palm, tallow, lard or a butter, or a little coconut. An iodine value above about 70 also signals a soft, rancidity-prone bar. Curing helps, but formulation is the real fix.