How to use a soap calculator
What every field means, where the number comes from, and how to tell when a result is wrong before you have mixed anything.
Enter each oil in your recipe and its share of the blend, enter your total oil weight, then choose a superfat and a water discount. The calculator multiplies each oil by its own saponification value, adds those together, and reduces the total by your superfat — 142.9 g of lye for a one-kilo beginner blend at 5% superfat.
A soap calculator — a lye calculator, in other words — does one job: it tells you how much sodium hydroxide a specific set of oils needs. It is not optional and it is not something you can eyeball, because the answer changes every time the recipe does. Too little lye and you get soft, oily soap that never firms up. Too much and you get a bar that burns skin.
The reason a single figure will not do is that oils differ more than people expect. Coconut oil needs about 36% more lye per gram than olive oil does, so the same kilo of fat takes 174 g of lye as pure coconut and 128 g as pure olive. A calculator resolves that by working each oil out separately and adding the results, which is exactly what averaging cannot do.
This page walks through what each field is asking for and what the number it returns actually means. The lye calculator itself shows the same working as you type.
A worked batch, oil by oil
A one-kilogram beginner blend before any superfat is applied. Each oil is saponified on its own and the results are summed — the calculator never averages a blend.
What the calculator hands back
Everything the oils could consume, with nothing left over. This is the starting figure, not a recipe — no body bar should be made at it.
The figure you actually weigh. The 7.5 g held back is what leaves 50 g of oil unsaponified in the bar.
Decided entirely separately. Water changes how the batch behaves, never how much lye it needs.
What each field is asking for
Enter the recipe as a ratio rather than as weights and it scales to any batch size without being re-entered. If the percentages do not reach 100 the calculator still keeps your ratio, but it is almost always a typo worth fixing.
The weight of fat in the batch, before lye, water or fragrance. Every other figure the calculator returns is derived from it, so this is the one number worth checking twice.
The share of oils deliberately left unsaponified. 5% is the cold process default; below 3% there is no margin left for measuring error. See what superfat actually does to a bar.
NaOH for solid bars, KOH for liquid and soft soap. KOH values are the sodium ones multiplied by 1.4028, so the same batch needs 223 g rather than 143 g — and flake KOH is only about 90% pure, which the calculator divides by.
The last field and the least consequential to safety. It sets how fast the batch traces and how soon it unmoulds — see water discounts explained.
Getting a result you can trust
A lye weight carried over from a different blend is the single commonest way a batch goes wrong. Change one oil and the figure changes with it.
Palm oil and palm kernel oil are different entries with different values, as are olive and pomace olive. Selecting the wrong one from a dropdown produces a confident, wrong answer.
Your superfat absorbs a small shortfall of lye. Nothing absorbs an excess. If your scale reads in whole grams, work in a bigger batch so a gram matters less.
For a recipe you plan to make repeatedly, run it through a second tool once. Two independent calculators agreeing is a cheap check against a mistyped percentage.
Run your own recipe
Build your blend, set a superfat and a water discount, and get every oil weighed out with the lye and water for the batch.
Open the lye calculator →Print the safety checklist
Lye handling, kit and the order of operations, on one page you can pin up where you work. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.