Free, and always will be

Lye calculator

Tell it which oils you're using and how much, and it works out the sodium hydroxide and water. Every oil here is plant-based, and the scents are essential oils and absolutes — no fragrance oils, no animal fats.

Run every recipe through a second calculator before you make it, and weigh everything on a scale — never by volume. If you haven't handled lye before, read the safety guide first.

How much oil are you using?

Not the finished bar — just the oils and butters.

grams

Your oils

100% of 100%

Olive oil
Gentle and conditioning. A soft bar on its own that hardens with a long cure — a 100% olive bar is castile.
%
500.0 g
Coconut oil (76°)
Hard bar, big bubbles. Drying above about a third of the batch unless you raise the superfat.
%
250.0 g
Shea butter
Hardness without the drying. Creamy rather than bubbly.
%
200.0 g
Castor oil
Draws lather out of everything else. Sticky and soft past 10%.
%
50.0 g

Add an oil

Or start from one of mine

Superfat

The share of oil left unsaponified, so the bar is never lye-heavy. 5% suits most recipes. Go higher for a gentler bar, lower for a harder, longer-lasting one.

Water

How strong the lye solution is. 33% is the usual starting point. Higher means less water, a faster trace and a shorter cure.

Scent, if you want it

Essential oils and absolutes only. Percentages are of your oil weight, and the ceilings come from the IFRA limits for bar soap.

Add a scent

Your recipe

Sodium hydroxide — NaOH: 137.28g
Water: 278.7g

That's a 33% lye solution. Total batch weight 1,416.0 g.

Weigh out

Olive oil500.0 g
Coconut oil (76°)250.0 g
Shea butter200.0 g
Castor oil50.0 g

How the bar should behave

Worked out from the fatty acids in your oils. Guidance, not law — plenty of lovely soap sits outside these ranges.

hardness3729–54
cleansing1712–22
conditioning5944–69
bubbly2114–46
creamy2516–48

What it's doing, and where the numbers come from

Sodium hydroxide, not potassium

This calculates NaOH, for solid bars. Liquid soap uses potassium hydroxide and different maths — don't use these weights for it.

How the lye is worked out

Every oil has a SAP value: the grams of lye it takes to turn one gram of that oil into soap. Each oil in your recipe is multiplied by its own value, the results are added up, and then the superfat is taken off the total.

Why my numbers may sit slightly under other calculators

SAP values are ranges, not constants. The same oil varies with the crop, the region and the refining, so published charts disagree by a percent or two. The two ways of being wrong are not equal: too little lye leaves a soft bar with more superfat than you planned, which is harmless, while too much lye leaves free alkali in the bar, which burns skin.

So wherever the published charts disagree, this one takes the lowest credible value. Cross-checked against four independent sources, listed below.

The essential oil ceilings

Bar soap is IFRA category 9. Most essential oils have no limit of their own — what caps them is a single constituent, and that limit applies to your whole blend rather than to each oil separately. Two oils that are each fine on their own can go over the line together, so the calculator adds them up.

You'll notice lemongrass and litsea are far lower here than on most soapmaking charts. Both are mostly citral, and citral is one of the tightly limited ones. If you're selling your soap, check your own supplier's paperwork — these are a safe starting point, not a compliance document.

The bar behaviour numbers

Those come from the fatty acids in your oils, not from the lye. Hardness and creaminess come from the saturated fats and the long-chain ones; cleansing and lather come from lauric and myristic acid, which is why coconut does both; conditioning comes from the unsaturated fats. They're a useful sanity check, not a score to chase.

Sources

Found something that looks wrong? Please tell me — I'd far rather fix it than have someone make a bad batch.