About
Lye math, done properly.
Free soapmaking calculators where every saponification value names its source, the arithmetic is shown, and the results are checked against the calculator everyone else already trusts.
Why this site exists
Ask the internet how much lye your recipe needs and you get three kinds of unhelpful answer. A blog recipe gives you a lye weight with no way to change the batch size and no hint where the number came from. A supplier's calculator gives you a number built on that supplier's assumptions, unstated. And SoapCalc — which is genuinely good, and which this site checks itself against — gives you the right answer through an interface built in 2001 that can't be used on a phone, can't share a recipe as a link, and never tells you which saponification value it used or how much that value varies between crops.
The underlying data is not a secret. Saponification values, fatty-acid profiles and iodine numbers are published reference figures. What's missing is a tool that shows you which number it used, where that number came from, how wide the published range is, and what the arithmetic did with it — on a page you can open one-handed while the oils melt.
Who's behind it
I'm Brian Hamilton. I'm not a chemist and I'm not a professional soapmaker, and I'm not going to pretend to be either. What I do is build calculation tools, and soapmaking turns out to be a problem of exactly that shape: the correct numbers are published and well established, and what goes wrong happens in the gap between the published number and the bowl in front of you — an oil that isn't quite the one you selected, a scale that reads in whole grams, a water convention that means something different in two different books.
So the domain knowledge here comes from sources I name rather than expertise I claim, and where a figure is uncertain the site says so instead of hiding it behind a decimal point. That's why every oil shows the range of published values alongside the one we use, and why the standing advice on this site is to check any recipe — including one from here — against a second calculator before the lye goes in.
The tool-building is what I do for a living: I have an internet engineering degree and thirty years of software development behind me, a good part of it building mathematical models for sports prediction companies, which is the same discipline as this — published inputs, transparent arithmetic, and a result somebody else can check. SoapmakingToolbox is one of a family of calculator sites made by Hamilton Digital Media, the company I run from Dorset in the UK, alongside its puzzle games and apps. There's a fuller profile on the company site.
How you can trust the numbers
Sourced, not copied
Every one of the 94 oils carries its saponification value with the source named, and 39 of them also carry the range of values published in the literature — the honest measure of how much a real batch of that oil can differ.
One engine, no contradictions
Every calculator imports the same calculation modules. Two pages on this site cannot disagree about the same recipe, because there is only one implementation of the math to disagree with.
140+ automated checks
The engine is covered by a test suite that includes a dozen reference recipes hand-checked against SoapCalc — castile, the classic bar, liquid soap at 90% KOH, dual-lye shave soap. If a check fails, the site doesn't deploy.
The working is shown
Every result has a "show the math" panel with your own numbers substituted into it. You should be able to reproduce any figure here on paper, and if you can't, that's a bug worth reporting.
The sources we build on
Saponification values, fatty-acid profiles, iodine values and INS numbers come from the SoapCalc oil list, retrieved 2026-08-19. The soap-quality indices — hardness, cleansing, conditioning, bubbly, creamy — follow SoapCalc's method of summing fatty acids, so the numbers you see here are comparable with the ones you're used to. The conversion from potassium to sodium hydroxide uses SoapCalc's constant of 40 ÷ 56.1 (0.71301), which is within 0.03% of the exact molecular-weight ratio.
The plan for this site was to verify every oil against two independent references, SoapCalc and SoapmakingFriend. On checking, they turned out to publish the same dataset — of the oils both list, 145 of 148 are identical to three decimal places and the rest differ only in the fourth. So they are one source, not two, and anyone "cross-checking" one against the other is checking nothing.
That's why the independent check here is the published literature range instead: for 39 oils we record the spread of saponification values reported in soapmaking and lipid references, and a test fails the build if our value falls outside it. It caught a bad range on the first run, which is the point of having one.
What's ours is the engine, the curation, the interface and the words. The saponification values themselves are facts about oils — nobody owns them, and we credit where we took ours from. Usage rates for fragrance follow the supplier's IFRA certificate for the specific oil in your hand, which is why the fragrance calculator asks you to type that limit in rather than pretending to know it.
Corrections are welcome — genuinely
If a number here is wrong, tell me: [email protected]. That's not politeness. A site whose entire claim is "you can check this" has to act like it, so corrections get fixed quickly and the page gets a note saying what changed and why.
The most useful correction is a specific disagreement: "your lye calculator says 143.8 g and mine says 148.2 g for this recipe." Sometimes the answer is an assumption — NaOH purity, or KOH at 90% as sold — and the fix is to explain it better. Sometimes it's a bug. Either way it belongs on the page, and paste the recipe link when you write, because the URL carries every number in it.
How the site makes money
Eventually, through affiliate links — buy something through one and it earns a commission at no extra cost to you. Right now there are none at all, and the site earns nothing. There are no ads, no sponsored posts, no paid reviews and no paywall, and there won’t be. The affiliate disclosure sets out the rules in full, but the one that matters is this: the math picks the recommendation, and we say so when the cheapest option is the right one. In soapmaking it usually is — lard from the supermarket makes a better bar than most boutique butters, and a $15 stick blender saponifies exactly as well as a $60 one.
What's here, and what's coming
The recipe builder is the centerpiece: oils and percentages in, and lye, water, fragrance, cost per bar and the soap-quality bars out, on one screen, with a printable sheet and a link that is the recipe. Around it are 13 calculators — lye, mold volume, batch resizing, lye concentration, superfat, fragrance load, liquid soap, dual lye, masterbatch, essential-oil blending, cost and pricing, and bar count.
Next are the 19 guides in the plan — lye safety first, then saponification, superfat, water discounting and the troubleshooting cluster — each paired to the calculator it feeds. Candle tools are deliberately last; the seller pages on pricing and labeling matter more to the people using this site.
Everything you enter stays in your browser. Nothing is sent anywhere, there's no account to make, and the privacy page lists exactly what's stored with a button that deletes all of it.
Safety statement
Sodium hydroxide and potassium hydroxide are caustic. They burn skin and eyes, the reaction with water produces heat and fumes, and a recipe with too little oil for its lye produces soap that will hurt whoever uses it.
- Wear eye protection and gloves whenever lye — solid, dissolving or in fresh batter — is out.
- Add lye to water. Never the other way.
- Work in a ventilated space, away from children and animals. Keep vinegar nearby for surfaces, and running water — not vinegar — for skin.
- Check every recipe against a second calculator before you pour, including recipes from this site.
- Cure and test finished soap before it touches skin. Lye-heavy soap gets rebatched or binned.
The calculators here are guidance derived from published saponification data. They can't see your oils, your scale or your measuring. You are responsible for the soap you make and sell; SoapmakingToolbox accepts no liability for recipes produced with these tools. The terms of use set this out in full, including what these tools do not do for anyone selling soap.
Questions, corrections, or a batch that went wrong?
All welcome — get in touch. The questions people ask most become the next guides on the site, so a good one helps everybody.