About Lye.rs

What it does

• Calculates exact NaOH or KOH from your oils and superfat, with 150 oils in the database.
• Water three ways - as a percent of oils, by lye concentration, or by water-to-lye ratio - plus the resulting concentration and ratio.
• Live soap qualities: hardness, cleansing, conditioning, bubbly and creamy lather, longevity, iodine, and INS, against the commonly suggested ranges.
• Recipe insights for trace speed, unmolding, gel phase, and cure time, derived from your recipe's fatty acid makeup.
• A library of 36 starting recipes across bathing, gentle, shampoo, shaving, laundry, and dog soap.
• Enter oils by percentage or by weight, in grams or ounces, and switch freely.
• Save recipes in your browser, then export or import them as files to back up, move devices, or share a formulation with someone else.
• Additives and notes travel with each recipe.

What saponification actually is

A fat is a triglyceride: three fatty acid chains attached to a glycerol backbone. Lye dissolved in water is a strong alkali, and it attacks the bonds holding those chains in place. Each chain breaks free and pairs with the metal ion from the lye - sodium from NaOH, potassium from KOH - becoming a salt of a fatty acid. That salt is soap. The glycerol is released and stays in the bar, which is why handmade soap feels different from most commercial bars, where the glycerin is often extracted and sold separately.

A soap molecule has two ends that want different things: a fatty tail that clings to oil and grease, and a charged head that clings to water. Wash with it and the tails bury themselves in the oil on your skin while the heads face outward, so the whole bundle rinses away in water that would otherwise leave the grease behind. That is the entire trick, and it hasn't changed in four thousand years.

This is also why the lye amount matters so much. Every oil needs a specific quantity of alkali to convert fully - its saponification value. Too little and you leave unreacted oil, which is fine in small amounts and is exactly what superfatting does deliberately. Too much and unreacted lye remains in the bar, which is caustic on skin. The calculation is the safety step, not a formality.

Sodium hydroxide produces a hard bar; potassium hydroxide produces a soft or liquid soap. Curing is not saponification - that finishes within a day or two. Curing is water evaporating and the soap's crystalline structure settling, over four to six weeks or far longer for high-olive recipes, which is what turns a usable bar into a good one.

Where the words come from

Saponification comes from the Latin sapo, soap, by way of the French saponifier, which entered English chemistry writing in the early 1800s. The Roman naturalist Pliny the Elder described sapo in the first century as a Gaulish invention of tallow and ash, used to redden hair rather than to wash with.

A popular story traces the word to Mount Sapo near Rome, where animal fat from sacrifices supposedly mixed with wood ash and rainwater to form soap in the riverbank clay below. It is a lovely story with no archaeological or textual support - there is no record of such a mountain - and it appears to be a nineteenth-century invention. The Latin word is more likely borrowed from a Germanic source related to English soap itself.

Lye is older and closer to home: Old English lēag, from a Proto-Indo-European root meaning to wash, which also gives us lather and the Latin lavare, to bathe. For most of history lye meant the alkaline liquid leached from wood ash, not the purified sodium hydroxide sold today - which is why old recipes are so imprecise about it and why modern soap makers can be exact.

Castile names a region of Spain whose olive oil soaps became the benchmark for mildness. Trace is the soap maker's own coinage: the moment the batter thickens enough that a drizzle leaves a visible trail on the surface.

Safety

Lye is caustic and deserves respect. Wear gloves and eye protection, work somewhere ventilated, keep children and pets out of the room, and always add lye to water - never water to lye, which can erupt.

Verify your lye amounts independently before mixing a batch. Lye.rs provides calculations as-is, without warranty.

Where your recipes live

Everything you enter stays in your own browser. No accounts, no servers storing recipes, no analytics on what you formulate, and no ads.

Because of that, clearing your browser data will remove your saved recipes. Export a backup file from Backup & Sharing if they matter to you - that file also lets you move to another device or send a formulation to another soap maker.

About the oil data

All 150 oils have verified saponification values, so lye and water amounts are exact for every one of them. 143 currently have complete fatty acid profiles, which is what soap quality scores are built from.

When a recipe uses an oil whose fatty acid data isn't verified yet, Lye.rs says so and shows what percentage of the recipe the quality scores actually cover, rather than guessing. That list grows as data is confirmed against primary sources.

Calculations are checked against established soap calculators on every build, and the test suite fails if results drift. Even so, soap making involves caustic chemicals and your own measurements - treat these numbers as a careful starting point, not a guarantee.