SAP Lye Calculator

Every batch of natural soap starts with one non-negotiable calculation: how much lye do you need to convert your specific blend of oils into soap without burning your skin or leaving harsh residue behind? The number depends on what fats you're using, because the molecular structure of each fat determines exactly how much alkali it takes to break it apart and rebuild it as soap. This calculator does that math — precisely — so you never have to guess.

What saponification actually is

Saponification is the reaction between a triglyceride (fat or oil) and a strong alkali — sodium hydroxide for bar soap, potassium hydroxide for liquid soap — that breaks the ester bonds in the fat molecule and recombines the fatty acid chains with the alkali to form soap molecules, releasing glycerin as a byproduct. The reaction is exothermic: the mixture heats up, which is normal and expected.

Each oil has a published saponification value — its SAP number — that tells you how many grams of NaOH are needed to fully saponify one gram of that oil. Olive oil is 0.134 g/g. Coconut oil is 0.190 g/g. The difference exists because coconut oil contains a higher proportion of shorter-chain lauric and myristic fatty acids, which have a lower molecular weight relative to their ester content, requiring more alkali per gram to convert.

The superfat: your built-in safety margin

A superfat percentage — also called a lye discount — intentionally reduces the amount of lye you use so that a portion of your oils remain un-saponified in the finished bar. A 5% superfat on a 500g batch means roughly 25g of oil was never converted. Those free fatty acids sit in the finished soap and contribute to its skin-feel, conditioning quality, and perceived mildness.

The tradeoff is shelf life: unsaponified oils can go rancid, particularly oils high in polyunsaturated fatty acids like sunflower or hemp. High-oleic oils (olive, avocado, high-oleic sunflower) superfat more gracefully because oleic acid oxidizes slowly. Most soap makers work in the 4–8% range. Going above 10–12% risks a soft bar that sweats; going below 2% risks a lye-heavy bar that can irritate skin if any lye remains uncombined.

NaOH vs. KOH: bar soap vs. liquid soap

Sodium hydroxide (NaOH) produces a hard bar because sodium soaps form tightly packed crystal structures. Potassium hydroxide (KOH) produces soft paste or liquid soap because potassium soaps are more soluble and hygroscopic. The lye calculation differs because KOH has a higher molecular weight (56.1 g/mol vs. NaOH's 40.0 g/mol), meaning you need more of it per gram of oil. The conversion factor is approximately 1.4025 for pure KOH.

Commercial KOH is typically sold at 90% purity. This calculator accounts for that: if your KOH is 90% pure, the required weight is divided by 0.90, so you're measuring out more material to deliver the correct amount of active KOH. Always confirm purity with your supplier and adjust accordingly.

Water: concentration and working time

Water is the medium that carries lye into contact with your oils. The standard approach in this calculator is water as a percentage of total oil weight — 38% is a reliable default that gives adequate working time and produces a firm bar. You can also think of it in terms of lye concentration: 38% water on oil weight is roughly equivalent to a 27–30% lye-to-water concentration depending on batch size.

Less water (32–35% of oil weight) accelerates trace — the point at which the soap batter thickens enough to hold a drizzle on its surface — and produces a denser, quicker-hardening bar. More water (40–45%) slows things down and gives you more time to work with additives, swirls, or complex molds. Water has no effect on the finished soap's quality, only on the process.

Reading this calculator's output

The results show lye weight and water weight in the same unit you entered your oils — grams or ounces — to the nearest 0.1. Use a scale that reads to 0.1g precision; a 1g error in a small batch is meaningful. The oil breakdown table shows how much lye each individual oil consumes, which is useful for understanding why coconut-heavy recipes require so much more lye than pure olive oil batches.

Total batch weight is the theoretical weight before any water evaporates during curing. A standard 4–6 week cold-process cure will reduce the bar's weight by 15–25% as water leaves the soap.

SAP values used in this calculator

The SAP values here are drawn from published fatty acid composition data and peer-reviewed soap-making references, including the standard tables in Kevin Dunn's Scientific Soapmaking (2010) and the Brambleberry Lye Calculator reference dataset. Values represent NaOH per gram of oil at standard purity (100% NaOH).