Saponification Value Calculator

What Saponification Value Measures

Saponification value (SAP value) is the number of milligrams of potassium hydroxide (KOH) needed to fully convert one gram of a fat or oil into soap. Wikipedia's saponification value entry describes it as a direct measure of the average molecular weight of the fatty acids present, oils built from shorter fatty acid chains, like coconut oil, have a high SAP value and need proportionally more lye, while oils built from longer chains, like olive oil, have a lower SAP value and need less.

This calculator covers both directions people actually need: figuring out how much lye a soap recipe needs from a blend of oils with known SAP values, and working the number backward from real titration data in a lab setting.

How to Calculate Lye Needed for a Soap Recipe

Each oil's published NaOH SAP value tells you how many grams of sodium hydroxide fully saponify one gram of that specific oil. Multiply each oil's weight by its own SAP value and add the results together to get the total lye needed for a blend, exactly how the Soap Guild's lye calculator handles multi-oil recipes. A recipe with 400 g olive oil (0.135), 150 g coconut oil (0.190), and 150 g palm oil (0.141) needs 400×0.135 + 150×0.190 + 150×0.141, roughly 103.7 g of NaOH before any superfat adjustment.

NaOH vs KOH: Solid Soap vs Liquid Soap

Sodium hydroxide (NaOH) produces the firm bar soap most people picture, while potassium hydroxide (KOH) produces a softer paste that becomes liquid or gel soap once diluted with water. Certified Lye's guide to using lye in soap notes the two lyes are not interchangeable gram for gram, KOH has a higher molecular weight than NaOH, so a KOH SAP value runs about 1.403 times higher than the equivalent NaOH figure for the same oil. Selecting the correct lye type in the calculator applies that conversion automatically rather than requiring a separate lookup table.

Why Superfat (Lye Discount) Matters

Superfat, also called lye discount, is the percentage of oils deliberately left unsaponified so the finished bar retains some free, moisturizing oil and carries no risk of leftover unreacted lye. Wholesale Supplies Plus's lye calculator guidance recommends a 5% superfat as a reliable default for most cold-process recipes, with 3 to 8% covering the range most soap makers actually use depending on how rich or how cleansing they want the final bar to feel. The calculator applies this discount directly to the calculated lye total, not to the oil weights, which is the standard method every major soap calculator follows.

Determining Saponification Value by Titration

In a lab setting, saponification value is measured rather than looked up, using the formula SV = (Blank − Sample) × Molarity × 56.1 ÷ Sample Weight, where the blank and sample titration volumes come from titrating excess KOH with standardized HCl before and after saponifying a weighed fat sample. Omni Calculator's saponification value tool confirms 56.1 is KOH's molar mass in this equation, converting the titration's mole difference into the standard mg KOH per gram unit. If you need to prepare a titrant at a specific molarity from a concentrated stock solution first, our Dilution Calculator handles that step, and our Normality Calculator converts between molarity and normality if your lab's titration protocol is written in normality instead.

SAP Values Vary by Oil, and Even by Batch

Published SAP value tables are averages, the actual fatty acid profile of any given batch of oil shifts slightly with the crop, extraction method, and refining process. The Soap Kitchen's guide to SAP values recommends checking your specific oil supplier's data sheet when precision matters, since two bottles labeled "olive oil" from different producers can carry SAP values that differ enough to matter for a lye-heavy recipe. The values built into this calculator reflect widely published industry averages, a reasonable starting point rather than a substitute for supplier-specific data on a commercial batch.

Common Lye Calculation Mistakes

The most consequential mistake is confusing NaOH and KOH SAP values, using a sodium hydroxide figure while actually measuring out potassium hydroxide produces a batch with dangerously unreacted lye. Auntie Clara's lye calculation tutorial also flags forgetting to apply superfat at all as a common beginner error, since a 0% superfat recipe leaves no margin for measurement error and can result in a genuinely lye-heavy bar. Always weigh both oils and lye on a gram-accurate scale rather than estimating by volume, and add lye to water, never water to lye, wearing gloves and eye protection throughout.