What is a saponification (SAP) value?

A SAP value is the amount of lye needed to fully saponify one gram of a specific oil. Each oil has its own value because of its fatty-acid makeup. The tool multiplies each oil's weight by its SAP value and sums the results to get total lye.

What is superfat and why does it matter?

Superfat is the percentage of oils left unsaponified on purpose, by using slightly less lye than the full amount. It makes the finished soap gentler and guarantees no leftover caustic lye. Five percent is a common, safe default for bar soap.

What is the difference between NaOH and KOH?

NaOH (sodium hydroxide) makes solid bar soap; KOH (potassium hydroxide) makes liquid and soft soap. KOH has a higher molar mass, so it takes about 1.4 times as much by weight to saponify the same oils. The tool converts automatically based on your choice.

Why does the calculator ask for water as a percent of oils?

Water dissolves the lye so it can react with the oils, then evaporates as the soap cures. A common starting point is water at about 38 percent of the oil weight. The tool also shows the resulting lye concentration so you can adjust to your method.

Is this calculator safe to make soap from directly?

Use it as a planning aid and always verify your recipe against a second trusted lye calculator before mixing. Lye is caustic: add lye to water, never water to lye, wear gloves and eye protection, and work in a ventilated space.

Soap Lye (Saponification) Calculator

Calculate lye for any soap recipe

Every cold-process or hot-process soap recipe lives or dies by its lye amount. Too much lye leaves a harsh, caustic bar; too little leaves it soft and greasy. This calculator uses each oil’s saponification value to compute the exact NaOH or KOH needed for your blend, then applies your superfat and water percentages.

How it works

Each oil has a published SAP value — the grams of lye needed to saponify one gram of that oil. The tool sums the lye contribution of every oil in your recipe:

lye (NaOH) = Σ (oil weight × oil SAP value)

If you are making liquid soap with potassium hydroxide, the figure is scaled by the KOH-to-NaOH molar-mass ratio (about 1.403), because KOH is heavier per molecule. Superfat then reduces the lye so a chosen fraction of oils stays unsaponified for a gentler bar:

lye used = lye × (1 − superfat %)
water    = total oils × water %

The tool also reports the resulting lye concentration so you can match it to your preferred working method.

Tips and example

A 1,000 g recipe of 500 g olive, 300 g coconut, 200 g palm, NaOH, 5% superfat:

Lye = 500×0.134 + 300×0.178 + 200×0.141 = 67 + 53.4 + 28.2 = 148.6 g at full strength
After 5% superfat = 148.6 × 0.95 = ~141 g NaOH
Water at 38% of oils = 380 g

Always weigh in grams on an accurate scale, and verify the recipe against a second trusted lye calculator before you mix. Lye is caustic — add lye to water (never the reverse), wear gloves and goggles, and keep vinegar nearby for spills.