Lye Calculator

Calculate the exact lye (NaOH or KOH) for your soap recipe from each oil’s saponification value, with superfat, KOH purity and a water:lye ratio. An educational estimate that runs entirely in your browser.

Oils & fats (g)
KOH purity only applies to liquid soap; NaOH is treated as pure.

Lye needed

-

Water-
Total lye solution-
Total oils-
NaOH at 0% superfat-

Show the math

Each oil needs a set amount of lye to turn into soap, its saponification (SAP) value. The recipe's lye is the sum across all oils, then trimmed by your superfat:

NaOH = Σ(oil weight × SAP)  ·  lye = NaOH × (1 − superfat ÷ 100)

Liquid soap uses KOH, which is heavier per molecule and rarely pure:

KOH = NaOH × 1.403 ÷ (purity ÷ 100)

Lye (sodium/potassium hydroxide) is caustic and can cause severe burns. Always add lye to water (never water to lye), wear goggles and gloves, and work in a ventilated space. This is an educational estimate, confirm every batch against a trusted lye calculator before mixing.

What this does

A lye calculator finds the exact lye (NaOH or KOH) for a soap recipe from each oil’s saponification value, with superfat, KOH purity, and a water:lye ratio.

How to use it

  1. Add each oil and its weight.
  2. Choose NaOH or KOH.
  3. Set your superfat percentage.
  4. Read the lye and water amounts.

How it works

Each oil has a saponification value — grams of lye per gram of oil. KOH is about 1.403× the NaOH amount and usually ~90% pure. Lye is caustic, so always confirm against a trusted calculator before making soap.

lye = total(oil × SAP) × (1 − superfat)

Understanding your result

Lye is caustic and getting the amount wrong makes a soap that is either lye-heavy and skin-burning or oily and soft, so always confirm this figure against a second trusted calculator before mixing a batch. Wear goggles and gloves, add lye to water and never the reverse, and remember the superfat setting is your safety buffer against a harsh bar.

Example

500 g of olive oil (SAP 0.134) needs about 67 g of NaOH at 0% superfat.

Sources & methodology

Last updated .

Frequently asked questions

How is the lye amount worked out?

Each oil has a saponification (SAP) value — the grams of lye needed per gram of oil. We sum oil weight × SAP across your recipe, then reduce it by your superfat percentage.

What is the difference between NaOH and KOH?

NaOH makes hard bar soap; KOH makes soft or liquid soap. KOH is heavier per molecule (about 1.403× the NaOH amount) and usually around 90% pure, both of which we account for.

Is this safe to rely on?

Treat it as an educational estimate. Lye is caustic and recipe errors are dangerous — always confirm your numbers against a trusted lye calculator before making soap.

Do you store my recipe?

No — every calculation runs locally in your browser with no tracking.