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Crafts Calculators

Soap Lye Calculator

Verified formula Updated Jun 2026 Private — runs on your device

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g
Verified formula Private

Lye needed

135.0

For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.

How to use the Soap Lye Calculator

The Soap Lye Calculator works out your lye needed in an instant. Enter oil weight and sap value (lye per g oil) and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.

  1. Enter the oil weight.
  2. Enter the sap value (lye per g oil).
  3. Read off your lye needed — the calculator updates automatically, with no button to press.

Formula

The Soap Lye Calculator uses the formula:

Lye needed = Oil weight × SAP value (lye per g oil)

Worked example

For example, with oil weight of 1,000 g and sap value (lye per g oil) of 0.135, the lye needed is 135.0.

Inputs used
Oil weight 1,000 g
SAP value (lye per g oil) 0.135
Results
Lye needed 135.0

Results are estimates for educational use, not professional advice.

Frequently asked questions

Multiply the oil weight by the oil's SAP value. 1,000 g of oil with a SAP value of 0.135 needs about 135 g of lye.

It is the saponification value: the lye needed to turn one gram of a specific oil into soap. Each oil has its own value.

Superfatting reduces the lye slightly so some oil stays unsaponified, making a gentler bar. Lower the lye by your chosen percentage.

Yes. Lye is caustic, so wear gloves and eye protection, work in a ventilated area and always add lye to water, not water to lye.

The Soap Lye Calculator uses the formula: Lye needed = Oil weight × SAP value (lye per g oil). For example, with oil weight of 1,000 g and sap value (lye per g oil) of 0.135, the lye needed is 135.0.

Enter the oil weight. Enter the sap value (lye per g oil). Read off your lye needed — the calculator updates automatically, with no button to press.

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