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Soap Making Lye Calculation Calculator

Work out soap making lye calculation instantly with clear inputs, formula shown and shareable results.

About Soap Making Lye Calculation Calculator

Lye is the oil weight times the weighted saponification value, reduced by the superfat percentage, with water added as a multiple of the lye weight. Superfat leaves a few percent of oils unsaponified for a milder bar, and getting the lye wrong in the other direction produces a caustic soap that must be discarded.

Formulas

Lye quantity

lye = total oils x SAP value x (1 - superfat)

Water

water = lye x water to lye ratio

Worked example

With the default values below, Soap Making Lye Calculation Calculator returns Sodium hydroxide needed of 128.25. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total oilsminimum 1 g
1,000 g
Weighted SAP value for sodium hydroxideminimum 0.01
0.135
Superfat (lye discount)accepts 0–20 %
5 %
Water as a multiple of lyeminimum 0.5
2.2

Results

Sodium hydroxide needed
128.25 g
Water needed
282.2 g
Total batch weight
1,410.4 g
Lye solution concentration
31.25%

Step by step

  1. 1Values used
  2. 2Lye quantity
  3. 3Water
  4. 4Sodium hydroxide needed= 128.25 g
  5. 5Water needed= 282.2 g
  6. 6Total batch weight= 1,410.4 g
  7. 7Lye solution concentration= 31.25

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Sodium hydroxide is caustic. Always verify quantities against a dedicated soap calculator and the SAP values for your specific oils before mixing.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Sodium hydroxide is caustic. Always verify quantities against a dedicated soap calculator and the SAP values for your specific oils before mixing.

Frequently Asked Questions

How is the Soap Making Lye Calculation figure worked out?

lye = total oils x SAP value x (1 - superfat). Lye is the oil weight times the weighted saponification value, reduced by the superfat percentage, with water added as a multiple of the lye weight.

Why does this matter?

Superfat leaves a few percent of oils unsaponified for a milder bar, and getting the lye wrong in the other direction produces a caustic soap that must be discarded.

What do I need to enter?

Enter total oils, weighted sap value for sodium hydroxide, superfat (lye discount), water as a multiple of lye. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.

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