What is lye and why do I need it?

Lye (sodium hydroxide for bar soap, potassium hydroxide for liquid soap) is essential for saponification - the chemical reaction that turns oils into soap. Without lye, you cannot make soap from scratch.

Superfat is the percentage of oils that remain unsaponified in the finished soap. A 5% superfat means 5% of the oils will not react with lye, making the soap more moisturising. Most cold process soaps use 3-8% superfat.

Is this safe for a single-oil soap?

This calculator works for single-oil soaps. For multi-oil recipes, calculate each oil separately and add the lye amounts together, or use the Oil Ratio Calculator for blended recipes.

How do I handle lye safely?

Always add lye to water (never water to lye). Wear safety goggles and rubber gloves. Work in a well-ventilated area. Keep vinegar nearby. The lye solution will heat to 90°C+ so use a heat-safe container.

What concentration should my lye solution be?

Most soapmakers use a 28-33% lye concentration (67-72% water by weight of oils). A 38% water-to-oil ratio is a good starting point. Higher water means softer soap that needs longer to cure.

CraftCalcs

Lye Calculator — NaOH & KOH for Soap Making

Calculate sodium hydroxide (NaOH) or potassium hydroxide (KOH) amounts for cold process soap.

Oil Weight

Oil Type

Lye Type

Superfat

Water (% of oil weight)

33-38% is typical

How We Calculate This

How the Calculation Works

Each oil has a saponification (SAP) value - the amount of lye needed to convert 1 gram of that oil to soap.

Lye amount = oil weight x SAP value x (1 - superfat%). For KOH, multiply the NaOH SAP value by 1.403.

Water amount = oil weight x water ratio%. The total batch weight is oils + lye + water.

Frequently Asked Questions

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Last updated: 2025-02-20

All calculations are estimates. Always verify quantities before purchasing materials.