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

Total weight of oils in the recipe

Oil Type

Lye Type

Superfat

Extra oil percentage beyond what lye can saponify

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.