How to Use a Lye Calculator for Cold Process Soap

What Is a Lye Calculator?

Making cold process soap requires an exact chemical reaction between lye (sodium hydroxide) and oils. A lye calculator takes the weight of each oil in your recipe and tells you exactly how much NaOH and water you need. Get it wrong and your soap will be lye-heavy and caustic, or too soft to cure properly. A good lye calculator removes the guesswork entirely.

NaOH vs KOH

Sodium hydroxide (NaOH) is used for solid bar soap. Potassium hydroxide (KOH) is used for liquid soap and soft soap. Each oil has a different SAP value — the amount of lye required to fully saponify one gram of that oil. Your lye calculator uses these values automatically, so you just enter your oil weights and choose your lye type.

Superfat Percentage

Superfat is the percentage of oils left unsaponified in your finished soap. A 5% superfat means 5% of your oils are left free, giving a more moisturising bar. Most cold process soap makers use a superfat of 5–8%. Your lye calculator reduces the lye amount to achieve this automatically.

Safety First

Always follow these rules when working with lye.

• Wear gloves, goggles, and long sleeves
• Always add lye TO water — never water to lye
• Work in a ventilated area — lye fumes are caustic
• Keep vinegar nearby to neutralise spills
• Keep children and pets out of the room

Ready to Calculate?

The LatherForge free lye calculator supports 20+ oils and butters, NaOH and KOH, adjustable superfat from 0–20%, and three water ratio presets. No account needed — just enter your recipe and get your lye weight instantly.