Free Lye Calculator
Calculate exact NaOH or KOH amounts for your soap recipe. Supports cold process, hot process and superfatting. Free, instant, no signup required.
Ready to Calculate
Add your oils and weights on the left, then click Calculate Lye.
How to Use This Lye Calculator
Choose your lye type
Select NaOH (sodium hydroxide) for bar soap or KOH (potassium hydroxide) for liquid soap. NaOH is used in cold process and hot process bar soap making. KOH at 90% purity is standard for liquid soap.
Select your soap making method
Cold process soap is made at room temperature and cured for 4–6 weeks. Hot process soap is cooked and can be used sooner. The lye amount is the same — the method changes how you handle the batter.
Add your oils and weights
Enter each oil or butter in your recipe with its weight in grams. Each oil has a unique saponification value (SAP value) — the calculator uses these to determine exactly how much lye is needed.
Set your superfat percentage
Superfat is the percentage of oils left unsaponified in your finished soap. A 5% superfat is standard for a mild, moisturising bar. Higher superfat means more free oils but a softer bar.
Frequently Asked Questions
What is a lye calculator?
A lye calculator determines the exact amount of sodium hydroxide (NaOH) or potassium hydroxide (KOH) needed to saponify a specific blend of oils and butters. It uses saponification values (SAP values) for each oil to calculate the correct lye amount so no lye remains in your finished soap.
What is superfatting in soap making?
Superfatting is using slightly less lye than required to saponify all oils, leaving a small percentage of free oils in the finished soap. This results in a milder, more moisturising bar. A 5% superfat is the most common choice for handmade soap.
What is the difference between NaOH and KOH?
NaOH (sodium hydroxide) produces hard bar soap and is used for cold process and hot process soap. KOH (potassium hydroxide) produces soft or liquid soap. Most bar soap makers use NaOH. Liquid soap makers use KOH, typically at 90% purity.
How much water should I use?
The standard water to lye ratio for cold process soap is 2:1 — two parts water to one part lye by weight. This is the default used in most soap recipes. Some makers reduce water to speed cure time. The calculator recommends water based on your lye amount.
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