Soap Making Lye Calculator

Enter your oil weight, choose your lye type (NaOH for solid soap or KOH for liquid soap), set your superfat percentage and water ratio, then select your oil blend to get the exact lye amount and water amount needed for your batch. The Soap Making Lye Calculator breaks down your full recipe including total batch weight and a visual split of lye vs. water vs. oils.

NaOH (sodium hydroxide) makes bar soap. KOH (potassium hydroxide) makes liquid soap.

Enter the total weight of all oils/fats in your recipe.

%

Superfat is the excess oil left unsaponified. 5% is common for skin-friendly bar soap.

%

Standard water amount is 33–38% of total oil weight.

Select your primary oil. SAP value is the saponification number used to calculate lye.

Only used when 'Custom SAP Value' is selected above. Enter the NaOH SAP number for your oil.

Typical usage is 0.5–1 oz of fragrance per pound of oils.

Results

Lye Required

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Water Required

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Fragrance Amount

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Total Batch Weight

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Lye Concentration

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Unit

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Batch Composition

Frequently Asked Questions

What is a SAP value and why does it matter?

SAP (saponification) value is the amount of lye required to fully saponify one unit of a specific oil or fat. Different oils have different fatty acid profiles, so each has its own SAP number. Using the correct SAP value ensures your soap has the right lye amount — too much lye makes a harsh soap, too little leaves excess oil.

What is superfat (lye discount) and what percentage should I use?

Superfat is the percentage of oils deliberately left unsaponified in the final soap. It adds moisturizing properties and acts as a safety buffer. A 5% superfat is standard for most cold process bar soaps. Higher superfat (8–10%) makes a more conditioning soap but shortens shelf life due to potential rancidity.

What is the difference between NaOH and KOH?

Sodium hydroxide (NaOH) produces hard bar soap and is used in cold and hot process soap making. Potassium hydroxide (KOH) produces soft or liquid soap. KOH typically comes in 90% purity, while NaOH is commonly 97–100% pure, which affects how much you need to use.

How much water should I use in my soap recipe?

Water is typically calculated as a percentage of total oil weight. A range of 33–38% is most common. Less water (33%) produces a harder bar faster but can make tracing and working time shorter. More water (38%) gives more time to work but requires longer cure time.

Why does lye concentration matter?

Lye concentration is the ratio of lye to the total lye-water solution. A concentration between 25–35% is typical. Higher concentrations accelerate trace and can cause issues with fragrances or colorants. Lower concentrations give more working time but produce a softer bar initially.

Can I mix different oils and still get an accurate lye calculation?

Yes — when using a blend of oils, each oil's weight should be multiplied by its own SAP value, and the results summed for total lye needed. This calculator uses the SAP value of your selected primary oil. For multi-oil recipes, use an advanced lye calculator that allows individual oil inputs.

Is it safe to make soap at home with lye?

Lye (NaOH or KOH) is caustic and must be handled with care. Always wear gloves and eye protection, add lye to water (never water to lye), and work in a well-ventilated area. Once fully cured (4–6 weeks for cold process), the finished soap contains no active lye.

What is a fragrance usage rate in soap making?

Fragrance usage rate is the amount of fragrance or essential oil added per weight of oils. A common guideline is 0.5 to 1 oz of fragrance per pound of oils (roughly 3–6% of oil weight). Always check the supplier's recommended maximum usage rate for each fragrance to avoid acceleration or separation.

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