Lye & Water Calculator

How It Works

Cold process soap is made through saponification — a chemical reaction between oils (fatty acids) and a strong alkali (lye). Each oil has a unique SAP value (saponification value) that tells us exactly how much lye is needed to fully convert that specific oil into soap.

The formula is straightforward:

• Per-oil lye = oil weight (g) × SAP value
• Total lye = sum of all per-oil lye amounts × (1 - superfat%)
• Water = total lye × (1 - lye concentration%) / lye concentration%

The SAP values used in this calculator are based on averages published by oil suppliers and verified against multiple soap-making references. Since natural oils can vary slightly between batches, a superfat buffer of 3-5% is recommended.

How to Use This Calculator

1. Select your lye type — NaOH for bar soap, KOH for liquid soap
2. Add your oils — choose from 30+ common soap-making oils and enter the weight in grams
3. Set your superfat — 5% is standard for most recipes; increase for more moisturizing soap
4. Adjust lye concentration — 33% is the most common; higher concentrations speed up trace
5. Read your results — the calculator updates in real time as you adjust inputs
6. Export your recipe — click Export PDF to save a printable recipe card

Tips & Safety Notes

• Always add lye to water, never water to lye. Adding water to dry lye can cause a violent exothermic reaction.
• Wear chemical-resistant gloves and safety goggles when handling lye.
• Work in a well-ventilated area — lye solution produces fumes when first mixed.
• Let your lye water cool to approximately 38-43°C (100-110°F) before combining with oils.
• A superfat below 3% risks having free lye in your finished soap — not skin safe.
• Cure cold process soap for 4-6 weeks to allow water evaporation and full saponification.

FAQ

What is the correct lye to water ratio for soap making?

A common lye concentration is 33%, meaning a 1:2 ratio of lye to water by weight. This can be adjusted between 25-50% depending on your recipe and technique. Higher concentrations (less water) lead to faster trace and harder soap, while lower concentrations give more working time.

What does superfat mean in soap making?

Superfat is the percentage of oils that remain unsaponified in the finished soap. A 5% superfat means 5% of your oils won't react with lye, leaving them free to moisturize skin. Most soap makers use 3-8% superfat.

Can I substitute NaOH with KOH?

NaOH (sodium hydroxide) makes bar soap while KOH (potassium hydroxide) makes liquid soap. They are not directly interchangeable as KOH requires about 40% more by weight for the same amount of oil. Use this calculator's lye type toggle to get the correct amount for either.

How accurate are the SAP values in this calculator?

The SAP values used are averages published by major oil suppliers and verified against multiple soap-making references (Bramble Berry, Soap Queen, Certified Lye). Natural oils vary ±2-5% between batches, which is why a superfat buffer of 3-5% is always recommended.

Can I use this calculator for hot process soap?

Yes. The lye and water amounts are identical for hot process and cold process soap — the saponification chemistry is the same. The difference is that hot process cooks the soap to complete saponification faster, reducing cure time but producing a rustier texture.

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Last reviewed: June 2026