Lye & Water Calculator

How It Works

Cold process soap is made through saponification — a reaction between a triglyceride (oil or fat) and a strong alkali. Each fat has a distinct SAP value, the milligrams of alkali needed to fully saponify one gram of that fat. Because coconut, olive, palm, tallow, and shea each have different fatty acid profiles, total lye for a recipe is the sum of the per-oil requirements — not a fixed ratio applied to total oil weight.

The formulas the calculator uses:

• Per-oil lye = oil weight (g) × SAP value for that oil
• Total lye at 0% superfat = Σ per-oil lye
• Total lye at superfat S% = Σ per-oil lye × (1 − S / 100)
• Water = total lye × (1 − C / 100) / (C / 100), where C is the target lye concentration

SAP values are drawn from the Bramble Berry Saponification Value Chart and cross-checked against the Certified Lye SAP reference table and USDA National Nutrient Database fatty acid data. Where sources differ by more than 2%, the median is used — the two-source approach recommended in Kenna Cote's Modern Soapmaking, 2nd ed., Ch. 4: Fatty Acids and SAP Values, which notes that no single table is authoritative because regional oils vary.

Superfat is a deliberate deficit of lye. A 5% superfat means 5% less alkali than the theoretical maximum, so roughly 5% of the fat stays unsaponified. That free fat makes handmade soap feel less stripping and doubles as a safety buffer against SAP variation between oil batches.

Lye concentration sets how much water dissolves the lye. At 33% the solution is roughly one part lye to two parts water by weight — the common starting point in published cold-process recipes. Higher concentrations (37-40%) speed up trace and allow earlier unmolding; lower concentrations (25-28%) give more working time for swirls.

How to Use This Calculator

1. Select your lye type — Choose NaOH for bar soap or KOH for liquid soap.
2. Add your oils — Choose from 30+ common soap-making oils and enter each oil's weight in grams.
3. Set your superfat — 5% is standard for most recipes; increase for a more moisturizing bar.
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.

Real-World Use Cases

Adjusting superfat on a coconut/olive/palm recipe

A small-batch soaper with a 500 g recipe — 100 g coconut, 300 g olive, 100 g palm — wants a gentler bar for sensitive skin. Moving from 5% to 8% superfat trims the NaOH weight by roughly 3% of the un-superfatted lye number, about 2 g in a 500 g batch. The bar saponifies cleanly but leaves noticeably more free fatty acid on the skin.

Converting a bar-soap recipe to liquid castile

An established soaper turns a 300 g pure-olive recipe into a liquid castile paste and toggles from NaOH to KOH. Because potassium (39.10 g/mol) is heavier than sodium (22.99 g/mol), KOH weighs about 40% more than NaOH for the same saponification. The 300 g of olive that needed roughly 39 g of NaOH now calls for about 55 g of KOH at the same superfat.

Troubleshooting a zap-test-positive bar

A beginner cures a batch for four weeks and touches the bar to their tongue — a light zap means free lye is still present, so the recipe used too little superfat. Recalculating with a corrected 5% superfat instead of the accidental 0% trims the NaOH weight by roughly 5%, and the reformulated batch cures without zapping. Both the Bramble Berry chart and Certified Lye reference back 3-5% as the standard skin-safety buffer.

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 free lye in the finished soap and is not skin safe.
• Cure cold process soap for 4-6 weeks to allow water evaporation and full saponification.

Limitations & What This Tool Cannot Do

• SAP value variation. Regional and seasonal variation of ±2-5% between oil harvests is not captured by the tool's fixed SAP values. A 3%+ superfat buffer compensates.
• Additive weights not calculated. Fragrance, colorant, and clay or exfoliant weights are not computed. Standard practice is to add them as a percentage of oil weight separately after the lye number is set.
• Purity assumption. The tool assumes standard lab-grade NaOH or KOH purity (98%+). Lower-purity lye (drain-cleaner variants) needs a manual concentration correction of a few percent and is not recommended for skin-safe soap.

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 lead to faster trace and harder soap; 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; KOH (potassium hydroxide) makes liquid soap. They are not directly interchangeable — 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 are averages published by major oil suppliers and verified against multiple references (Bramble Berry, 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 — the saponification chemistry is the same. Hot process simply cooks the soap to complete saponification faster, reducing cure time.

Do I need to adjust my recipe for a water discount at high superfat?

Water discounts (reducing water below the 33% concentration default) speed up trace and shorten unmold time, and are common at superfat 5-8%. Combining a high superfat with an aggressive water discount still works but can produce a stiffer trace requiring quick pouring. If you're new to cold process, keep the concentration at 33% until you're comfortable with faster trace.

Related Tools

• Soap INS Predictor — Predict the hardness, lather, and conditioning profile of the same recipe
• HLB Calculator for Emulsifiers — Take the same carrier oils into lotion and cream formulation
• Candle Wax & Fragrance Calculator — Chemistry-adjacent tool for the same maker

References

• Bramble Berry Saponification Value Chart
• Kenna Cote, Modern Soapmaking, 2nd ed., Ch. 4: Fatty Acids and SAP Values
• USDA FoodData Central (formerly the National Nutrient Database) — Fatty Acid Composition of Common Fats and Oils — fdc.nal.usda.gov
• Certified Lye SAP reference table

Reviewed by the Craft Calc Lab team on July 29, 2026.