Soap Lye Calculator (NaOH / KOH)
Enter your oil weights in grams, choose your lye type, and calculate safe lye and water amounts for cold process soap or liquid soap formulations.
| Oil / Butter | Weight (g) |
|---|---|
| Olive Oil | |
| Coconut Oil (76°) | |
| Palm Oil | |
| Castor Oil | |
| Shea Butter | |
| Sunflower Oil |
Tip: Most soapmakers use 3-8% superfat for bar soap. Always run final recipes through your own safety checks before production.
What Is a Lye Calculator?
A lye calculator is a recipe tool used by soapmakers to determine exactly how much alkali (lye) is needed to saponify oils and butters. Each fat has a different saponification value, which is the amount of lye required to fully turn that fat into soap. If you use too little lye, the soap may be overly soft or oily. If you use too much, the finished product can be harsh or unsafe.
This calculator helps you estimate:
- Total oil weight
- Required NaOH or KOH amount
- Water amount from a water-to-lye ratio
- Lye concentration
- Estimated total raw batch weight
How the Calculator Works
1) Oil Weights
You enter how many grams of each oil you want in your batch. The calculator multiplies each oil by its SAP value.
2) Lye Type
Choose NaOH for hard bar soap and KOH for liquid soap paste. SAP values differ between these alkalis, so this choice matters.
3) Superfat Discount
Superfat reduces lye to intentionally leave a small amount of unsaponified oils for skin feel and margin of safety. A 5% superfat means the calculator uses 95% of the full lye required.
4) Water Ratio
The water-to-lye ratio controls how concentrated your lye solution is. Lower water can speed trace and unmolding; higher water can provide more working time but may increase cure time and risk of warping.
NaOH vs KOH: Which Should You Use?
- NaOH (Sodium Hydroxide): Best for solid bar soaps.
- KOH (Potassium Hydroxide): Used for liquid soap and cream soap systems.
Do not swap them 1:1 without recalculation. Their molecular weights and SAP conversions are different.
Recommended Starting Ranges
- Superfat: 3-8% for most body bars
- Water:Lye Ratio: 1.8:1 to 2.5:1 common for CP soap
- Lye Concentration: roughly 28-40% depending on technique
Beginners often start around 5% superfat and 2.2-2.5 water:lye ratio, then adjust as they gain experience.
Practical Formulation Tips
Balance Hardness and Conditioning
High-coconut formulas clean strongly but can feel drying unless you use a higher superfat. Olive, sunflower, and high-oleic oils add conditioning but can make bars softer unless balanced with harder fats and enough cure time.
Don’t Skip Cure Time
Even when a bar is technically safe after saponification, cure improves hardness, lather quality, and mildness. Four to six weeks is common for many cold process recipes.
Track Every Batch
Record oils, temperatures, additives, fragrance behavior, and mold type. Good notes accelerate improvement and reduce costly mistakes.
Common Mistakes to Avoid
- Using volume measurements instead of weight
- Ignoring oil-specific SAP values
- Entering ounces in a grams-only calculator
- Forgetting to account for superfat
- Changing oils without recalculating lye
- Unsafe handling of lye solution
Safety Checklist
- Wear goggles, gloves, and long sleeves
- Use heat-safe containers and stainless/silicone tools
- Label lye and keep away from children/pets
- Add lye to water slowly with stirring
- Keep vinegar for cleanup surfaces only (not skin treatment)
- Rinse accidental skin contact with cool running water
Final Notes
This lye calculator is designed for quick, practical batch planning. It is ideal for hobby formulation, test batches, and concept checks. For commercial production, always validate your full process, ingredient specs, purity assumptions, and regulatory requirements in your region.
When in doubt, recalculate and verify before mixing. Precision is one of the best habits a soapmaker can build.