Soap Lye Calculator
Calculate precise lye and water amounts for cold process soap making. Supports NaOH (bar soap) and KOH (liquid soap).
Soap Lye Calculator
Calculate lye and water for cold/hot process soap
Oils & Butters
Sodium Hydroxide (NaOH)
3.88 oz
lye needed
Distilled Water
7.75 oz
33.3% lye solution
28.00 oz
base oils
1.40 oz
5% of oils
39.63 oz
oils + lye + water
33.3%
concentration
Oil Breakdown
Understanding Saponification
Saponification is the chemical reaction between lye (NaOH or KOH) and fats/oils that produces soap. Each oil requires a specific amount of lye based on its SAP (saponification) value.
Key Concepts
- SAP Value: Grams of NaOH needed to saponify 1 gram of oil
- Superfat: Percentage of oils left unsaponified
- Lye Concentration: Lye ÷ (Lye + Water) × 100
- Trace: When soap batter thickens and emulsifies
Popular Oil Combinations
Basic Beginner
- Olive: 50%
- Coconut: 25%
- Palm: 25%
- 5% superfat
Luxury Bars
- Olive: 40%
- Coconut: 20%
- Shea Butter: 20%
- Castor: 10%
- Sweet Almond: 10%
Vegan Alternative
- Olive: 40%
- Coconut: 25%
- Cocoa Butter: 15%
- Rice Bran: 10%
- Castor: 10%
Frequently Asked Questions
What is superfat and why is it important?
Superfat (or lye discount) is the percentage of oils left unsaponified in your soap. 5% superfat is standard for most soaps, providing moisturizing properties and a safety margin. Increase to 7-10% for facial soaps or shampoo bars.
What's the difference between NaOH and KOH?
Sodium Hydroxide (NaOH) makes solid bar soap. Potassium Hydroxide (KOH) makes liquid soap. KOH requires about 1.4× more lye than NaOH for the same amount of oils because of its molecular weight.
What water to lye ratio should I use?
Standard is 2:1 water to lye (33% lye solution). Less water speeds up trace and cure but runs hotter. More water slows trace but requires longer cure time. Beginners should stick with 2:1.
How do I calculate lye for a custom oil?
You need the SAP (saponification) value for that oil. Multiply oil weight by SAP value to get lye needed. Our calculator includes 25+ common oils with accurate SAP values. For exotic oils, check the supplier's data sheet.
Why is my soap too soft or taking forever to cure?
Soft soap usually means too many liquid oils (olive, sunflower) or not enough hard oils (coconut, palm, tallow). Aim for at least 30-40% hard oils. Cure time is typically 4-6 weeks; using less water can reduce this.
Can I substitute oils in a recipe?
You can, but you MUST recalculate the lye. Different oils have different SAP values. Never substitute oils without recalculating, or you'll end up with lye-heavy (caustic) or oil-heavy (soft, greasy) soap.
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