Soap Making Lye Calculator

How soap lye calculations work:

Soap is made by combining oils/fats with an alkali (lye). The chemical reaction is called saponification. Each oil has a unique SAP value — the grams of lye needed to fully saponify 1 gram of that oil.

Basic formula:

Lye (g) = Oil weight (g) × SAP value × (1 − Superfat %)

Superfat (also called lye discount) is the percentage of oils left unsaponified, giving the soap extra moisturising properties. A 5% superfat is the most common starting point.

Common SAP values (NaOH / bar soap):

• Coconut oil: 0.190
• Palm oil: 0.141
• Olive oil: 0.134
• Castor oil: 0.128
• Shea butter: 0.128
• Sweet almond oil: 0.136
• Lard / tallow: 0.138

Worked example — simple 3-oil bar soap:

• 300g coconut oil × 0.190 = 57.0g
• 300g palm oil × 0.141 = 42.3g
• 400g olive oil × 0.134 = 53.6g
• Total lye before superfat = 152.9g
• Apply 5% superfat: 152.9 × (1 − 0.05) = 145.3g NaOH

Water amount: Use 33–38% of total oil weight. For 1,000g oils: 330–380g distilled water. Less water = harder bar faster; more water = easier to mix.

Safety — always observe these rules:

• Add lye TO water, never water to lye — adding water to lye causes a violent reaction
• Lye solution reaches 200°F (93°C) instantly — use heat-safe containers
• Wear goggles and gloves at all times
• Mix in a ventilated area — fumes are caustic
• Lye is safe in cured soap — saponification is complete after 4–6 weeks

This calculator uses the simple single-oil approach. For full multi-oil recipes, use a dedicated lye calculator tool with your complete recipe.