Soap Making Calculator

Soap making requires precise calculation of lye (sodium hydroxide) and oil quantities. Every oil has a unique SAP value (saponification value) — the grams of sodium hydroxide needed to fully saponify 1 gram of that oil. Using too little lye leaves unsaponified oils (rancid soap); too much leaves caustic, skin-irritating soap.

Core formula: Lye Required = Oil Weight (g) × SAP Value Total Lye = Σ (Each Oil Weight × Its SAP Value) Lye Discount = Total Lye × (1 − Superfat Percentage)

Water formula: Water = Lye Amount × Water-to-Lye Ratio (Standard ratio: 2:1 to 2.5:1 water-to-lye by weight)

What each variable means:

• SAP Value (NaOH) — saponification value for sodium hydroxide (bar soap). Each oil has a fixed SAP value derived from its fatty acid profile.
• Superfat Percentage — the intentional lye discount (typically 5–8%). Leaves a small percentage of oils unsaponified, making a milder, more moisturizing soap.
• Water — dissolves the lye before mixing with oils. Too little water causes seizing; too much extends cure time.
• Cure time — handmade cold-process soap must cure 4–6 weeks to allow water to evaporate and the soap to harden fully.

Common oil SAP values (NaOH):

Worked example: Recipe: 400g coconut oil + 300g olive oil + 100g castor oil (total 800g oils). Superfat 6%. Water-to-lye ratio: 2.3:1.

Lye requirement:

• Coconut: 400 × 0.190 = 76.0g
• Olive: 300 × 0.134 = 40.2g
• Castor: 100 × 0.128 = 12.8g
• Total lye (no discount): 129.0g

With 6% superfat: 129.0 × (1 − 0.06) = 121.3g lye Water = 121.3 × 2.3 = 278.9g water

Safety note: Lye (NaOH) is highly caustic. Always wear gloves, eye protection, and long sleeves. Add lye to water (never water to lye) to avoid dangerous heat surges. Work in a ventilated area.