How a soap lye calculator works

Safety: lye (sodium/potassium hydroxide) causes chemical burns. Wear gloves and eye protection, add lye to water (never water to lye), and always verify the lye weight with a second calculator before making soap. Results below are estimates from typical SAP values; oils vary by supplier.

The core formula

Every oil has a saponification (SAP) value: the grams of lye needed to turn 1 g of that oil into soap. For sodium hydroxide (NaOH) olive oil is about 0.135 g/g, so 1,000 g of olive oil needs about 135 g NaOH at 0% superfat.

NaOH = Σ(oil g × SAPNaOH) × (1 − superfat) ÷ purity

Superfat

Superfat means using less lye than the oils could consume, leaving some free oil in the bar. 5% is a common starting point for bar soap; 0% risks a harsh bar, higher values feel richer but go rancid sooner. Never use a negative superfat.

Water

Water does not change the chemistry; it changes how quickly the batter thickens and how long the bar takes to dry. Calculators express it three ways: percent of oils (e.g. 33%), water-to-lye ratio (e.g. 2:1) or lye concentration (e.g. 30%). They are convertible: at 5% superfat, 33% water of an olive-oil recipe is roughly a 2.6:1 ratio.

NaOH, KOH and mixed soap

Bar soap uses NaOH; liquid soap uses KOH. KOH flakes are usually about 90% pure, so the calculated weight is divided by 0.90. Mixed soaps split the lye by a percentage.

Why two calculators can disagree

SAP tables differ slightly between sources (olive oil is listed as 0.134, 0.135 or 0.136). That can move the result by a gram or two per kilogram of oil. This is why you should always use the same table consistently and cross-check with a second tool.

Try the free lye calculator.