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:

There are two ways to say the same thing, and recipes use both, which is a common source of confusion.

Water = Lye × Water-to-lye ratio (traditional: 2:1 to 2.5:1 by weight)

Lye concentration % = Lye ÷ (Lye + Water) (what this calculator asks for, and what most modern recipes quote)

They convert directly. A 30% lye concentration means the solution is 30% lye and 70% water, so the water-to-lye ratio is 70 ÷ 30 = 2.33:1. Concentration is the more useful of the two because it says something about the solution itself rather than about a ratio you have to work backwards from. The result panel gives both.

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 the calculator reproduces:

800g of olive oil, 30% lye concentration, 5% superfat. Put those three numbers in above and you get:

• Full lye: 800 × 0.134 = 107.2g
• Less 5% superfat: 107.2 × 0.95 = 101.84g lye
• Total solution at 30%: 101.84 ÷ 0.30 = 339.47g
• Water: 339.47 − 101.84 = 237.63g water, a 2.33:1 water-to-lye ratio
• Superfat left free in the bar: 107.2 − 101.84 = 5.36g of oil

A blend, worked by hand:

The calculator takes one oil at a time, so a real multi-oil recipe is done oil by oil and added up. Recipe: 400g coconut + 300g olive + 100g castor, 800g total, 5% superfat, 30% concentration.

• Coconut: 400 × 0.190 = 76.0g
• Olive: 300 × 0.134 = 40.2g
• Castor: 100 × 0.128 = 12.8g
• Total lye before discount: 129.0g

With 5% superfat: 129.0 × 0.95 = 122.55g lye. Water at 30% concentration: 122.55 ÷ 0.30 − 122.55 = 285.95g.

Note that the blend needs 129.0g where 800g of straight olive oil needs 107.2g, 20% more, entirely because of the coconut. Running a coconut-heavy recipe against olive’s SAP value is how a batch ends up with unsaponified oil in it.

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.