What Is a SAP Value?

The Saponification Value (SAP value) tells you exactly how much lye is needed to turn a specific oil or butter into soap. Every oil has a different SAP value based on its fatty acid composition. Oils high in short-chain fatty acids (like coconut oil) have higher SAP values, meaning they need more lye per unit of weight.

NaOH vs KOH

There are two types of lye used in soap making:

• Sodium Hydroxide (NaOH) — used for solid bar soaps. Creates a hard, crystalline soap.
• Potassium Hydroxide (KOH) — used for liquid soaps. Creates a soft paste that dissolves in water.

Each ingredient in Apothecary Wizard has both an NaOH value and a KOH value. The calculation engine uses the correct one based on your recipe type.

How the Calculation Works

For a solid soap, the formula is:

Total NaOH = Sum(Oil Weight × NaOH value) × (1 - Superfat% / 100)

For example, if you have 16 oz of olive oil (NaOH value 0.135) and 12 oz of coconut oil (NaOH value 0.191) at 5% superfat:

NaOH = (16 × 0.135 + 12 × 0.191) × 0.95 = (2.16 + 2.292) × 0.95 = 4.23 oz

Apothecary Wizard does this math automatically. You just select your oils, enter the weights or percentages, set your superfat, and the engine calculates the exact NaOH and water you need.

Water Calculation

Water is typically calculated as a percentage of the total oil weight. The standard is 33-38%. Less water means a faster trace and a harder bar sooner, but is less forgiving. More water gives you more time to work but requires a longer cure.

Why Accuracy Matters

Lye calculation is one area where precision is non-negotiable. Too much lye creates a harsh, drying soap. Too little lye means your soap will be overly soft and may go rancid. Always weigh your ingredients (never measure by volume) and double-check your calculations. Apothecary Wizard handles the math, but you should always verify the numbers make sense before pouring.