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Soap Math

SAP Values

The Science Behind Soap Calculations

Every oil and fat has a unique saponification (SAP) value that determines exactly how much lye is needed to turn it into soap. Understanding SAP values is the foundation of accurate soap formulation — they are the numbers your soap calculator relies on to produce a safe, balanced bar.

What Is a SAP Value?

The SAP value is defined as the number of milligrams of potassium hydroxide (KOH) required to completely saponify one gram of a fat or oil.

KOH SAP Value

The base measurement used in chemistry — mg of KOH needed per gram of oil.

NaOH SAP Value

Derived by dividing the KOH value by 1.403. Since NaOH is a lighter molecule, you need less of it.

Example: Olive oil has a KOH SAP value of ~190. Its NaOH SAP value is 190 ÷ 1.403 = 0.1354.

NaOH vs KOH SAP Values

Which lye you use depends on the soap type:

NaOH (Sodium Hydroxide)

Used for bar soap. Produces a hard, solid bar.

KOH (Potassium Hydroxide)

Used for liquid soap and cream soap. Produces a soft paste.

Dual Lye

Uses a mix of both. The calculator splits the requirement proportionally.

Fatty Acid Profiles

The blend of fatty acids determines the soap's properties:

  • Lauric Acid: Big, fluffy lather and cleansing. (Coconut oil)
  • Oleic Acid: Conditioning and moisturizing. (Olive oil)
  • Stearic & Palmitic: Hardness and creamy lather. (Palm, Tallow)
  • Ricinoleic Acid: Boosts lather and adds slip. (Castor oil)
  • Linoleic & Linolenic: Silkiness, but softer bars and shorter shelf life.

Iodine and INS Values

Iodine Value

Measures unsaturation. Higher iodine means softer bars more prone to DOS (orange spots). Aim for below 70.

INS Value

Calculated as SAP − Iodine. An INS near 160 is considered ideal for a balanced bar.

Tip: SoapMath uses SAP values automatically

Every oil in SoapMath has its SAP value built in. Just enter your oil percentages and the calculator handles all the lye math.

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