Understanding 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. Whether you use sodium hydroxide (NaOH) for bar soap or potassium hydroxide (KOH) for liquid soap, SAP values ensure your lye calculations are precise.

What Is a SAP Value?

The saponification value (SAP value) is defined as the number of milligrams of potassium hydroxide (KOH) required to completely saponify one gram of a given fat or oil. This is the standard measurement used in chemistry and is always expressed in terms of KOH.

KOH SAP Value: The base measurement — mg of KOH needed per gram of oil.

NaOH SAP Value: Derived by dividing the KOH SAP value by 1.403 (the ratio of their molecular weights: 56.11 ÷ 39.997). Since NaOH has a lower molecular weight, you need less of it.

Example: Olive oil has a KOH SAP value of approximately 190. Its NaOH SAP value is 190 ÷ 1.403 = 0.1354 (expressed as grams of NaOH per gram of oil).

NaOH vs KOH SAP Values

Which SAP value you use depends on the type of soap you are making:

NaOH (Sodium Hydroxide)

Used for bar soap (cold process, hot process). Produces a hard, solid bar. The NaOH SAP value is always lower than the KOH SAP value because NaOH is a lighter molecule.

KOH (Potassium Hydroxide)

Used for liquid soap and cream soap. Produces a soft paste that is diluted with water. The KOH SAP value is the original, standard measurement.

Dual Lye

Some recipes use a combination of NaOH and KOH. The calculator splits the lye requirement proportionally and uses the appropriate SAP value for each portion.

How the Soap Calculator Uses SAP Values

The calculator follows three key steps to determine your lye and water amounts:

Step 1 — Calculate lye per oil: For each oil in your recipe, the calculator multiplies the oil weight (in grams) by its SAP value. This gives the total lye needed to fully saponify that oil.
Step 2 — Apply superfat discount: The total lye is reduced by your superfat percentage. For example, at 5% superfat the calculator uses only 95% of the total lye, leaving some oil unsaponified for conditioning.
Step 3 — Calculate water: Based on your chosen water method (lye concentration, water:lye ratio, or % of oil weight), the calculator determines how much water to dissolve the lye in.

Example: 500g olive oil × 0.1354 NaOH SAP = 67.7g NaOH needed. At 5% superfat: 67.7 × 0.95 = 64.3g NaOH. At 33% lye concentration: 64.3 ÷ 0.33 − 64.3 = 130.5g water.

Fatty Acid Profiles

Every oil is made up of different fatty acids, and the blend of fatty acids determines the properties of your finished soap. Understanding these profiles helps you design a balanced recipe:

Lauric Acid: Produces big, fluffy lather and excellent cleansing. Found in coconut oil and palm kernel oil. Too much can be drying.

Oleic Acid: Provides conditioning and moisturizing. Found in olive oil, avocado oil, and sweet almond oil. Creates a stable, creamy lather.

Stearic & Palmitic Acids: Contribute to bar hardness and a stable, creamy lather. Found in palm oil, tallow, lard, cocoa butter, and shea butter.

Ricinoleic Acid: Boosts lather and adds a stable, dense foam. Found almost exclusively in castor oil. Typically used at 5–10% of the recipe.

Linoleic & Linolenic Acids: Provide conditioning and silky skin feel, but produce softer bars with a shorter shelf life. Found in sunflower, soybean, hemp, and grapeseed oils.

INS and Iodine Values

Two additional numbers help you evaluate the quality and shelf life of your soap recipe:

Iodine Value

Measures the degree of unsaturation in your oils. Higher iodine values mean more unsaturated fats, which produce softer bars that are more prone to rancidity (dreaded orange spots, or DOS). A recipe iodine value below 70 is generally recommended for a long-lasting bar.

INS Value

Calculated as SAP value minus iodine value (INS = SAP − Iodine). A recipe INS near 160 is considered ideal, indicating a good balance of hardness, cleansing, conditioning, and lather. This is a guideline, not a strict rule — many excellent recipes fall outside this range.

Tips for Working with SAP Values

Always use a reliable soap calculator rather than doing SAP math by hand — small errors in lye amounts can result in unsafe soap.
If you source oils from different suppliers, SAP values may vary slightly. A 5% superfat provides a comfortable safety margin for natural variation.
Aim for a balanced fatty acid profile: roughly 25–35% hard fats (palmitic/stearic), 40–50% conditioning oils (oleic), 15–25% cleansing oils (lauric), and 5–10% castor oil for lather.
Keep your recipe iodine value below 70 and INS value near 160 for a well-balanced bar, but don't let numbers override practical testing.
When substituting oils, choose replacements with similar SAP values and fatty acid profiles to maintain the recipe's balance.

Frequently Asked Questions

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