Saponification Value Calculator | Free Soap Making Tool

What is a saponification value calculator?

A saponification value calculator finds the saponification (SAP) value of an oil or fat blend used in soap making. It takes the weight of each oil in a recipe and returns the weighted-average amount of potassium hydroxide (KOH), a strong alkali also called lye, needed to fully react with the fat.

What is saponification value?

Saponification value is the number of milligrams of KOH needed to fully saponify one gram of a given fat or oil. Saponification is the chemical reaction between a fat and an alkali that produces soap and glycerin.

Every oil has its own saponification value, because oils are made of different fatty acids. An oil built from shorter fatty-acid chains, such as coconut oil, packs more molecules into each gram than an oil built from longer chains, such as olive oil. More molecules per gram means more lye is needed, so coconut oil has a higher saponification value (260 mg KOH/g) than olive oil (190 mg KOH/g).

Soap makers use saponification value to work out exactly how much lye a recipe needs. Too little lye leaves the soap greasy and prone to spoiling. Too much lye leaves harsh, caustic soap that can irritate skin.

Saponification value formula

When a recipe mixes several oils, the blend's saponification value is the weighted average of each oil's value, weighted by how much of that oil is in the recipe:

$S = \frac{\sum (w \times v)}{W}$

where S is the blend's saponification value, w the weight of one oil, v that oil's own saponification value and W the combined weight of all the oils.

Σ means "sum of." Each oil's weight is multiplied by its own SAP value, those products are added together, and the total is divided by the combined weight of all the oils.

Worked example

A recipe uses 400 g of coconut oil (260 mg KOH/g), 300 g of olive oil (190 mg KOH/g), and 300 g of shea butter (180 mg KOH/g).

Saponification value = 215,000 ÷ 1,000 = 215 mg KOH/g.

That means the full recipe needs 215 mg of KOH for every gram of oil, or 215 g of KOH for the full 1,000 g of oil (1,000 g × 215 mg/g ÷ 1,000).

Saponification values of common oils and fats

The table below lists standard saponification values in mg KOH per gram of oil.

Real oils vary a little by growing region, harvest, and processing, so lab-measured values can differ from these standard figures by a few percent. Soap makers who need an oil not on this list can enter a custom saponification value from a supplier's data sheet. The custom field accepts 0 to 500 mg KOH/g.

What the calculator shows

The calculator reports the blend's saponification value in mg KOH/g, rounded to two decimal places, and the total weight of oil it was worked out from. It also lists each oil's weight times its saponification value, so the sum divided by the total weight can be checked by hand.

A row only counts once the calculator knows its saponification value. A custom oil with an empty value field is left out of both the total weight and the average, and the field is marked as required until a value is entered.

Converting KOH values to NaOH

Bar soap is usually made with sodium hydroxide (NaOH) instead of potassium hydroxide, while liquid soap is made with KOH. Because NaOH is a lighter molecule than KOH, less of it is needed for the same reaction. To convert a KOH saponification value to the matching NaOH value, multiply by 0.713:

$N = K \times 0.713$

where N is the NaOH value and K the KOH value.

For the 215 mg KOH/g example above, the matching NaOH value is 215 × 0.713 ≈ 153.3 mg NaOH/g.

Accounting for superfat

Most soap recipes are not saponified 100%. Soap makers often hold back 5% to 8% of the calculated lye amount, called "superfat," so a little unreacted oil stays in the finished bar for extra moisturizing. To apply a superfat discount, reduce the calculated lye weight by that percentage. A recipe that calls for 150 g of lye at 5% superfat would use 142.5 g instead (150 g × 0.95).

Frequently asked questions

What happens if the wrong saponification value is used?

The lye weight is worked out from the saponification value, so an error carries straight through. Too low a value calls for too little lye, and unreacted oil is left in the bar, which makes it soft and quicker to spoil. Too high a value calls for too much lye, and the leftover alkali can burn skin.

Why does coconut oil have a higher saponification value than olive oil?

Coconut oil is rich in lauric acid, a fairly short chain of 12 carbon atoms. Shorter chains are lighter, so more molecules fit into each gram, and each molecule needs its own share of lye. Olive oil is mostly oleic acid, an 18-carbon chain, so fewer molecules fit into a gram and less lye is needed.

Can a custom oil be added if it is not on the list?

Yes. Enter its saponification value directly using the custom oil option. Saponification values for uncommon oils are usually available from the oil supplier or a soap-making reference table.

Does the calculated value work for both cold process and hot process soap?

Yes. Saponification value depends only on the fatty-acid makeup of the oil, not on how hot the mixture gets or how long it reacts, so the same value applies to cold process, hot process, and liquid soap recipes.

Does temperature change an oil's saponification value?

No. Saponification value is a fixed property of the oil's fatty-acid composition. Temperature changes how fast the reaction happens, not how much lye the reaction needs.