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What is a SAP Number?


Photo: C Squared Studios / Getty Images

Photo: C Squared Studios / Getty Images

Photo: C Squared Studios / Getty Images
Definition: When looking at oils for soap making, or calculating a recipe for soap, you'll see a number associated with each oil called the SAP number. The SAP number is short for the saponification number. This is the units (ounces, grams, pounds) of lye needed to completely react with one unit (ounce, gram, pound) of an oil.

(Now...sodium hydroxide (which is used to make solid/bar soaps) and potassium hydroxide (which is used to make liquid soaps) each have different SAP numbers for oils. For the purposes of the example below, I'm going to use the sodium hydroxide numbers.)

For example, to make bar soap with sodium hydroxide, the SAP number for olive oil is .135.

To calculate the amount of lye needed, you multiply the amount of oil by the SAP number.

So, to completely saponify 10 ounces of olive oil:

10 (ounces) X .135 = 1.35 (ounces) of lye

If you're doing pounds or kilograms, just substitute that measure on both sides.

To calculate a more complex recipe, you do the same calculation for each oil in the recipe...and add up the total amount of lye needed.

But what about a lye discount or superfatting? Good question! If you're going to superfat your recipe (discount the amount of lye) you multiply the discount percentage by the final calculation.

For example, to put a 5% lye discount onto the 10 ounces of olive oil we're saponifying above, you'd multiply the 1.35 ounces of lye by 5% to find the amount to subtract. (You could also multiply it by 95% to find out the amount to keep.)


1.35 ounces of lye X 5% = .0675 ounces

Discounted amount of lye = 1.35 - .0675 = 1.28 ounces (rounded down)

Hopefully this will help you understand the math behind calculating the lye for a recipe...and also make you appreciate the programming and calculations behind the many different lye calculators that we have to use as soap makers.

The SAP number for olive oil is .135

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