Classic Bells > Soapy stuff > Glycerin made by saponification

Glycerin made by saponification

When soap is made by reacting fats with sodium hydroxide (NaOH) or potassium hydroxide (KOH), a certain amount of glycerin is naturally created by the saponification reaction. You can easily calculate the amount of glycerin produced.

Glycerin from NaOH saponification

In typical cold-process or hot-process soaps made with NaOH, 0.77 grams of glycerin are created for every 1 gram of NaOH used to make the soap. (Or 0.77 ounces glycerin for every 1 ounce NaOH.)

Glycerin weight = 0.77 X (NaOH weight)

Total paste weight = Fat weight + NaOH weight + Water weight

% glycerin in the newly made soap = Glycerin weight / Total paste weight X 100

This relationship between NaOH and glycerin is accurate regardless of the amount and type of fats in the recipe. (See also "Limitations" below.)

Glycerin from KOH saponification

In a typical liquid soaps or shaving soaps made with KOH, 0.55 grams of glycerin are made for every 1 gram of KOH. (Or 0.55 ounces glycerin for every 1 ounce KOH.)

Glycerin weight = 0.55 X (KOH weight)

Total paste weight = Fat weight + KOH weight + Water weight

% glycerin in the newly made soap = Glycerin weight / Total paste weight X 100

This relationship between KOH and glycerin is accurate regardless of the amount and type of fats in the recipe. (See also "Limitations" below.)

Glycerin from dual lye saponification (recipe uses KOH and NaOH)

Recipes for cream soaps, some types of shaving soaps, and dual-lye bar soaps use both KOH and NaOH. For these dual-lye recipes, separately calculate the glycerin created by each alkali and add the two numbers together.

Glycerin weight = 0.77 X (NaOH weight) + 0.55 X (KOH weight)

Total paste weight = Fat weight + NaOH weight + KOH weight + Water weight

% glycerin in the newly made soap = Glycerin weight / Total paste weight X 100

This relationship between NaOH, KOH, and glycerin is accurate regardless of the amount and type of fats in the recipe. (See also "Limitations" below.)

Glycerin content after cure

During cure, water evaporates from the soap, but the weight of the other ingredients, including the glycerin, will not change. I estimate the percentage of glycerin in a typical cured bath soap will be about 2% higher than in the newly made soap.

Limitations of this method

These formulas are based on the assumption that ALL of the lye reacts with triglyceride fats to make soap, so they will not be accurate if your recipe includes one or more of these characteristics --

Unusually lye heavy. The recipe can have a zero % superfat, a slight negative superfat % (down to about -3%), or any positive superfat %.
Fatty acids (example: stearic acid)
Ingredients that consume lye, but do not make soap (examples: rosin, pine tar)

Any soap that is very lye-heavy or made with fatty acids or other unusual ingredients will contain less glycerin than you will calculate from these formulas.

Example for NaOH

Here's a simple, classic soap recipe:

Coconut oil 30 grams
Olive oil 70 grams

superfat 5%
28% lye solution concentration

I calculated these weights for lye and water for the recipe:

NaOH 14.6 grams
Water 37.7 grams

How much glycerin will be in this soap from the saponification process?

Glycerin weight = (14.6 grams) X (0.77) = 11.2 grams

What percent of glycerin is in this newly made soap?

Total paste weight = 30 + 70 + 14.6 + 37.7 = 152.3 grams

% glycerin in the newly made soap = 11.2 / 152.3 X 100 = 7.4%

% glycerin in the cured soap ~ 7.4% + 2% = 9.4%