What Is a Saponification Value?

If you make soap and have decided to start formulating your own recipes, I’m guessing you’ve already come across the term saponification value.

Trying to understand the connection between this number and the amount of lye you need can be confusing. Especially when the numbers seem to be different from one blog to another. But the idea behind these numbers is actually quite simple: the saponification value helps us determine how much caustic soda (NaOH) is needed to turn an oil into soap. These numbers are essential when you want to create or modify your own natural soap recipes, so you need to learn how to use them.

If you're on my blog, chances are you're making natural solid soap, which means you're using caustic soda (NaOH). And we both know that, to make soap, you need to mix the lye with water and then let it react with the oils you've chosen for your recipe. The reaction produces soap and glycerin, like in the diagram below:

Oil or butter + NaOH + water → soap + glycerin

Looking at this equation, you might think that all you have to do is mix your oils with your lye solution and you’ll get soap. The problem is that every oil has a different saponification value.

So, each oil needs a specific amount of lye to react properly, without leaving excess lye in the finished soap. If I have 100 g of coconut oil, I cannot use the same amount of caustic soda as I would for 100 g of olive oil. And that is why, when you decide to formulate your own recipes, you need to start working with these values.

But how do you use saponification values?

The saponification value tells us how much NaOH is theoretically needed to fully saponify a specific amount of oil. So, if I have 100 g of olive oil and use the amount of NaOH given by its saponification value, the resulting soap would theoretically be at 0% superfat. (If you don't know what superfat is, check out my other article, “What Is Superfatting?”)

In simple terms, you take the amount of oil you have and multiply it by its saponification value to calculate the amount of NaOH needed. And each oil has its own multiplier.

More concretely, the saponification value for coconut oil is around 0.183. If I have 100 g of coconut oil, I simply multiply 0.183 by the amount of oil:

100 g coconut oil × 0.183 = 18.3 g NaOH

So, if I use 100 g of coconut oil, 18.3 g of NaOH would theoretically be enough to fully saponify it. But, of course, this would give me a soap with 0% superfat. A 100% coconut oil soap at 0% superfat would generally be far too harsh for use on the skin.

If you want to use a 100% coconut oil soap on your body, you would normally need to use a higher superfat. (And just to be clear, superfat levels of 20% or more are mainly relevant to 100% coconut oil recipes. For most soap recipes, a superfat somewhere around 5–10% is much more common.)

Now, you might be wondering: Why don't all oils have the same saponification value?

Why can't the 18 g of NaOH calculated for coconut oil be used for 100 g of olive oil or castor oil?

To understand this, you first need to know that not all oils are chemically identical. Even though we call them all “oils” or “fats,” they are made up of different molecules called fatty acids. And these fatty acids don't all have the same size, weight, or structure.

Coconut oil contains a large proportion of shorter-chain fatty acids, such as lauric acid. Because these molecules are relatively small, there are more of them by number in a given weight of oil.

And since each fatty acid molecule needs its share of NaOH to react, coconut oil requires a relatively large amount of lye for the same weight of oil. That's why it has a relatively high NaOH saponification value, around 0.183.

Other oils contain a larger proportion of longer-chain fatty acids. For the same weight of oil, there are fewer molecules overall, so less NaOH is needed to saponify them. This is exactly why we cannot use the same number for every oil.

If you put the 18 g of NaOH calculated for coconut oil into 100 g of olive oil, you would be using far too much lye. Some of that lye could remain unreacted, making the soap excessively caustic and potentially capable of causing chemical burns. That's why each oil has its own saponification value: we need the right amount of NaOH for the particular fats we're using.

Some commonly used values

Here are some approximate NaOH saponification values commonly found in soap calculators.

One important thing to remember is that saponification values are not always exactly the same from one source to another. This is why soap calculators may sometimes give you slightly different numbers.

When formulating your own recipes, the important thing is to understand what the number represents and, above all, to calculate your NaOH accurately and use a reliable lye calculator.