Superfatting Soap – An Explanation
Soapmaking is a science at heart. When you add color and fragrance, it turns into an art. Each oil has its own saponification value, or the amount of lye it takes to turn 1 gram of oil into 1 gram of soap.
When we make cold process soap, it’s a mathematical formula that looks like this: (oil amount) x (SAP value) = lye amount needed. Find a list of SAP values in this post Beginner's Guide to Common Soapmaking Oils. An example is (10 oz. Olive Oil ) x (.134) = 1.34 oz. lye. So, it takes 1.34 ounces of lye to turn 10 ounces of olive oil into soap.
Using the exact amount of lye you need to make the exact amount of soap with nothing leftover is a 0% superfat or lye discount. Many makers like to have some leftover oils in their recipe that aren’t bound to lye, which can be anywhere from a 1-20% superfat.
There are a few ways to calculate superfat. The easiest method is to plug your recipe into the Lye Calculator. All you have to do is select the percentage you want and it will calculate for you.
You can also use the following equation: (1 – % superfat you want). So, if you want to superfat your olive oil soap in the above example by 4%, it would be: (1.34 oz. lye) x (1 – .04) = 1.29 oz. lye. You can also simplify that – for a one pound batch of soap, a 5% superfat works out roughly to 0.8 oz. of extra oils per pound of soap.
We almost always superfat our recipes at 5% because it adds luxury to the soap without making it too soft or inhibiting lather.
That said, superfat is a totally personal thing. Some soapmakers go up to 15% and swear by it. It also depends on the recipe. For instance, we recommend superfatting coconut oil soap at 20% so it isn’t too harsh on the skin. Experimenting with a few different superfats is the best way to find the amount you love.
This Coconut Oil and Annatto Soap is superfatted at 20%.
This post was updated in May 2018.