Superfatting Soap - An Explanation

I just got a fun question on superfatting for cold process soap over on TeachSoap and thought I'd expound a bit on superfatting cold process soap for everyone. First of all, if you haven't seen the SoapQueen.TV video on superfatting, I have a handy dandy little explanation in there at minute 3:26 (completel with clay - yes, clay! to make my point).

Soapmaking is at its heart, a science. When we add colors and fragrance to the process, we turn the science 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. For a real example, it looks like this (10 oz. Olive Oil) x (.134) = .134 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 left over (no extra oils) is called a 0% superfat or a 0% lye discount.


Many soapmakers like to have some extra oils in their soap that are leftover and not bound to lye. Any extra oil left in the soap and not attacked by the lye is called a 'superfat.' The terms 'superfat' and 'lye discount' can be used interchangeably.

To figure out a superfat, you can do it a few different ways. My favorite is to let the lye calculator figure it out for you (!) or you can take the amount of lye that you are supposed to use and multiply it with the following equation (1 - % superfat you want). So, if you wanted to superfat your Olive Oil soap in the above example by 4%, you would do the following (.134 oz. lye) x (1 - .04) = .1286 oz. lye. Or you can do the rough 'n' easy way; for a one pound batch of soap, a 5% superfat works out roughly to .8 oz. of extra oils per pound of soap.

When I superfat my cold process soap I normally keep my superfat to 5% or less. I find that much more than that and my soaps are a wee bit soft, don't lather quite as much as I like and also tend to go rancid more quickly. That said, superfat is a totally personal thing. I know some soapmakers that go up to 15% and swear by it.  I'd recommend experimenting a bit until you find the amount you love.  Soapmakers, I'd love to get your opinion on what your favorite superfatting range is.