Here's another way, adapted from a post I made in Songwind's shaving soap thread:
Three molecules of KOH are needed to saponify one molecule of fat. Three molecules of NaOH are needed to saponify one molecule of fat. Sounds easy peasy, right? But there's a catch -- one molecule of KOH weighs more than one molecule of NaOH. So if you want to make a soap using, for example, 40% NaOH and 60% KOH, what is the correct way to figure the weights for the KOH and NaOH lyes?
There are several ways to do this. Some people use the summerbeemeadow.com
soap calculator. It will allow you to specify the percentages of the two lyes and will calculate the weights for the two lyes for you. Others will do the figuring by hand or create their own spreadsheet to calculate the lye amounts.
A third way to figure the lye weighs is to use your favorite
soap recipe calculator, such as soapcalc.net. Calculate the exact same soap recipe twice. The only difference between the two versions is your choice of lye -- use NaOH as the lye for the first version and KOH as the lye for the second version.
For the first version, the recipe calculator will tell you how much NaOH that you would need to use if you wanted NaOH as ALL of the lye. In the second version, it will tell you how much KOH you would need to use if KOH was ALL of the lye.
Since you want a mixture of 40% NaOH and 60% KOH for the recipe, multiply the NaOH weight times 0.40. And multiply the KOH weight times 0.60. This will give you the correct weights of each lye to mix together to make the recipe with 40% NaOH and 60% KOH.
