When using a recipe out of a book or internet how do you know what size mold to use? How do you know if the recipe is 2.5 pounds, five pounds or something else? Do you add up the weight of the oils to get the pounds?
The best method I have found is to find a soap mold you like. One that will make however many bars of soap you would like and adjust any recipe to fit that mold.
I don't suggest trying to find molds to fit the recipes since there are so many out there and each one is a different size. You would have way too many soap molds in the end.
Lets say for example you want to make 10 bars of soap each time you make a batch. Lets assume that each bar is 1 inch thick by 3 inches wide by 2 inches tall. One option for a soap mold for this batch would be a log mold that has an interior measurement of 10 inches long by 3 inches wide by 2 1/2 or 3 inches high.
In order to figure out how much oil is needed for this soap mold you will need to use the formula located at the bottom of this page
or at the bottom of this page
Once you know how much oil you will need to fill your soap mold, you can use a lye calculator to adjust the recipe to the right size.
This will require figuring out what percent of each oil is used in the original recipe and plugging those numbers into the lye calculator.
In order to make soap it is really important to learn how to use lye calculators as they are an essential tool to making soap.