These are some of my favorite online soap-making lye calculators. They provide varying degrees of information and differ in ease of use for the novice soap maker.
www.thesage.com/calcs/LyeCalc.html
www.brambleberry.com/pages/lye-calculator.aspx
Although I highly recommend using an online lye calculator, there may be times when it is necessary to calculate the amount of lye needed by hand.
In this appendix, I have listed all the oils I use in this book and their corresponding fatty acid profiles and values needed to do the lye calculations. The SAP values are given in ranges, as the actual values vary from one batch of oil to the next and from one supplier to the next. If the actual SAP number is known for a particular oil, that number should be used. The SAP number is expressed as the number of milligrams of KOH (potassium hydroxide) required to saponify 1 gram of fat. A Certificate of Analysis (COA) generally shows this number.
In order to use this number to calculate the lye needed, the SAP value needs to be converted into a universal multiplier. There is a difference in numbers because the molecular weights of KOH and NaOH (sodium hydroxide) are not the same.
To simply things, I have already done the math using an average SAP value for particular oils. I have included those converted values next to the actual SAP value for both sodium hydroxide (NaOH) and potassium hydroxide (KOH).
In order to calculate the lye needed, using the converted SAP value for the desired oil, simply multiply the desired weight of oil by the converted SAP number.
For example, to make the Bar Soap with Honey recipe, these are the calculations:
For liquid soap (KOH) = the SAP value ÷ 1000
For solid soap (NaOH) = the SAP value ÷ 1402.50
This number is not the same as the amount of lye shown in the recipe. That’s because the number calculated above will convert all of the oil into soap. However, because most of the SAP numbers used are just averages of an acceptable range of lipids for a particular oil, it is recommended that a buffer is calculated in, about 2 to 3 percent. On top of that, this soap would probably be very drying, so it would be helpful to have some oil remaining in the soap to act as a moisturizer, let’s say about 5 percent, bringing the total buffer to 7 percent. This 7 percent number is called the superfat. I generally work with 6 to 7 percent superfat in my bar soap recipes.
So, with the above example, we take: