How Much Lye to Use?
NOTE: Sometime in the spring of 1999, the Lewis Lye company started packaging their lye crystals in 18 ounce containers with reclosable child-proof lids. Be sure you notice how many ounces are in the can you have purchased before you dump it all into your batch! You may need to get a postal scale in order to weigh out the proper amount. I have still seen 12 ounce containers, but don't know if they are old stock or whether they are still packaging it both ways.
The first consideration in designing your soap recipe is that the balance of lye to fats be correct so that you will have a mild end product. This is determined by the saponification values of the oils. Elaine White has been very generous in providing this useful information to people via her website. Her chart is widely accepted, but some people think that relying on it alone can yield a harsher soap than using the Lye Calculator at Majestic Mountain Sage (Link Below) which displays the amount needed for a milder product. You can use either...but measure carefully if you're using the original saponification numbers.
All recipes are based on WEIGHTS, not volume. You will need to weigh oils and lye with a good scale. Water can be measured with a liquid measuring cup with no problems.
In the chart below I have put up the original SAP numbers (Elaine's) and a second set that has a built-in cushion of 5% extra fat. People who make soap all the time sometimes come up with their own favorite numbers, but this will give you something fairly reliable to work from if you don't want to go online every time you create a recipe. If you want to add a bit of superfatting oil at the end, you should still be able to do that with the lighter numbers, but I wouldn't get carried away since some superfatting is already there.
To use the following numbers, take the amount of fat you plan to use in your recipe (in ounces) and multiply it by the decimal number assigned to that type of fat. The resulting answer will be the amount of lye needed (in ounces). You can round your numbers up when you finish calculating them, but I'd wait and round up the whole thing after adding the numbers for each fat you are using in your recipe.) If you are combining fats, you can add the results of the calculations and see how close it comes to the standard 12 ounces of lye that come in a can (I don't like measuring lye crystals or having partial cans left over so the ideal recipe for me will use 12 ounces...if you are buying lye in bulk, this will not be a concern for you). Adjust your fats accordingly. At least 1/4 of your fat content should be a fat that is hard at room temperature but this is not written in stone. When designing your own recipe, a rule of thumb for the water used is approx. 1/3 of the total weight of the fats (in other words, add up the weight of the fats and oils and divide that by 3 for the ounces of water needed). Many people like to have a small cushion of extra fat in a recipe for mildness and usually strive for about 5-8%. If you use the Majestic Mountain Sage Lye Calculator link below, it will make that easier. Here is a list of most of the fats you might think to try in your soap, and a few others you may never want to use! ...