Superfatting is a commonly used but much-misunderstood soapmaking technique. If you are interested in doing it, most lye calculators will figure this for you automatically. Use this explanation so you can understand just why you should do it.
Superfatting is either adding an extra amount of oil to your soap recipe but keeping the lye amount the same, or using the same amount of oil and less lye. For example, using 9 1/2 ounces of lye instead of 10 ounces would amount to percent superfatting.
The Reason for Superfatting
There are a few reasons why soapmakers may choose to superfat their soap. The first is, a small amount of extra "free" oil in the soap adds extra moisturizing qualities to the soap. This can often be referred to as "emollient" qualities. Basically, it makes the soap more moisturizing to the skin. Another common reason is that by superfatting your soap recipe, you will have a bit of a "fudge factor." Having extra lye in your soap is a big problem, much bigger than having extra oil.
Additionally, many oils will vary slightly in their true SAP (saponification value). Superfatting builds in some safety if your scale is not completely precise. Having a little extra oil in the recipe assures that every one of the lye molecules will have more than enough opportunity to pair up with an oil molecule.
Mistakes to Avoid
The thing you need to watch out for with superfatting is that while soap doesn't spoil quickly, oil does. The more "free" oil you have in your soap, the more likely you are to have spoilage. This is often called DOS or Dreaded Orange Spots, which are small orange spots where a pocket of free oil has gone bad. These can ruin your soap.
Superfatting your soap is easy. In fact, most lye calculators will calculate it for you. If not, you can just figure your lye, and then 'discount' it by your percentage. For example, if your recipe calls for 10 ounces of lye and you want a 5 percent discount, multiply that by .95 (95 percent). That would give you 9 1/2 ounces of lye. Follow the rest of the recipe and make the soap as you otherwise would.
Once you have a recipe you like, you may also want to experiment with a higher or lower superfat percentage. Try a batch at 3 percent and try a batch at 10 percent and see which outcome you prefer.