Spend five minutes with a lye calculator and you'll run into a box labeled "superfat," usually already set to 5%. It's one of the most important numbers in soapmaking, and one of the least explained. Here's what it means and how to choose it.
What is superfat in soap making?
Superfat is the percentage of oil in your recipe that's deliberately left without enough lye to turn it into soap. Every oil needs a specific amount of lye to fully react. Soapmakers use a little less than that, so some oil stays as oil in the finished bar.
A 5% superfat means you're using about 5% less lye than your oils could fully react with. The leftover oil stays free in the bar.
Why superfat matters
It does two jobs. The first is safety. Scales aren't perfect, lye isn't always exactly full strength, and oils vary a little from batch to batch. A superfat is a cushion that makes sure every bit of lye gets used up and none is left in the bar.
The second is feel. A little free oil makes a bar feel softer and more conditioning on the skin and less squeaky. Too much, and the bar gets soft, lathers less and can go rancid sooner.
How much superfat should you use?
For most cold process bar soap, somewhere around 5% is the classic starting point, and plenty of soapmakers stay between about 3% and 8%. A few common exceptions are worth knowing.
- Coconut-heavy bars (like laundry or salt bars) often use a much higher superfat, sometimes 15 to 20%, because coconut soap on its own can feel stripping.
- Laundry soap sometimes goes very low or to 0%, since nobody wants extra oil on their clothes.
- Bars heavy in soft oils, like olive, usually do fine at the standard 5%. Going much higher can make them slow to firm up.
When in doubt, use 5% and see how you like the bar after it cures.
Superfat vs. lye discount
You'll sometimes see the term "lye discount." It's essentially the same idea from the other direction. Superfat describes the extra oil, and lye discount describes the reduced lye. Most calculators, including ours, ask for superfat and do the conversion for you.
How to calculate superfat
You don't need to do the math by hand. Enter your oils and their weights into a lye calculator, set the superfat, and it will tell you exactly how much lye and water to use. Change the superfat and watch the lye amount drop or rise. That's all superfat is doing.
A word of caution. Always run your own oils and weights through a calculator, even if you're following a recipe. Swapping one oil for another changes how much lye you need.
Can you add superfat after the soap traces?
Some soapmakers add a special oil or butter at trace, hoping it stays as free oil rather than becoming soap. In practice the lye reacts with whatever oil is around, so the effect is smaller than people hope. It's fine to do, but the calculator setting is what actually controls your superfat.
New to all of this? Start with our beginner's guide on how to make soap, and our guide to lye covers the rest. Or let us do the measuring, and browse the soap shelf.