Soap making · Soap Lye Calculator
What is superfat in soap making
Superfat is the percentage of oil in a soap recipe deliberately left unreacted after saponification. A 5 percent superfat means the lye is reduced so 5 percent of the oil stays as free oil in the bar, which is why it is also called a lye discount. Typical bar soap values are 3 to 8 percent.
Superfat and lye discount are the same adjustment
Saponification is the reaction between an oil and an alkali (sodium hydroxide for bars, potassium hydroxide for liquid soap). Each oil has a saponification value that says how much lye converts one gram of that oil to soap. If you use exactly that amount, the bar has 0 percent superfat: no free oil, and no margin for error.
Superfatting means leaving some oil unreacted. Historically makers did this two ways. A lye discount reduces the lye: if the calculation says 100 g of lye is needed, a 5 percent discount uses 95 g. Superfatting in the older sense adds extra oil at trace instead. Research summarized by the Handcrafted Soap and Cosmetic Guild found the oils extracted from discounted and superfatted bars were virtually identical, so the timing does not matter and the terms are now used interchangeably.
Every lye calculator applies superfat as a percentage reduction on the computed lye. The formula is lye needed times (1 minus superfat). Bramble Berry's example: 1.34 oz of lye at 4 percent superfat becomes 1.34 times 0.96, which is 1.29 oz.
Why some free oil is always left in a bar
The first reason is safety. Saponification values published for an oil are averages, and a real batch of oil can differ. The Guild cites a palm oil listed at a saponification value of 203 mg KOH per g whose measured value was 196, about 3 percent lower. With 0 percent superfat that batch would have contained unreacted lye, which makes a harsh, skin irritating bar. A 5 percent margin absorbs supplier variation and scale error.
The second reason is feel. Free oil in the bar adds emollience and a milder wash. Soap Queen describes 5 percent as adding richness without making the bar too soft or hurting lather. Humblebee and Me gives the same standard and warns not to go much above 10 percent, because the bar becomes soft and the free oil can go rancid.
The trade off is shelf life and lather. Soap is stable for years, but unsaponified oil is not. High superfat bars, especially those made with oils rich in polyunsaturated fats, are more prone to dreaded orange spots, the rancidity marks that appear months after curing. Excess oil also coats the lather and reduces bubbles.
A worked example with olive oil
Humblebee and Me gives a clean example. 500 g of olive oil pomace needs 67.7 g of sodium hydroxide at 0 percent superfat. At 5 percent superfat the lye drops to 64.3 g. That 3.4 g reduction in lye leaves about 25 g of the 500 g of oil unreacted, so the finished bar contains 475 g of saponified olive oil plus 25 g of free oil.
Work it yourself with the formula: 67.7 g times (1 minus 0.05) equals 64.3 g. For a 10 percent superfat, 67.7 times 0.90 is 60.9 g. Notice that the superfat percentage is a share of the oil, not of the lye: a 5 percent lye discount and a 5 percent superfat are numerically the same because lye scales linearly with oil.
Bramble Berry offers a rough rule for visualizing it: a 5 percent superfat works out to about 0.8 oz of extra oil per pound of soap. That is the amount of oil that never becomes soap and stays behind to condition skin.
Choosing a superfat percentage by soap type
General cold process bars: 3 to 8 percent is the common working range, and 5 percent is the default most calculators load. It gives a safe margin without noticeably softening the bar.
100 percent coconut oil bars: coconut soap is very cleansing and can be drying, so makers push superfat to 20 percent to make it usable on skin. Soap Queen recommends 20 percent for this recipe. Salt bars, which are coconut heavy, are handled the same way.
Castile and high olive bars: these are already mild and slow to lather, so they gain nothing from high superfat and many makers keep them at 3 to 5 percent. Shaving and liquid soap made with potassium hydroxide usually run lower, often 0 to 3 percent, because free oil kills the dense lather those products depend on and because commercial KOH is only about 90 percent pure, which itself acts as a hidden discount if the calculator does not correct for it. A dual lye calculator like Benchlet's has a purity field so the superfat you enter is the superfat you get.
What superfat does not do
It does not make an unsafe recipe safe. If the oil weights are mismeasured by more than the superfat margin, or the lye is weighed on a scale that is off by 10 g, the bar can still be lye heavy. Superfat is a buffer, not a substitute for accurate weighing.
It does not choose which oil stays free. In a blended recipe, the lye reacts with whatever it meets, so the unreacted 5 percent is a mix of all the oils, not the expensive oil added at trace. This is the practical conclusion of the Guild's research: mix all oils together before adding lye rather than holding one back.
It does not fix a poorly balanced recipe. A bar with a bad fatty acid profile stays soft, sticky, or non lathering regardless of superfat. Adjust the oil blend first, then set superfat at 5 percent unless a specific soap type calls for more.
Frequently asked questions
Is superfat the same as lye discount?
Yes in practice. Both leave a percentage of oil unreacted. A 5 percent superfat equals a 5 percent lye discount, meaning the calculator uses 95 percent of the theoretical lye.
What superfat percentage should a beginner use?
5 percent. It is the default in most lye calculators and gives a safety margin against oil variation without making the bar soft.
Can superfat be too high?
Yes. Above about 10 percent the bar softens, lathers less, and the free oil is more likely to go rancid and form orange spots. The exception is 100 percent coconut soap, which is often made at 20 percent.
Does adding oil at trace give a better superfat?
No. Testing found the free oil in discounted and trace superfatted bars was virtually identical. Mix all oils together before adding lye.
Does superfat apply to liquid soap?
Yes, but lower, often 0 to 3 percent, because free oil clouds liquid soap and reduces lather. Also account for KOH purity, which is typically around 90 percent.
