What superfat actually buys you in a bar

Superfat is the share of oil in a recipe that is deliberately left unsaponified. Five percent is the usual default, and most explanations stop at the word moisturising.

That is the smaller half of what it is doing.

It is a safety margin first

Saponification values are averages. The lye needed to turn a gram of olive oil into soap varies slightly with the crop, the pressing and the age of the oil, so the SAP figure in any calculator is a good central estimate rather than a constant.

Weighing is imperfect too. A scale reading to whole grams on a 1,000 g batch, an oil container that holds back a little more than you thought, a fragrance oil that affects trace, and the real ratio drifts from the calculated one.

Superfat absorbs all of that. At 5%, a recipe can be meaningfully off in the direction of too much lye and still produce a safe bar, because there is surplus oil waiting to take it up. At 0% the same error leaves free sodium hydroxide in a bar that people are going to wash with.

What it costs

Surplus oil is not free. It makes a softer bar that wears away faster in the dish, it cuts lather slightly, and because the free oil is not bound into soap it can oxidise, which is where the orange spots known as dreaded orange spots come from on an older bar.

So the sensible range sits around 3 to 8% for a bar, and nudging toward the top of it buys conditioning at the cost of longevity. Pushing past 10% generally gives a bar that feels greasy rather than gentle and that does not keep well.

The coconut exception

The one place people deliberately run high is a bar heavy in coconut oil. Coconut makes a hard bar with big lather and is noticeably stripping on skin, so recipes built around it often use 15 to 20% superfat to take the edge off.

That is a specific correction for a specific oil rather than a general technique. Applying a 20% superfat to an olive-heavy recipe produces a soft, slow-curing bar that takes months to firm up and never gets especially good.

Which lye, and why it matters

Sodium hydroxide makes bar soap. Potassium hydroxide makes liquid and soft soap, and the SAP values are entirely different, roughly 1.4 times higher for the same oil.

The two are not interchangeable and no amount of adjustment makes one behave as the other. Potassium hydroxide is also frequently sold at 90% purity rather than 100%, which has to be accounted for or the batch comes out lye-heavy. The soap lye calculator handles the oil-by-oil saponification and applies the superfat as a discount on the total lye, which is the arithmetic that otherwise has to be done per oil by hand.

Water is a different lever

Water content does not change the chemistry, only the pace. Less water means a faster trace, a harder bar sooner and a shorter cure. More water gives longer working time, which matters for swirls and layers.

It is worth separating the two in your head, because recipes often change both at once and then it is impossible to tell which one caused the result you got.

The number to be careful with

Superfat is forgiving upward and dangerous downward. A bar at 8% when you meant 5% is softer than planned. A bar at minus 3%, which is what a weighing error on a 0% recipe gives you, is caustic.

That asymmetry is the whole reason the default exists. Treat 5% as the number you deviate from deliberately, with a reason you could state out loud, rather than as a setting to tune for its own sake.