What is superfat in soap making?
The oil left deliberately unsaponified in a bar — what it is for, what each setting changes, and why the number matters more as a safety margin than as a skin treatment.
Superfat is the share of your oils left unsaponified, achieved by using less lye than the oils could consume. Most cold process soap runs at 5%, which on a one-kilogram batch leaves 50 g of free oil in the bar — enough to condition the skin and, more importantly, to absorb ordinary measuring error.
Soap is the product of a reaction between fat and lye. If you put in exactly enough lye to consume every gram of fat, the reaction finishes with nothing left of either — in theory. Superfatting deliberately puts in less lye than that, so a measured share of the oils survives the reaction and stays in the bar as oil.
That leftover oil is usually sold as the thing that makes a bar feel conditioning rather than stripping, and it does. But its more important job is quieter: it is a margin of error. Published saponification values sit in a small range rather than at a single point, scales drift, and sodium hydroxide draws moisture out of the air the moment the tub is opened. A superfat means none of those has to be perfect.
The table below shows what each common setting costs in lye and leaves in oil, worked on a one-kilogram batch of 40% olive oil, 30% coconut oil (76°), 25% palm oil, 5% castor oil. The superfat calculator does the same for your own blend, and will also read the superfat back out of a recipe that only states a lye weight.
What each setting actually changes
A one-kilogram beginner blend at every common superfat. The last column is the difference from the setting above — the weights are small, which is why a tenth-of-a-gram scale matters more than the setting you choose.
The numbers worth holding onto
Enough slack to absorb a mistyped gram or an oil whose real value runs high, without costing you noticeable lather.
Three percentage points is a difference most people cannot weigh reliably on a kitchen scale. It is a real change to the bar and a tiny change to the lye.
On a one-kilogram batch. If your scale reads in whole grams you cannot hit a superfat target to better than about one point.
What a superfat decides
The real reason the default is 5% rather than zero. Below 3% a batch assumes every value is exact and every weight is perfect, and neither is reliably true.
Free oil is what stops a bar feeling squeaky. The effect is real but modest — a well-chosen oil blend does far more for the feel of a bar than a couple of superfat points.
Oil suppresses bubbles. Every point of superfat costs you a little lather, which is why shampoo and shaving bars that need high superfat also need a bubbly oil like castor to compensate.
Saponified oil is stable; free oil is not. The superfat is the part that goes rancid first, so a high-superfat bar made with an unstable oil will show dreaded orange spots long before a lean one does.
In cold process the lye reacts with whatever it meets and the leftovers are whichever oils were slowest, not the expensive one you had in mind. Only hot process lets you choose.
Choosing a number
It is the default because it works for almost every bar. Change the oils before you change the superfat — the blend has far more influence on how a bar feels.
Seven per cent makes a noticeable difference on a facial bar. On a body bar the same change mostly costs you lather and shelf life for a benefit you will struggle to feel.
A superfat absorbs a gram of slop. It does not rescue a recipe whose lye weight was wrong to begin with, and raising it will not make an unverified recipe safe.
A recipe that states a lye weight but no superfat is ambiguous. Enter its oils and its lye weight into the calculator in reverse and find out before you make it.
Set it, or read it out of a recipe
See what each superfat costs your blend in lye — or enter a recipe’s stated lye weight and find out what it is really running at.
Open the superfat calculator →Print the safety checklist
Lye handling, kit and the order of operations, on one page you can pin up where you work. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.