Superfat in plain terms
Superfat is the percentage of oils or butters in a soap recipe that are left unreacted after the lye has done its work. In other words, the recipe contains slightly more fat than the lye can convert into soap. A “5% superfat” means the lye is calculated to react with roughly 95% of the fats, leaving about 5% free.
It applies to cold process and hot process soap, where you make the soap yourself from oils and lye. It is not something you adjust in melt-and-pour, because that base has already been saponified.
Why soap makers superfat
- A safety buffer. Scales are never perfect and saponification values for oils are averages, not exact numbers. Leaving a little extra fat helps ensure no free lye remains in the finished bar. A lye-heavy bar can be harsh on skin.
- Skin feel. The leftover oils may make a bar feel gentler and less stripping.
- Taming cleansing oils. Very cleansing recipes, such as those high in coconut oil, are often given a higher superfat to offset a drying feel.
Lye discount vs. adding fat after the cook
There are two ways to reach a superfat.
- Lye discount. Reduce the lye in the recipe so it cannot convert all the oils. This is by far the most common approach and is what most lye calculators do when you enter a superfat percentage.
- True superfatting. In hot process, some makers hold back a specific oil or butter and stir it in after the cook, so that exact oil is the one that remains unsaponified.
The total amount of oil in your batch is the same either way. Superfatting does not save oil, it just changes how much lye is used.
Common superfat percentages
Figures vary by maker and recipe, so view these as typical ranges rather than rules.
| Type of bar | Commonly seen range |
|---|---|
| General body bar | Around 5% |
| Facial or sensitive-skin bar | Often higher, roughly 8-15% |
| High coconut oil bar | Often higher, sometimes up to around 20% |
| Laundry or household soap | Very low, around 1-2% or none |
A simple example
Suppose a recipe uses 500 g of oils and the lye needed to convert all of them is 70 g. At a 5% superfat, a calculator would reduce the lye to about 66.5 g. The unused portion of lye leaves a small share of oil unconverted. These numbers are illustrative only; always use a calculator with your exact oils and double-check your weights.
Too much superfat
Going higher is not always better. Possible downsides include:
- A softer bar that wears away faster.
- Less lather.
- A higher risk of rancidity, sometimes seen as dreaded orange spots. Soap makers disagree about how much superfat contributes compared with oil freshness and the oils chosen, so the safest approach is fresh oils, good storage and sensible percentages.
What happens with too much lye?
If a bar has too little superfat or a measuring error, it can be lye heavy. It may feel harsh, crumbly or irritating, and can be unsafe on skin. If you suspect a bar is lye heavy, do not use it on skin. Home testing methods are not fully reliable, so when in doubt, discard it safely or ask an experienced soap maker. Never taste-test soap.
Choosing oils and a number
The right percentage depends on the oils in your blend. A mild, olive-heavy recipe may be fine at a lower figure, while a coconut-heavy one often gets more. Our guide to oils used for soap making explains what different oils do, and jojoba oil in soap shows an oil many makers use specifically because it stays largely unreacted.
Frequently asked questions
Which hard oils are best for soapmaking? Common choices include coconut oil, palm oil, tallow, lard, and butters like shea and cocoa. Each has different effects on hardness and lather.
Do they still use animal fat in soap? Yes. Tallow and lard remain in use among some makers and manufacturers, though many bars are entirely plant-based.
Does tallow soap actually work? Tallow has a long history in soap and typically makes a hard, stable bar with a creamy lather. Whether it suits you is largely a matter of preference.