🧼 SoapLog · soap & candle batch records
What is superfat in soap?
Superfat is the small amount of oil in your recipe that is deliberately left unsaponified — that is, not turned into soap by the lye. It's expressed as a percentage. A 5% superfat means roughly 5% of your oils stay as free, nourishing oil in the finished bar, while the rest reacts with the lye. The same idea is sometimes called a lye discount, because you can achieve it by using slightly less lye than would be needed to saponify 100% of the oils.
Open SoapLog — free →Why leave oil behind?
Two reasons. First, that leftover oil makes the bar feel milder and more conditioning on skin. Second, and just as important, it's an insurance buffer: oil weights, lye purity and measuring are never perfect, so a little superfat ensures there's no leftover free lye in the bar. Soap with zero superfat (or, worse, a lye excess) can be harsh or even lye-heavy and unsafe to use. A modest superfat protects against small errors.
Typical percentages
- Cold/hot process bar soap: ~5% is the common all-round choice
- Gentle / facial / "luxury" bars: 6–8% for extra conditioning
- High-cleansing or laundry-style bars: 1–3%, kept low so they're not greasy
- Liquid soap (KOH): often a low 0–3%, since excess oil can cloud the soap
Going much above ~8–10% can leave a bar soft, prone to a greasy feel, and faster to develop rancid spots (the "dreaded orange spots") because there's more free oil to oxidize. Most beginners are well served staying near 5%.
How to set it
You don't guess superfat — you set it in a lye calculator before you make the batch. You enter your oils and choose a superfat %, and the calculator returns the exact reduced lye amount. Then record the % you actually used, because superfat is one of the biggest levers on how a bar feels. If a batch came out too drying or too soft, your logged superfat is the first number to adjust next time.