What superfat is, in one paragraph
Superfat (or lye discount — the same thing seen from the other side) is the percentage of the oils you deliberately leave unsaponified by using less lye than the oils could take. A 5% superfat uses 95% of the full-saponification lye; 5% of the oil stays oil in the finished bar. It is there for two reasons: a safety margin against scale and SAP-value error, so a small slip leaves free oil rather than free lye; and a conditioning cushion on the skin. It does not change how long the soap takes to cure.
Because lye at any superfat is just the 0% figure × (1 − superfat), you can move between superfats with two multiplications. The classic bar calls for 143.8 g of NaOH at 5%. Its 0% figure is 143.8 ÷ 0.95 = 151.4 g; at 8% that is 151.4 × 0.92 = 139.3 g — 4.5 g less. The oils, the fragrance and everything else stay the same; if you keep a 33% lye concentration, the water drops to 282.7 g to match.
How much lye for each superfat — the classic bar at 1,000 g
| Superfat | NaOH | Water at 33% | Oil left unsaponified | What it is for |
|---|---|---|---|---|
| 0% | 151.4 g | 307.3 g | 0 g | Liquid soap paste; never bar soap |
| 3% | 146.8 g | 298.1 g | 30 g | Shave soap, laundry soap, shampoo bars |
| 5% | 143.8 g | 292.0 g | 50 g | The default for bar soap |
| 8% | 139.3 g | 282.7 g | 80 g | A gentler everyday bar; high-olive recipes |
| 10% | 136.2 g | 276.6 g | 100 g | Drier skin; lard/tallow bars |
| 15% | 128.7 g | 261.2 g | 150 g | High-coconut recipes |
| 20% | 121.1 g | 245.9 g | 200 g | 100% coconut and salt bars |
Olive 35 / coconut 30 / palm 30 / castor 5; computed by this site's engine. Each 1% of superfat is 1% less lye — 1.51 g here.
What if the lye weighed a few grams off?
This is the question the "what I actually got" mode answers, and it is the reason superfat exists. On the classic bar, a gram of lye is about 0.66 percentage points of superfat. Two grams heavy on a 5% recipe still leaves you at 3.7% — a slightly more cleansing bar nobody would notice. Five grams heavy leaves 1.7%, which is still soap but getting thin on margin. The line is at 151.4 g: past that, there is more lye than the oils can take, the bar is lye-heavy, and it goes back in the pot (rebatch) or in the bin — never on skin.
| Scale reads | Error | Superfat you made |
|---|---|---|
| 138.8 g | -5 g | 8.3% |
| 141.8 g | -2 g | 6.3% |
| 142.8 g | -1 g | 5.7% |
| 144.8 g | +1 g | 4.3% |
| 145.8 g | +2 g | 3.7% |
| 148.8 g | +5 g | 1.7% |
Classic bar, 1,000 g oils, written at 5%. Smaller batches are less forgiving: the same 2 g on a 300 g batch is three times the swing.
Published SAP values are averages over crops and suppliers, and an oil can sit a few percent either side of the number in any calculator. A 5% superfat covers that; a 0% superfat does not, which is why bar soap is never made at 0% even when the scale is perfect. Liquid soap is the exception — KOH paste at 0–3% stays clear, and a lye-heavy paste is corrected by adding oil or citric acid before dilution.
What superfat should I use?
5% is the default because it is enough margin for a careful maker and not so much that the bar goes soft or the free oil goes rancid (dreaded orange spots feed on excess unsaturated oil). Go higher when the recipe is cleansing or the oils are stable: a 100% coconut bar at 5% would strip skin, so it is made at 15–20% — 146.6 g of NaOH per kilo instead of 174.1 g. Go lower when you want hardness and lather over gentleness (shave and laundry soaps at 2–3%) or when free oil is a problem (high-linoleic recipes prone to DOS). Above about 8–10% in an ordinary bar you are trading lather and shelf life for a conditioning you may not be able to feel.
Superfat vs lye discount
Same number, two words. A 5% lye discount means using 5% less lye than full saponification, which leaves 5% of the oils unreacted — a 5% superfat. Older books and some calculators say "lye discount"; most soapers today say "superfat". Enter either here.