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 the oil it leaves in the finished bar, which most soapers find makes a milder bar. 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 |
|---|---|---|---|
| 0% | 151.4 g | 307.3 g | 0 g |
| 3% | 146.8 g | 298.1 g | 30 g |
| 5% | 143.8 g | 292.0 g | 50 g |
| 8% | 139.3 g | 282.7 g | 80 g |
| 10% | 136.2 g | 276.6 g | 100 g |
| 15% | 128.7 g | 261.2 g | 150 g |
| 20% | 121.1 g | 245.9 g | 200 g |
Olive 35 / coconut 30 / palm 30 / castor 5, lye from the oils table's SAP values at each superfat, water at a 33% lye concentration. 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 and too small a change to 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 skin bars aren't made at 0% even when the scale is perfect (laundry bars commonly are, at 0–1%). Liquid soap is the other exception: KOH paste at 0–3% stays clear. A lye-heavy paste is corrected either by cooking in a little more oil or by neutralizing the excess with a citric acid or borax solution; neutralizing leaves salts that can cloud the soap, so many makers prefer oil, and soapmakers disagree on which is better.
Choosing the number
This calculator moves a recipe from one superfat to another; it doesn't pick one for you. The superfat guide does: which percentage suits which soap, what a higher one costs in lather, hardness and shelf life, and why a 100% coconut bar is made at 15–20%. If an older recipe gives a "lye discount", that is the same number; enter it here as the superfat.