What does a 5% superfat setting mean?
It asks for 5% less lye than the oil table predicts for complete saponification. The result is a nominal lye discount, not a measurement of the free oil left in a finished bar. Actual SAP values, purity and weighing errors all affect that result. The superfat guide explains the range; this tool adjusts an existing recipe's lye.
Keep the oil blend, oil weight, lye type and purity unchanged when using the shortcut. If any of those changes, recalculate in the recipe builder instead. Enter a weighed error only when you know the original recipe and what went into the pot.
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 becomes 139.3 g. Keeping the lye concentration at 33% also reduces the water to 282.7 g. This adjusts the written recipe before mixing; it is not an instruction to add dry lye to finished soap.
How much lye for each superfat: the classic bar at 1,000 g
| Superfat | NaOH | Water at 33% | Nominal oil allowance |
|---|---|---|---|
| 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 calculated result rather than a guarantee about the finished bar. 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 smaller batch produces a larger swing.
A positive result depends on the recipe inputs being right. It cannot detect the wrong oil, inaccurate purity or unmixed lye pockets. If the amount weighed is known, the rebatch calculator can work out a correction. Do not guess at an acid dose to neutralize a suspect batch or use pH alone as proof it is safe.
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.