After a distillation, you often want to know how much alcohol was in the still. This calculator helps here.
The calculation of the %abv of the boiler content before distillation is very accurate when current 0%abv is given at the end of distillation. If distillation is stopped earlier, the calculation of the %abv of stillage assumes that the still does not rectify. However, if it does rectify, the %abv of stillage will be lower in reality and therefore (by an even smaller amount) the %abv in the boiler before distillation will also be lower than calculated. However, in practical calculations, i.e. when distilling most of the alcohol, this makes almost no difference.
The calculator does not provide a check if the inputs are plausible. So it shows a result even if the inputs are measured incorrectly. Namely, the result with which the distillate and stillage poured together again make the mash.