Alcohol by volume without an original gravity (combined method)

Every brewer forgets to measure the original gravity one day: too keen to start chilling, a broken trial jar, a reading written on a lost scrap of paper. Without it, you cannot calculate the alcohol content with the usual formula.

The hydrometer and the refractometer do not react to alcohol in the same way. The first reads the true gravity, the second a skewed value. The gap between the two depends on the amount of alcohol, and therefore on the starting gravity: it is this gap that lets you recover it.

The method also tells you the strength of a beer you know nothing about, for example a bottle from another brewer.

The tool looks for the original gravity that makes the two readings consistent. It starts from the final gravity and goes up in steps of 0.001:

for OG from FG to 1.200:
    Bi = OG converted to °Brix
    if corrected FG(Bi, Bf) ≤ hydrometer FG: OG found

The corrected FG is that of the refractometer calculation. The alcohol content is then calculated with the hydrometer formula.

• Accuracy is about ± 0.003 on the original gravity, or ± 0.4% alcohol.
• Both readings must be taken on the same beer, degassed and at 20 °C.
• No solution is sought beyond an original gravity of 1.200.