Alcoholmeter temperature correction calculator

How to use it

1. Take the reading. Fill a trial jar, lower the alcoholmeter in gently and let it settle without touching the sides. Read it at eye level, where the flat surface of the spirit meets the stem - not the little curve that climbs it.
2. Measure the temperature. Of the spirit in the jar, straight after the reading. A difference of a few degrees matters.
3. Check the calibration. It is printed on the meter, usually on the paper scale: 20 °C for most alcoholmeters, 60 °F for US proof hydrometers.
4. Read the true strength. Use it for your records, or carry it straight into the proofing calculator with the button under the results.

How it works

An alcoholmeter is a float: it sinks until it displaces its own weight of liquid, and its scale is marked in the strength that corresponds to each density at its calibration temperature. Warm the spirit and it expands, its density falls, the meter sinks further - and reads too strong. Cool it and the opposite happens. The glass of the meter expands a little with heat too.

The calculator works out what density the reading stands for, allows for the glass, and then finds the mixture of alcohol and water that has exactly that density at the temperature you measured. It uses the formula behind the international (OIML) alcoholometric tables, valid from 0 to 40 °C, and reports the strength at 20 °C - or at 60 °F, the basis of US proof.

A common rule of thumb is about 0.3–0.4% ABV per degree Celsius, but the real figure changes with strength and temperature, which is why it is worth calculating.

Worked examples

An alcoholmeter calibrated at 20 °C reads 40% in spirit at 25 °C. The true strength is 38.0%. The same reading in a cold room at 15 °C means 42.0%.

Spirit fresh from the still is often warm: 70% read at 30 °C is really 66.7%.

A US proof hydrometer (60 °F) reading 80 proof at 75 °F means a true 73.3 proof.

Questions

Why does warm spirit read too strong?

Warm spirit is less dense, so the alcoholmeter floats lower and the scale shows a higher strength. Spirit that is cooler than the calibration temperature reads too weak.

Can I correct the reading of a sweetened or flavoured spirit?

No. Sugar, fruit and anything else dissolved in a spirit make it denser, so any alcoholmeter reads low - whatever the temperature. Measure the strength before sweetening or flavouring, or distil a sample first.

What is the difference between an alcoholmeter and a hydrometer?

They work the same way, but an alcoholmeter is scaled in % ABV (or proof) for spirits, while a hydrometer is scaled in gravity for washes and wines. For a wash, use the hydrometer calculator.

Can I read the alcoholmeter in the parrot?

Yes - that is what a parrot is for - but the temperature of the spirit running from the condenser changes, so treat the reading as a guide and correct a jar sample when it matters.

How accurate is an alcoholmeter?

A good one reads to within about 0.1–0.2% ABV when it is clean, the reading is taken at eye level and the temperature is corrected.