Why an alcoholmeter lies when the spirit is warm
Alcoholmeters are calibrated at 20 °C. At any other temperature the reading needs correcting.
An alcoholmeter does not measure alcohol. It measures density: a weighted glass float sits in the liquid, and how deep it sinks is read off as a strength. The scale is calibrated for one temperature only — 20 °C.
What happens at any other temperature
Liquids expand as they warm, so their density falls. A float in thinner liquid sinks deeper, and the instrument reads that as more alcohol.
- Warm spirit reads higher than it really is.
- Cold spirit reads lower.
So the correction always runs opposite to the deviation:
true strength = reading − 0.33 × (temperature − 20)
For example, 52 %ABV read at 25 °C is really 52 − 0.33 × 5 = 50.4 %ABV. Five degrees costs a percent and a half — easily enough to leave a bottle intended for 50% sitting at 48.
Where the approximation breaks down
That 0.33 coefficient is a simplification. The real relationship is not linear, and it depends on the strength as well as the temperature: a 40% spirit behaves differently from a 70% one.
Near room temperature and in the spirit range, the formula is good to a few tenths of a percent. The further from 20 °C you go, the worse it gets — which is why the calculator refuses temperatures below 0 °C or above 40 °C rather than extrapolating into nonsense.
For anything official or commercial, use the OIML alcoholometric tables, not this formula.
Measuring properly
- Pour the spirit into a tall, narrow test cylinder rather than reading it in the pot.
- Wait for the temperature to settle — after dilution that can take half an hour.
- Measure the temperature of the liquid in that same cylinder, not the air.
- Spin the float gently so it does not cling to the cylinder wall.
- Read the scale at the bottom of the meniscus, at eye level.
The most common mistake is not skipping the correction. It is impatience: measuring freshly diluted spirit while it is still warm.