How do you correct a hydrometer reading for temperature?
A hydrometer measures density, and the density of anything liquid falls as it warms. The instrument is calibrated to be exact at one temperature — so a sample warmer than that reads low, a colder one reads high, and the correction is simply the ratio of water's density at the two temperatures. Read 1.048 at 78°F on a 60°F hydrometer and the wort is really 1.050 — +2.1 points, enough to turn a "missed my OG" into "hit it".
The formula every calculator uses (including this one) is a fit to water's density in °F. It's worth printing correctly, because at least one popular page prints it with the powers missing:
true = reading × ρ(Tsample) ÷ ρ(Tcalibration)
ρ(T) = 1.00130346 − 1.34722124×10⁻⁴·T + 2.04052596×10⁻⁶·T² − 2.32820948×10⁻⁹·T³ (T in °F)
Because the correction is a ratio, it barely depends on the reading itself: at 100°F it adds 5.8 points to a 1.000 reading, 6.1 to 1.050 and 6.4 to 1.100. That's why a single chart column works for the whole beer range.
Hydrometer temperature correction chart
Points to add to what the hydrometer shows (subtract when negative), for the two common calibrations. Read across from your sample temperature.
| Sample °F | Sample °C | 60°F / 15.6°C hydrometer | 68°F / 20°C hydrometer |
|---|---|---|---|
| 40°F | 4°C | −1.1 | −1.9 |
| 45°F | 7°C | −0.9 | −1.8 |
| 50°F | 10°C | −0.7 | −1.5 |
| 55°F | 13°C | −0.4 | −1.2 |
| 60°F | 16°C | 0 | −0.8 |
| 65°F | 18°C | +0.5 | −0.3 |
| 68°F | 20°C | +0.8 | 0 |
| 70°F | 21°C | +1.1 | +0.2 |
| 75°F | 24°C | +1.7 | +0.9 |
| 80°F | 27°C | +2.4 | +1.6 |
| 85°F | 29°C | +3.3 | +2.4 |
| 90°F | 32°C | +4.1 | +3.3 |
| 95°F | 35°C | +5.1 | +4.3 |
| 100°F | 38°C | +6.1 | +5.3 |
| 110°F | 43°C | +8.4 | +7.6 |
| 120°F | 49°C | +11.0 | +10.1 |
| 130°F | 54°C | +13.8 | +12.9 |
| 140°F | 60°C | +16.8 | +15.9 |
Gravity points (0.001 = 1 point) at a 1.050 reading; within ±0.3 pt for any reading from 1.000 to 1.100. Standard water-density correction. Above about 100°F / 38°C, cool the sample instead.
What temperature is my hydrometer calibrated to?
Look at the paper scale inside the tube — it says "60°F", "68°F/20°C" or similar near the top or bottom. Most triple-scale hydrometers sold in the US are 60°F (15.6°C); most sold in metric markets are 20°C (68°F); a few older or lab-grade ones are 15°C (59°F). The difference is not academic: the two common calibrations disagree by 0.8 points on the same wort, which is why the calculator asks.
Fill the trial jar with distilled water at the calibration temperature and float the hydrometer. It should read 1.000 exactly. Cheap hydrometers are often off by a point or two — the paper scale slipped in manufacture — and that offset applies to every reading forever. Write it on the tube. Then read at the bottom of the meniscus (the curved surface, where it meets the glass), spin the hydrometer to shed bubbles, and let foam and CO₂ settle before you trust the number.
How much does skipping the correction change my ABV?
More than most brewers expect, because the mistake is lopsided: OG is taken from wort still cooling, FG from beer at cellar temperature. Take a 1.050 wort read warm and uncorrected, with the FG read properly:
| OG sample at | Reading is low by | ABV understated by |
|---|---|---|
| 70°F / 21°C | 1.1 pts | 0.1% |
| 80°F / 27°C | 2.4 pts | 0.3% |
| 90°F / 32°C | 4.1 pts | 0.5% |
| 100°F / 38°C | 6.1 pts | 0.8% |
| 110°F / 43°C | 8.4 pts | 1.1% |
60°F calibration, FG 1.010 read correctly. Standard ABV formula, from this page's engine.
If you read OG and FG at the same warm temperature, both readings are low by nearly the same amount and the difference between them — which is all ABV cares about — barely moves. A pint at 100°F for the OG and a pint at 65°F for the FG hides 0.8% ABV; both at 80°F hides almost nothing. If you're going to be sloppy, be consistently sloppy.
Cool the sample, or correct the reading?
Both work; cooling is better. The correction models the density of water, and wort with 12% sugar in it doesn't expand at quite the same rate — close enough at 80°F, drifting by 100°F, and above that the glass and paper scale of the hydrometer are expanding too. Hot wort also foams, holds CO₂ after fermentation, and cracks plastic trial jars. Pull the sample into a jar, stand it in cold water for ten minutes, then read it near the calibration temperature and apply the small correction that's left. Use the big corrections in the chart for the reading you already took and can't retake, not as a routine.
Two things this correction does not cover. A refractometer has its own temperature compensation and, once alcohol is present, needs a completely different correction for the alcohol — don't put its readings through this calculator. And a floating digital hydrometer (Tilt, iSpindel and friends) reports temperature and applies a correction itself; the accuracy of that depends on how well you calibrated it in water, so do that check above for it too.