Hydrometer Temp Correction
Enter your measured gravity, the sample's temperature, and your hydrometer's calibration temperature — get the corrected SG so your OG and FG are the numbers they actually are, not the numbers a hot sample is lying to you about. Runs the standard cubic f(T) = 1.00130346 − 1.34722124e-4·T + 2.04052596e-6·T² − 2.32820948e-9·T³ by a former New Belgium yeast specialist.
corrected = measured × f(sample°F) ÷ f(calibration°F). Uses the standard cubic polynomial for water's thermal expansion — the same correction that's printed as a small table on most hydrometer inserts, computed exactly instead of interpolated off a chart.
Read hot, correct cold — get the real gravity.
A hydrometer is calibrated at one specific temperature — usually 60 °F or 68 °F. Read it at any other temperature and the liquid's density (and therefore the reading) is off, because water and wort expand as they warm. This tool takes your measured gravity, the actual temperature of the sample, and your hydrometer's calibration temperature, and gives you the corrected SG — the number your hydrometer would have shown if you'd cooled the sample down to calibration temp first.
Everything runs client-side. Nothing is stored, nothing is sent — change any input and the result updates live.
The correction formula, explainedThe cubic behind the chart on your hydrometer's insert.
corrected = measured × f(sample temp) ÷ f(calibration temp)
Both the sample and the calibration temperature get run through the same correction function, f(T), with T in °F. The ratio of the two tells you how much to scale the measured reading. Read at exactly your calibration temperature and f(sample) = f(cal), the ratio is 1, and no correction is applied — which is exactly what should happen.
f(T) = 1.00130346
− 1.34722124e-4 × T
+ 2.04052596e-6 × T²
− 2.32820948e-9 × T³
# any °C input is converted first
T_F = T_C × 9/5 + 32
corrected = measured × f(sample_F) ÷ f(cal_F)
1.050 read at 100 °F, hydrometer calibrated at 60 °F.
f(100) ≈ 1.005908, f(60) ≈ 1.000063.
corrected = 1.050 × 1.005908 ÷ 1.000063 ≈ 1.0561 — a correction of +6 gravity points.
Why the correction is almost always positive above calibration temp
Warm liquid is less dense than cold liquid, so a hot sample sinks a hydrometer deeper than it should — the reading comes back lower than the true gravity. The correction adds those points back. Read colder than calibration temp and the effect flips: the hydrometer floats a little high, and the correction is negative. Either direction, the fix is the same ratio.
The part no calculator farm writesWhen the standard number is wrong.
Temperature correction is one of the most misapplied fixes in a homebrewer's toolkit. Here's where it goes sideways.
You corrected against the wrong calibration temperature
Calibration temperature is a property of the hydrometer, not a universal constant. Older US-market hydrometers are commonly calibrated at 60 °F (15.6 °C); most hydrometers sold today — including most European and modern imports — are calibrated at 68 °F (20 °C). Assume 60 when yours is actually 68 (or the reverse) and you bake a small, permanent, silently-wrong offset into every single reading you ever take with it, OG through FG. It shows up nowhere as an error — attenuation and ABV both look internally consistent, just consistently off. Check the paper insert that came with the hydrometer, or the maker's spec sheet, once — then you never have to guess again.
You corrected using the room, the wort, or the fermenter's setpoint
The formula wants the actual temperature of the liquid in the sample tube at the moment you read it — not the ambient room temperature, not the kettle's target, not the fermenter's controller setpoint. A sample pulled from a 68 °F-controlled fermenter and left on the counter for ten minutes is not at 68 °F anymore. If you care about the tenths of a gravity point, put a thermometer in the sample, not near it.
Big corrections mean you're reading too hot — cool the sample instead
Right off the kettle or straight out of a warm fermentation, the correction can run +0.006 to +0.010 or more, and the cubic is being stretched well past the gentle, well-behaved range it was fit for near calibration temp. A big correction isn't wrong, exactly — but it's a sign you're leaning on the math instead of on good practice. Cool the sample to within 10–20 °F of your hydrometer's calibration temperature before you read it, and the correction (and its uncertainty) shrinks to something you don't have to think about.
Temperature correction can't fix a bad read
This tool corrects for thermal expansion. It does nothing for a misread meniscus (read at eye level, at the bottom of the curve), CO₂ bubbles clinging to the stem and floating the hydrometer artificially high, or a dirty, trub-coated hydrometer that's no longer displacing the volume it was calibrated for. Those errors are routinely bigger than the temperature effect, and no correction formula touches them. Spin the hydrometer gently to shed bubbles, wipe it clean, and read at eye level — then correct for temperature on top of a reading that's actually trustworthy.
Notes from the labOn the cellar floor at New Belgium, nobody eyeballed a hot gravity reading and called it done — you cooled the sample, or you corrected it properly, because a tenth of a Plato compounds across a QC log fast enough to make an attenuation trend look wrong when the yeast was fine the whole time. The lesson that travels to any brewery, home or commercial: a temperature correction is only as good as the calibration temperature you assumed and the sample temperature you actually measured. Get either one wrong and the math is precise about being precisely incorrect. Know your hydrometer, measure your sample, then trust the number.
Stop re-deriving OG and FG by hand, every batch.
A corrected gravity reading is only useful if it makes it into the batch record without a transcription error. Log OG, FG, and temperature once and let a QC dashboard carry the correction, the attenuation, and the ABV forward automatically — for every batch, not just the ones you remember to double-check.