Why does temperature affect a hydrometer reading?

Liquid expands as it warms, lowering its density. A warm sample is less dense than the same liquid when cool, so the hydrometer floats lower and reads a lower gravity than the true value. The correction adds back the density lost to thermal expansion.

What is the calibration temperature of my hydrometer?

Most homebrew hydrometers are calibrated at 60°F (15.6°C); many laboratory and European models calibrate at 20°C. The value is usually printed on the paper scale inside the hydrometer or on its packaging. Using the wrong calibration temperature shifts every correction.

How accurate is this correction formula?

It uses a widely-used polynomial fit to the density of water versus temperature, accurate to within about 0.001 SG across the normal brewing range of roughly 0–40°C. For very hot samples above 40°C the error grows, so cool the sample toward calibration temperature when you can.

Should I correct both OG and FG?

Yes, whenever the sample is not at the hydrometer's calibration temperature. Errors in OG and FG both feed into your ABV calculation, and a 10°C offset can shift a reading by several gravity points, changing the estimated alcohol noticeably.

Is it better to just cool the sample instead?

Reading at the calibration temperature is always most accurate because no correction is needed. But waiting for hot wort to cool is slow; this calculator lets you read warm and correct mathematically, which is accurate enough for homebrewing within the supported range.

Hydrometer Temperature Correction

The Hydrometer Temperature Correction calculator converts a specific-gravity reading taken at any temperature into the true gravity it would show at your hydrometer’s calibration point. Warm wort reads low; this tool adds back the density lost to thermal expansion so your OG, FG, and ABV figures are accurate.

How it works

A hydrometer measures density relative to water, but water’s density changes with temperature. Hydrometers are therefore calibrated at one fixed temperature (commonly 60°F / 15.6°C, sometimes 20°C). If you read a sample at a different temperature, the value is off.

The calculator uses a standard polynomial that models the density of water as a function of temperature in Fahrenheit:

factor = 1.00130346
       − 1.34722124e−4 × T
       + 2.04052596e−6 × T²
       − 2.32820948e−9 × T³

It evaluates this at the sample temperature and at the calibration temperature, then applies:

corrected SG = reading × (factor at sample temp ÷ factor at calibration temp)

Because the sample factor is smaller when the liquid is warm, the ratio is greater than 1 for hot samples, nudging the reading upward to its true value.

Example

You pull a sample of cooling wort and read 1.048, but the wort is at 35°C (95°F) while your hydrometer is calibrated at 60°F. Evaluating the polynomial at 95°F and 60°F gives a ratio of about 1.0046, so the corrected gravity is roughly 1.0524 — over four gravity points higher than the raw reading. Ignoring the correction would understate your starting gravity and your final ABV.

Tips

Always check whether your hydrometer calibrates at 60°F or 20°C; the wrong setting silently biases every reading.
The formula is most accurate within about 0–40°C. For very hot samples, let the wort cool toward calibration temperature first.
Correct both your original and final gravity for the most accurate ABV.