Hydrometer Temperature Correction Calculator

A hydrometer reads low when the sample is warmer than its calibration temperature, because warm liquid is less dense. The correction scales the reading by the change in the density of water between the two temperatures. A reading of 1.048 at 30 °C on a hydrometer calibrated at 20 °C is really about 1.051.

How the correction works

A hydrometer compares the density of the sample with the density of water at its calibration temperature. Wort and beer expand with temperature at nearly the same rate as water, so the calculator scales the reading by how much the density of water changes between the two temperatures:

SG_c = SG_r \times \frac{\rho(T_c)}{\rho(T_s)}

where $SG_r$ is the reading, $T_s$ the sample temperature, $T_c$ the calibration temperature and $\rho$ the density of water. The density of water comes from the formula recommended by Tanaka and colleagues for 0 to 40 °C:

\rho(t) = a_5 \left[1 - \frac{(t + a_1)^2\,(t + a_2)}{a_3\,(t + a_4)}\right]

with $t$ in °C, $a_1 = -3.983035$ , $a_2 = 301.797$ , $a_3 = 522528.9$ , $a_4 = 69.34881$ and $a_5 = 999.97495$ kg/m³.

For example, a reading of 1.048 at 30 °C (86 °F) with a hydrometer calibrated at 20 °C (68 °F) corrects to 1.051. With a 15.6 °C (60 °F) hydrometer, the same reading corrects to about 1.052.

The correction ignores the small expansion of the hydrometer’s own glass, and dissolved CO₂ or bubbles on the hydrometer make it read high. Degas beer samples and spin the hydrometer to dislodge bubbles before you read it.

Inside Foambyte, you record each batch’s gravity readings with their temperature and follow fermentation on the batch chart.

Reference table

Gravity points to add to a 1.050 reading by sample temperature (rows) and hydrometer calibration temperature (columns); negative values are subtracted
Sample temperature	60 °F (15.6 °C) hydrometer	68 °F (20 °C) hydrometer
50 °F (10.0 °C)	-0.7	-1.6
55 °F (12.8 °C)	-0.4	-1.3
60 °F (15.6 °C)	0.0	-0.9
65 °F (18.3 °C)	0.5	-0.3
68 °F (20.0 °C)	0.9	0.0
70 °F (21.1 °C)	1.1	0.2
75 °F (23.9 °C)	1.8	0.9
80 °F (26.7 °C)	2.5	1.7
85 °F (29.4 °C)	3.4	2.5
90 °F (32.2 °C)	4.3	3.4
95 °F (35.0 °C)	5.3	4.4
100 °F (37.8 °C)	6.3	5.5

Sources

Masami Tanaka, Georges Girard, Richard Davis, Arturo Peuto and Neville Bignell. “Recommended table for the density of water between 0 °C and 40 °C based on recent experimental reports”, Metrologia, 2001. Density of water as a function of temperature.

Frequently asked questions

Why does temperature change a hydrometer reading?

Liquids expand as they warm, so warm wort is less dense and the hydrometer sinks deeper and reads low. Cold wort is denser and reads high. The hydrometer is only exact at the temperature it was calibrated for.

What is my hydrometer's calibration temperature?

It is usually printed on the scale or the box. Common calibration temperatures are 20 °C (68 °F) and 15.6 °C (60 °F).

Can I measure hot wort?

This calculator accepts samples up to 40 °C (104 °F), the range of the water density formula it uses. Hotter samples also risk cracking the test jar or the hydrometer, and the correction becomes less accurate. Cool the sample first, for example in a covered jar in cold water.

How large is the correction?

Small near the calibration temperature and larger as the gap grows. With a 20 °C hydrometer, a 1.050 reading at 25 °C becomes about 1.051, and at 35 °C about 1.054.

Does this work for refractometers?

No. Most refractometers correct for temperature automatically (ATC). For refractometer readings during or after fermentation, use the refractometer calculator.