Hydrometer temperature correction calculator
Enter your hydrometer reading, the sample temperature and your hydrometer’s calibration temperature to get the corrected gravity. Works in SG and °Plato, °F and °C.
Reading
Printed on the scale. Usually 68 °F or 60 °F; some use 59 °F (15 °C).
Corrected gravity
1.051
- Measured
- 1.050
- Correction
- +1.2 pts
Runs in your browser. Nothing you enter is sent or stored.
Why temperature changes the reading
A hydrometer floats higher in denser liquid. Warm wort expands and gets less dense, so the hydrometer sinks deeper and reads low. Cold wort reads slightly high. The scale is only exact at the temperature it was calibrated for.
The calculator uses the polynomial most homebrew tools use: a fit of how the density of water changes with temperature. It compares the value at your sample temperature with the value at the calibration temperature and scales your reading by that ratio. Readings in °Plato are converted to SG, corrected and converted back.
CG = MG × f(T) / f(C)
f(t) = 1.00130346 − 0.000134722124·t + 0.00000204052596·t² − 0.00000000232820948·t³
- MG
- Measured gravity (SG) as read on the hydrometer
- CG
- Corrected gravity (SG)
- T
- Sample temperature (°F)
- C
- Calibration temperature (°F)
The coefficients are defined in °F. Temperatures in °C are converted first.
Find your hydrometer’s calibration temperature
Every hydrometer is calibrated at one temperature. Use that number in the calculator, not the room temperature or your fermentation temperature. If you pick the wrong one, the correction is off by up to a point or so.
- Look at the paper scale inside the stem or the box. It usually says something like “20 °C” or “60 °F”.
- Most current homebrew hydrometers are calibrated at 20 °C (68 °F).
- Older, lab and many US triple-scale hydrometers use 15 °C (59 °F) or 60 °F (15.6 °C).
- Can’t find it? Measure distilled water at a known temperature. It should read 1.000 at the calibration temperature.
How big the error gets for hot wort
At fermentation temperatures the correction is small: a 1.050 sample at 77 °F reads only about one point low on a 68 °F hydrometer. It grows quickly as the sample gets warmer. Pull a sample straight from the kettle at 140 °F and the hydrometer reads about 16 points low, so 1.050 is really about 1.066.
Because the correction scales with the reading, it is nearly the same in points for any normal wort gravity. The further you are from the calibration temperature, the more the result depends on the formula matching your wort, so for hot samples cooling is the better option.
| Sample temperature | Correction |
|---|---|
| 50 °F (10 °C) | −1.5 points |
| 59 °F (15 °C) | −0.9 points |
| 68 °F (20 °C) | 0 |
| 77 °F (25 °C) | +1.2 points |
| 86 °F (30 °C) | +2.6 points |
| 95 °F (35 °C) | +4.3 points |
| 104 °F (40 °C) | +6.2 points |
| 122 °F (50 °C) | +10.7 points |
| 140 °F (60 °C) | +15.9 points |
Reading the hydrometer, and when to cool the sample instead
A correct temperature correction does not help if the reading itself is off. A few habits make a bigger difference than any formula. When the sample is warmer than about 95 °F (35 °C), cool it first and correct only the last few degrees.
- Fill the test jar so the hydrometer floats freely and does not touch the wall or the bottom.
- Give the hydrometer a spin to knock off bubbles. CO₂ on the bulb lifts it and makes the reading too low.
- Read at eye level with the jar on a flat surface, not from above.
- Read where the flat liquid surface meets the scale, at the bottom of the meniscus, not the edge that creeps up the stem.
- To cool, set the jar in cold water or ice and cover it so nothing evaporates. Then measure and correct the remaining difference.
Built into every BrewForge recipe
This calculator is part of BrewForge’s calculator collection, next to your recipes. On brew day and during fermentation you log gravity and temperature readings on the brew, by hand or from a WiFi hydrometer, and BrewForge tracks the gravity curve, ABV and attenuation from them.
- Hydrometer correction in the built-in calculators
- Gravity and temperature readings logged on each brew
- Automatic readings from iSpindel, Tilt, RAPT Pill and GravityMon
Use this calculator via API
The same calculation is also a free JSON API: no account, no key, one GET request. Handy for scripts, spreadsheets and AI assistants that should use exact brewing math instead of estimating it.
Add units=imperial (or one unit parameter per value) for °F, gallons and pounds. Every answer lists the inputs, the units and the formula it used.
GET https://brewforge.sh/api/public/calc/hydrometer-correction?gravity=1.050&sampleTemp=25
FAQ
Questions & answers
How do I correct a hydrometer reading for temperature?
Note the reading and the sample temperature, then look up your hydrometer’s calibration temperature on its scale. Enter all three in the calculator. It scales the reading by how much denser water is at the calibration temperature than at the sample temperature and shows the corrected gravity.
Does a warm sample read high or low?
Low. Warm wort is less dense, so the hydrometer sinks deeper. You add the correction to the reading. A sample colder than the calibration temperature reads slightly high, and the correction is subtracted.
What is the calibration temperature of my hydrometer?
It is printed on the scale or the packaging. Most homebrew hydrometers today are calibrated at 20 °C (68 °F). Older and many US hydrometers use 60 °F (15.6 °C) or 15 °C (59 °F).
Can I measure hot wort with a hydrometer?
You can, but the reading will be far too low and the correction gets large: about 16 points at 140 °F (60 °C). For the most reliable number, cool the sample to near the calibration temperature first and correct only the small remaining difference.
How accurate is the temperature correction?
Within a few degrees of the calibration temperature the correction itself is under a point, so small inaccuracies hardly matter. The formula is based on the density of water, so for hot samples far from the calibration temperature the result is an estimate. That is why cooling the sample is better than correcting a big difference.
Do refractometer readings need temperature correction too?
Most refractometers have automatic temperature compensation (ATC), and a few drops reach room temperature in seconds, so you normally don’t need this calculator. Use the refractometer calculator instead to convert Brix and correct readings taken after fermentation has started.
Do the math once. BrewForge does it every brew.
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