Why a warm sample reads low
A hydrometer measures density by how deep it floats. Warm liquid is less dense than cold liquid, so the instrument sinks further into a warm sample and shows a lower number than the same liquid would show once cooled. Nothing has changed about the sugar content — only the temperature has.
Every hydrometer is therefore calibrated at one specific temperature, printed on the paper scale inside. Read at that temperature and the number is correct as it stands. Read at any other temperature and it needs adjusting. The two common calibrations are 60 °F (15.6 °C), which is usual on brewing hydrometers sold in the United States, and 20 °C (68 °F), which is usual elsewhere. A hydrometer calibrated at 20 °C read against a 60 °F assumption is already wrong by about a gravity point before temperature enters into it.
The correction matters most in the two places brewers are most impatient: a wort sample pulled before it has fully chilled, and a gravity check on a fermenter sitting warmer than the room. Both read low, both make a beer look weaker or less attenuated than it is.
The formula, and why it is a cubic rather than a rule of thumb
Water does not expand linearly with temperature. It is at its densest near 4 °C and expands increasingly quickly as it warms, so any correction that works across a useful range has to be a curve rather than a straight line. The standard brewing correction uses a cubic in degrees Fahrenheit:
P(T) = 1.00130346 − 1.34722124×10−4·T + 2.04052596×10−6·T² − 2.32820948×10−9·T³
The corrected gravity is the measured gravity multiplied by P(sample) / P(calibration). Because it is a ratio, the calibration temperature is not baked in — the same polynomial handles a 60 °F hydrometer and a 20 °C one, which is why this calculator asks which you have rather than assuming.
The linear rule of thumb — roughly one gravity point for every ten degrees Fahrenheit above calibration — is a fair approximation from about 60 °F to 80 °F and then drifts. At 100 °F the rule predicts four points and the polynomial gives six.
Worked example: 1.050 read at 80 °F on a 60 °F hydrometer
You pull a sample that has not fully chilled, and the hydrometer reads 1.050 at 80 °F. The scale says the instrument is calibrated at 60 °F.
- Evaluate the polynomial at the sample temperature. With T = 80: the linear term contributes −0.010778, the square term +0.013059, and the cube term −0.001192. Added to 1.00130346 that gives P(80) = 1.002393.
- Evaluate it at the calibration temperature. With T = 60 the same three terms give −0.008083, +0.007346 and −0.000503, so P(60) = 1.000063.
- Take the ratio. 1.002393 ÷ 1.000063 = 1.002330.
- Apply it. 1.050 × 1.002330 = 1.0524.
The true gravity is 1.0524, not 1.050 — a shift of about two and a half gravity points. On an original gravity that is roughly 0.3% ABV. Taken on a final gravity it is the difference between an apparent attenuation of 74% and one of 79%, which is the difference between deciding a beer has finished and deciding it has not.
When the correction is worth applying and when it is noise
Under about half a gravity point, ignore it. That is finer than most homebrew hydrometers can be read — the scale divisions are usually 0.001 and the meniscus costs you some of that. A sample within five degrees Fahrenheit of calibration falls into this category, and correcting it is false precision.
Between roughly one and three points, apply it if the number is going to be used for anything. That covers the common case of a sample in the seventies read on a 60 °F hydrometer. It shifts calculated ABV by a few tenths of a percent and attenuation by several points.
Above about 90 °F, correct it and then stop trusting it. The polynomial still returns a number, but at those temperatures the glass of the hydrometer has expanded measurably too, and a sample that hot is rarely thermally uniform. If the reading is going on a record, cool the sample instead of correcting it.
Correction for a 60 °F hydrometer
| Sample temperature | Add, gravity points | 1.050 becomes |
|---|---|---|
| 50 °F / 10 °C | −0.6 | 1.0494 |
| 60 °F / 16 °C | 0.0 | 1.0500 |
| 70 °F / 21 °C | +1.0 | 1.0510 |
| 77 °F / 25 °C | +1.9 | 1.0519 |
| 80 °F / 27 °C | +2.4 | 1.0524 |
| 90 °F / 32 °C | +4.1 | 1.0541 |
| 100 °F / 38 °C | +6.1 | 1.0561 |
Below the calibration temperature the correction goes the other way: a cold sample reads high and the adjustment is negative.
Check what your hydrometer is calibrated at before anything else
The calibration temperature is printed on the paper scale inside the stem, usually as Cal. 60°F or 20°C. Assuming the wrong one puts a fixed error into every reading you ever take with that instrument — about one gravity point between the two common values — and no amount of temperature correction will remove it, because the correction is measured from whichever baseline you tell it.
Reading a hydrometer accurately
- Let the sample settle. Rising CO2 bubbles cling to the stem and float it high. Give the hydrometer a spin in the cylinder to shed them and wait for it to stop moving.
- Read the bottom of the meniscus. Liquid climbs the glass; the true reading is the flat part of the surface, not the edge where it curves up.
- Get your eye level with the surface. Reading down at an angle is worth a point or two on its own, which is the same size as the correction you are here to apply.
- Measure the sample's temperature, not the fermenter's. A sample in a cylinder equilibrates towards room temperature within minutes, so the number on the fermenter's stick-on thermometer is not the number to use.
- Correct once, not twice. Some digital and electronic meters already apply temperature compensation internally. Applying this correction on top of that double-counts it.
Where this sits among the other gravity corrections
Temperature correction is one of three adjustments a gravity reading might need, and they are independent of one another.
This one applies to hydrometers and to any refractometer without automatic temperature compensation. The second is the wort correction factor, which applies only to refractometers and handles the difference between wort and the sucrose solution the instrument was calibrated in. The third is the alcohol correction, which applies only to refractometer readings taken after fermentation has begun — that one needs the refractometer final gravity calculator, because it cannot be expressed as a simple factor.
Once you have a gravity you trust, it feeds everything else: ABV and attenuation, brewhouse efficiency against your grain bill, and priming sugar at packaging. Correcting the reading first is what stops a two-point temperature error propagating into all three.
Terms used here
- Specific gravity
- The density of the sample divided by the density of water at a reference temperature. Water is 1.000; a wort at 1.050 is 5% denser.
- Gravity point
- One ten-thousandth of specific gravity — the last digit of 1.050. Brewers count in points because the differences that matter are that small.
- Calibration temperature
- The temperature at which a hydrometer's scale is true, printed on the paper insert. Commonly 60 °F or 20 °C.
- Degrees Plato
- Percentage of sugar by weight, used in commercial brewing. Roughly specific gravity points divided by four in the normal brewing range.