Hydrometer Calculator

A hydrometer only tells the truth at one temperature. If your must is warmer or cooler than the temperature printed on the stem, the number you read is a little off. This calculator corrects a single specific gravity reading for the temperature of the sample, then shows the potential alcohol and the Brix equivalent of the corrected figure.

Enter the reading, the temperature of the sample in the jar and the calibration temperature of your hydrometer. If you want the finished alcohol of a wine you need two readings, so use the wine ABV calculator for that. New to the instrument? Read how to use a hydrometer first.

Why Temperature Changes the Reading

A hydrometer floats because of the density of the liquid. Liquids expand when they warm up, so the same must is less dense at 80°F than at 60°F, the hydrometer sinks a little deeper and the stem reads lower than the true gravity. A cold sample does the opposite and reads high. The correction below adds back what the heat took away.

What your hydrometer is calibrated to

According to Brewer’s Friend’s hydrometer temperature adjustment calculator, old hydrometers are calibrated to 59°F (15°C) and newer ones are typically calibrated to 68°F (20°C). Craft Beer & Brewing’s hydrometer temperature correction tool puts it as usually 60°F or 68°F. Look on the stem or in the instructions to see which one you have. The calculator starts at 60°F and also offers 68°F and 59°F.

The formula

The calculator multiplies your reading by a ratio of two water density polynomials, one for the sample temperature and one for the calibration temperature. With T in degrees Fahrenheit:

Corrected SG = measured SG × f(sample T) / f(calibration T)

f(T) = 1.00130346 − 0.000134722124 × T + 0.00000204052596 × T² − 0.00000000232820948 × T³

These are the coefficients published in the open-source hydrometer-calculations project and in the Vinolab hydrometer temperature correction calculator, which says the expression is based on degrees Fahrenheit. Celsius inputs are converted first. Vinolab gives its accuracy range as roughly 0 to 60°C for instruments calibrated at 20°C.

Temperature Correction Table

This table was generated with the same formula for a hydrometer calibrated to 60°F. Read across to find what a wine at your sample temperature should really read. For a 59°F or 68°F instrument, use the calculator.

How big is the error in practice?

At cellar temperature the correction is almost nothing. At 70°F on a 60°F instrument it is about one point on the third decimal. It grows quickly once the sample is warm: a 1.090 must read at 80°F is really about 1.093, which is three points, or roughly 0.3% potential alcohol. That is enough to matter if you are deciding whether to add sugar.

The simplest fix

Cool the test jar sample to the calibration temperature before you read it, for example by standing the jar in a bowl of cool water for a few minutes. The table and the calculator are for the times when that is not practical, such as taking a reading from a warm must in summer.

Potential Alcohol From One Reading

Before the yeast goes in, a single reading tells you the most alcohol the must can make if it ferments fully dry. The calculator uses the common brewing shortcut: potential alcohol = (SG − 1) × 131.25. Brewer’s Friend’s ABV calculator write-up presents it as the basic formula most brewing sites use and notes that some versions use 131, with a pretty minor difference. It assumes the wine finishes at 1.000, so it is an upper estimate rather than a promise.

The Brix column uses the same polynomial as our Brix to specific gravity converter. If you measure with a refractometer instead, the refractometer calculator goes the other way. To raise a low reading, use the sugar calculator.

What counts as a good reading?

It depends on the stage. Before fermentation, a good reading is the one your recipe or kit asks for, and the table shows what alcohol it implies. After fermentation, a dry wine reads close to or just below 1.000, because alcohol is lighter than water. A wine that sits well above that with no airlock activity has probably stalled, so see stuck fermentation.

Getting ABV: You Need Two Readings

One reading gives potential alcohol, not the alcohol in the finished wine. Real ABV comes from the drop between the original gravity (before the yeast) and the final gravity (after fermentation has stopped). Take both, correct each for temperature here if needed, then enter the corrected numbers in the wine ABV calculator.

Do not mix a corrected figure with an uncorrected one. If the first reading was taken warm and the last one cold, the difference between them includes an error that has nothing to do with the yeast.

Reading Errors That Temperature Will Not Fix

Read the meniscus correctly

The liquid climbs the stem and forms a curve. Put your eye level with the surface and read the main liquid level, not the top of the curve. The full routine is in how to use a hydrometer.

Carbon dioxide on the stem

A fermenting sample is full of dissolved gas. Bubbles cling to the glass and lift the hydrometer, so it reads high. Spin it gently as you lower it in and let the bubbles clear before you read. Degassing the sample first helps a lot.

A sample that is not representative

Sugar settles. Stir the must before you draw the sample, and never return the sample to the batch after you have touched it with a dirty jar or hydrometer.

Related Tools and Guides

• Wine ABV calculator for the alcohol of a finished wine from two readings.
• Refractometer calculator for Brix readings.
• Sugar calculator to raise the starting gravity.
• Stuck fermentation if the gravity will not drop.
• All MyHomeWine tools.

You must be of legal drinking age to make and drink wine. Enjoy it responsibly.