How the hydrometer correction works
Hydrometers are calibrated to read accurately at one temperature, usually 60 or 68 degrees Fahrenheit. Liquid expands when warm and contracts when cool, which shifts the reading. This tool uses the standard correction formula to adjust your measured gravity to what it would read at the calibration temperature, so your original and final gravity numbers are accurate and comparable.
Why temperature matters
A warm sample is less dense, so the hydrometer floats lower and reads a gravity that is too low. A cold sample reads slightly high. The further your sample is from the calibration temperature, the bigger the error. For a few degrees the difference is tiny, but for a wort straight off the boil or a sample left in a cold garage it can shift the reading by several points.
Calibration temperature
Check the slip of paper inside your hydrometer or the maker documentation for the calibration temperature. Older and British hydrometers are often calibrated at 60 degrees Fahrenheit, while many newer ones use 68. Enter the right value here, since correcting to the wrong calibration point reintroduces the error you are trying to remove.
Reading a hydrometer accurately
Float the hydrometer in a sample tube, give it a gentle spin to shake off bubbles, and read at the bottom of the meniscus at eye level. Let a hot sample cool toward room temperature before reading for the most reliable result, then apply this correction for any remaining difference. Consistent technique matters as much as the math.
Frequently asked questions
Do I need to correct hydrometer readings? For a few degrees it barely matters; for larger gaps from the calibration temperature, yes.
What temperature are hydrometers calibrated to? Commonly 60 or 68 degrees Fahrenheit; check your specific hydrometer.
Why does a warm sample read low? Warm liquid is less dense, so the hydrometer sinks a little lower and shows a lower gravity than the true value.
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