A device for measuring the density of a liquid. A hydrometer will float higher in a more dense liquid than in a less dense one. Brewing hydrometers are usually calibrated to pure water at 20°C or 60°F. There are several methods of reading a liquid's density, with the metric standard being Specific Gravity. Specific Gravity is a ratio of the liquid's density compared to the density of water, giving water a Specific Gravity of 1. Wort is a sugar solution and is more dense than water. Readings taken after fermentation will give a lower reading since sugars have been converted to alcohol. From these numbers, alcohol percentage and yeast attenuation can be calculated. Alcohol readings on alcometers are only reliable if there are no additional substances dissolved in the mixture (like sugar); and no air or CO2 bubles adhere to the meter during the measurement.
 Using a Hydrometer
Make sure the hydrometer and trial jar are clean. Place the sample in the jar. Lower the hydrometer into the jar. Spin to remove clinging bubbles. Make sure the hydrometer is floating straight and not touching the sides. When the hydrometer is steady, read the scale at the lowest level (this viewpoint is disputed) of the liquid. Take the temperature of the liquid and adjust for rated temperature, usually 60°F or 20°C.
Take a sample using a sterilised implement (Wine thief, Turkey Baster, Syphon) Do not return the test sample to the vessel, Sample it for taste. Do not bottle the beer unless it is at or close to the target final gravity. This can cause a bottle to explode or at least a gusher.
 Temperature correction
Always take the temperature of the liquid being tested.
The following corrections are for a Hydrometer rated at 20°C / 68°F
- 10°C (50°F) -0.002
- 15°C (59°F) -0.001
- 20°C (68°F) -None
- 24°C (75°F) +0.001
- 28°C (82°F) +0.002
- 32°C (90°F) +0.003
The following corrections are for a Hydrometer rated at 15°C / 60°F
- 10°C (50°F) -0.0005
- 15°C (60°F) -None
- 21°C (70°F) +0.001
- 25°C (77°F) +0.002
- 29°C (84°F +0.003
- 35°C (95°F +0.005
The following measurements are commonly used.