As a homebrewer, your hydrometer may be one of the tools of your trade. Like any instrument, however, it must be calibrated correctly in order to provide accurate readings.
Hydrometers function by submerging them in liquid that has different density than their calibration liquid. Most hydrometers will be calibrated at specific temperatures; check with your instruction manual for details. Creating a simple calibration solution requires just four ingredients:
Once your hydrometer has been calibrated and brought back up to temperature, you will need to test it against the liquid you wish to measure its specific gravity in. Make sure the jar you are measuring with can accommodate both your hydrometer and some extra room; cleanliness of the environment is also important; bubbles that cling onto its stem may alter buoyancy readings, leading to inaccurate readings.
Adding some of the liquid you are testing into your test jar is also recommended, to prevent it from overfilling and provide more accurate readings from the bottom of the meniscus.
When taking specific gravity readings, be sure to take them from the same spot on the meniscus each time. Remember that temperature may alter the specific gravity of your wort or beer and account for this in your calculations.