Because of the delicate design of hygrometers, it is easy for them to become innacurate (e.g. with shipping or just over time). A simple method of calibrating your hygrometer will help ensure you are measuring the relative humidity as accurately as possible. Hygrometers should be calibrated yearly.
Hygrometers are available in both digital and dial models, and both should be checked for calibration. Some dial types will have a small screw or adjusment on the back so you can adjust them after calibration. If there is no way to adjust them, make a note of the calibration result, and then you can simply add or subtract from the actual reading to get a more accurate humidity level (this is discussed in greater detail after the calibration process is explained).
Tip: give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.
This series of diagrams will show you how to calibrate a hygrometer. The last page in the series provides a summary of the steps that you can print out for reference.