Crystal accuracy: PPM parameter

Manufacturers usually provides the accuracy or stability of the crystals as a PPM value which basically means how many pulses per million the crystal might drift from the central frequency.

To convert this value into how much hertz this crystal can drift, we can use the following formula

                      F x ppm
Delta F=   ---------------------
                        1E6

i.e. if you have a 16 Mhz with a 10 ppm

                     16 Mhz x 10
Delta F=  ----------------------
                          1E6

Delta F= 160 Hz

You can use a high precision frequency counter to measure the frequency of the crystal.