The refresh rate means the number of times an image is displayed in one second. It is expressed in Hertz (Hz), so a monitor with a refresh rate of 75 Hz "rewrites" the same image 75 times a second. Note that this rate of 75 Hz is not random, because it is the requisite minimum for a good image with no flickering. The refresh rate depends on the frequency of horizontal refreshing and the number of horizontal lines displayed (and therefore on the resolution used). The horizontal refresh rate is the number of times per second that the electron beam travels along a line and returns at the start of the next. It is expressed in kilohertz (kHz). A monitor with a refresh rate of 120 kHz travels 120,000 lines per second. The number of horizontal lines depends on the resolution used, so a resolution of 1600x1200 gives 1200 horizontal lines. To calculate the total time taken to travel the entire surface of the screen, you must include the time the beam takes to return from the bottom to the top of the screen. This means adding roughly 5% of the total time to travel the screen, so you have to multiply this total by 0.95 to calculate the maximum refresh rate for a given resolution.
The formula below is used:
Vf= horizontal frequency / number of horizontal lines x 0.95
where Vf represents vertical frequency, or the refresh rate.
For instance, a monitor with a horizontal refresh rate of 115 kHz used in 1024x768 will have a maximum refresh rate of 142 Hz (115,000/768 x 0.95).