The amount of time that the signal is at 0v (Time Low), is calculated by the following equation:

The amount of time that the signal is approximately 5v (Time High), is calculated by the following equation:

The total period is calculated either by adding together the time high with the time low, or by using this condensed equation:

Finally, the frequency can be calculated using this formula:

More information about 555 timers can be found here: ‘http://www.ti.com/lit/ds/symlink/ne555.pdf‘.