This calculator is used with the kind permission of Robert Davis of the University of London. His web site is http://homepages.gold.ac.uk/rdavis. The calculator allows you to choose resistor and capacitor values to produce the desired frequency and duty cycle.
Note: It is not possible to have the off period longer than the on period with this basic circuit. In other words, the duty cycle will be 50% or longer.
Resistors: Select R for ohms or select k or M. As a rule of thumb, it is best to choose resistor values above 1k and below 1Meg. Some report that it is ideal to keep R1 at or above 5k and R2 at or above 3k for best stability. That is not possible at higher frequencies.
Capacitors: USA users select the uF or pF ranges. Here we seldom see capacitors in the nF range. (.001uF = 1nF = 1000pF.)