555 as a monostable multivibrator
To use the calculator you need to enter any two of three values and the calculator will compute the third one (if it is mathematically possible). For example: you can enter the value of resistance R and the impulse length T (don't forget to choose proper units), select the unit for the result and click the [Compute] button which is connected to an unfilled field. More details
here.
Also check out our
calculator for astable multivibrator circuit.