The 555 timer is also called as NE555, is a most popular integrated circuit from earlier days. This circuit is still used as timer circuit in various toys, machines etc because of its high reliability. 555 timer formula allows you to calculate the frequency, time high, time low and duty cycle based on resistor and capacitor values. As per 555 timer astable circuit formula, the duty cycles can be calculated in percentage using time high and time low values as inputs.