555 Timer Calculator
Calculate 555 astable frequency, period, and duty cycle, or monostable pulse width, from resistor and capacitor values.
Frequency–
Period–
Output high time–
Output low time–
Duty cycle–
555 timer formulas
Astable (oscillator)
f = 1.44 / ((R1 + 2·R2) · C)
t_high = 0.693 · (R1 + R2) · C · t_low = 0.693 · R2 · C
Duty = (R1 + R2) / (R1 + 2·R2)
In astable mode the 555 free-runs as a square-wave oscillator. Because the output charges through R1 + R2 but discharges through only R2, the standard circuit's duty cycle is always above 50%; add a diode across R2 for a 50% (or lower) duty cycle.
Monostable (one-shot)
t = 1.1 · R · C
In monostable mode a trigger produces a single output pulse of width t, then the output returns low until triggered again.