555 Timer Calculator
Astable frequency & duty or monostable pulse width from your R and C values.
The 555 in two modes
Astable (free-running oscillator): the cap charges through R1+R2 and discharges through R2 between ⅓ and ⅔ of the supply, giving
so the duty cycle is — always above 50% unless you add a diode across R2. Monostable (one-shot): a trigger produces a single pulse of
Accuracy is dominated by the capacitor tolerance, so round R to the E-series and pick a stable cap (C0G/film) where timing matters.
Frequently asked questions
What is the 555 astable frequency formula?
f = 1.44 ÷ ((R1 + 2·R2) · C). The capacitor charges through R1+R2 and discharges through R2, so the high time is 0.693·(R1+R2)·C and the low time is 0.693·R2·C.
Why is the 555 astable duty cycle always above 50%?
In the standard configuration the capacitor charges through R1+R2 but discharges only through R2, so the high time is always longer than the low time. Duty = (R1+R2)/(R1+2·R2) > 50%. Adding a diode across R2 lets the charge path bypass R2 and allows duty below 50%.
How do I calculate a 555 monostable pulse width?
The one-shot pulse width is t = 1.1 · R · C, where R and C are the timing resistor and capacitor on the threshold pin. The output goes high for that time after a falling-edge trigger.
What values of R and C should I use?
Keep R between about 1 kΩ and 1 MΩ and avoid very small capacitors where leakage matters. Round to standard E-series values; the achievable timing accuracy is mostly set by the capacitor tolerance.