555 Timer Calculator
Calculate 555 timer astable frequency and duty cycle or monostable pulse width from R and C, with the classic 8-pin pinout and an output-waveform diagram.
Output (pin 3)
duty 52.4% · period 1.4553 ms
687.144 Hz
Frequency
1.4553 ms
Period
52.4%
Duty cycle
762.3 µs / 693 µs
High / Low
The two classic modes
The 555 charges and discharges a capacitor between ⅓ and ⅔ of the supply through external resistors. How you wire it decides the behaviour:
- Astable — free- running oscillator. It never stops, producing a square wave whose frequency and duty cycle are set by R1, R2 and C.
- Monostable — one-shot. A trigger produces a single output pulse of a fixed length set by R and C, then it waits for the next trigger.
The formulas
Astable: f = 1.44 / ((R1 + 2·R2)·C), t_high = 0.693·(R1+R2)·C, t_low = 0.693·R2·C.
Monostable: t = 1.1·R·C.
The 0.693 is ln(2) — the time for the RC node to swing one third of the supply between the comparator thresholds, which is also why the result is independent of supply voltage.
The duty-cycle quirk
In the standard astable circuit the capacitor charges through R1 + R2 but discharges through R2 only, so the high time is always longer than the low time — duty cycle is always above 50%. Getting close to 50% needs R1 ≪ R2, and a true or below-50% duty needs a steering diode across R2 or a different topology.
The 8 pins
- 1 GND, 8 VCC — power (typically 4.5–15 V for a bipolar 555).
- 2 TRIG, 6 THRES — the comparator inputs that watch the capacitor.
- 3 OUT — the output square wave / pulse.
- 4 RESET — active-low; tie to VCC if unused.
- 5 CTRL — control voltage; a 10 nF cap to ground if unused.
- 7 DISCH — discharge path for the timing capacitor.