⏲️ 555 Timer Calculator
Enter your resistors and capacitor to get the 555's output frequency and duty cycle (astable) or pulse width (monostable).
Frequency
6.857 Hz
Duty cycle
52.4%
High / low time
76.23 ms / 69.3 ms
Astable: f = 1.44 ⁄ ((R1 + 2·R2)·C); duty = (R1 + R2) ⁄ (R1 + 2·R2). The standard 555 duty is always above 50% — add a diode across R2 for a symmetric square wave. 🔒 In your browser.
How the 555 timer calculator works
The 555 is the classic timer chip. In astable (oscillator) mode it outputs a square wave at f = 1.44 ÷ ((R1 + 2·R2)·C), with a duty cycle of (R1 + R2) ÷ (R1 + 2·R2). In monostable (one-shot) mode a trigger produces a single pulse of width t = 1.1 · R · C. The tool computes these and the high/low times, handling your R and C units automatically.
A standard 555 astable duty cycle is always above 50% because the capacitor charges through R1 + R2 but discharges through R2 alone. To get a symmetric (or under-50%) square wave, add a diode across R2 so charge and discharge use separate paths.
Frequently asked questions
How do I calculate 555 timer frequency?
For astable mode, f = 1.44 ÷ ((R1 + 2·R2)·C). With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF that's about 6.9 Hz. Larger resistors or capacitor give a lower frequency.
What is the difference between astable and monostable?
Astable runs continuously, producing an oscillating square wave (a clock, blinker or tone). Monostable is a one-shot: a trigger produces a single timed pulse of width 1.1·R·C, then it waits for the next trigger.
Why can't I get a 50% duty cycle from a 555?
Because the timing capacitor charges through R1 + R2 but discharges through only R2, the high time is always longer than the low time. Adding a diode across R2 lets it charge through R1 alone, allowing a 50% (or lower) duty cycle.
How do I calculate a 555 monostable pulse width?
t = 1.1 · R · C. For R = 100 kΩ and C = 10 µF that's about 1.1 seconds. Increase R or C for a longer one-shot pulse.
What do R1, R2 and C do in a 555 astable?
R1 and R2 set the charge/discharge paths and thus the timing and duty cycle; C is the timing capacitor. Bigger values slow the oscillation; the ratio of R1 to R2 sets how long the output stays high versus low.