555 Timer Calculator
Calculate 555 timer IC output frequency, period and duty cycle for an astable (oscillator) circuit, or pulse width for a monostable (one-shot) circuit, from resistor and capacitor values.
Input
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
The 555 timer is one of the most widely used integrated circuits ever made — a simple, cheap chip that can generate a repeating square wave (astable) or a single timed pulse (monostable) using nothing but a couple of resistors and a capacitor. This calculator works out the frequency, period and duty cycle of an astable circuit, or the pulse width of a monostable circuit, straight from your resistor and capacitor values.
How to use it
- Choose a circuit mode:
- Astable — the classic free-running oscillator, wired with two resistors (R1, R2) and a capacitor.
- Monostable — a one-shot pulse generator, wired with one resistor and a capacitor.
- Enter your resistor value(s) and capacitor value, picking whichever units match your components (Ω/kΩ/MΩ, pF/nF/µF/mF/F).
- Read off the results — frequency, period, high/low time and duty cycle for astable; pulse width for monostable.
The formulas
Astable:
- Time High = 0.693 × (R1 + R2) × C
- Time Low = 0.693 × R2 × C
- Period = Time High + Time Low
- Frequency = 1 / Period
- Duty Cycle = Time High / Period
In the standard 3-pin wiring this calculator models, the duty cycle is always above 50% — R1's charge time always adds to the high phase. To get below 50%, real designs typically add a diode across R2, which this basic formula doesn't account for.
Monostable:
- Pulse Width = 1.1 × R × C
FAQ
Why is my duty cycle always over 50%?
That's inherent to the standard astable 555 topology — the capacitor charges through both R1 and R2 but only discharges through R2, so the high time is always at least as long as the low time. A near-50% (or below-50%) duty cycle needs a diode-modified circuit, which isn't modeled here.
What's the difference between astable and monostable?
Astable free-runs continuously, generating a steady square wave — useful for clocks, LED flashers and tone generators. Monostable outputs a single pulse of a fixed width each time it's triggered — useful for debouncing, timed delays and one-shot triggers.
Related tools
For the underlying RC charge/discharge behavior these formulas build on, see the RC Circuit Time Constant Calculator. For picking the resistor value for an LED, see the LED Resistor Value Calculator.
Privacy
All calculations run entirely in your browser — the values you enter are never sent to a server.