NE555 Astable Calculator
Enter R1, R2, and C for an NE555 astable circuit to get frequency, period, duty cycle, and charge/discharge times, plus why swapping R1 and R2 cannot push duty below 50%.
Use the NE555 Astable Calculator
Component Values
Results
Need duty cycle below 50%?
Swapping R1 and R2 will not do it. The capacitor charges through R1 + R2 but discharges only through R2 no matter which resistor holds which label, so (R1 + R2) / (R1 + 2R2) can never drop under 50% on the standard astable circuit. To go below 50%, add a diode across R2 (anode toward the discharge pin, cathode toward the capacitor) so charging bypasses R2 — duty then approaches R1 / (R1 + R2).
Formulas Used
Summary
The NE555 is the most common part-number variant of the 555 timer family, and in astable mode it free-runs as a square-wave oscillator set by two resistors (R1, R2) and a timing capacitor (C). This calculator runs the standard datasheet equations on your component values to return frequency, period, duty cycle, and the individual high (charge) and low (discharge) times. It also flags a common design mistake: swapping R1 and R2 to try to force duty cycle under 50% has no effect on the NE555's standard astable topology.
How it works
- Enter R1 and R2 with their unit (Ω or kΩ).
- Enter the timing capacitor C with its unit (µF, nF, or pF).
- Click Calculate to get frequency, period, duty cycle, t_high, and t_low.
- Read the duty cycle floor note if you need a duty cycle under 50%.
- Adjust values and recalculate, or use Reset to start over.
Use cases
- Pick R1, R2, and C for a target oscillator frequency in an NE555 astable circuit.
- Check the duty cycle before building an LED flasher or tone generator.
- Confirm charge and discharge times for a PWM-style driver stage.
- Verify component values pulled from an NE555 datasheet reference design.
- Understand why a swapped R1/R2 pair did not lower duty cycle as expected.
- Teach astable 555 timing with live numeric feedback.