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
Frequency
—
Period
—
Duty Cycle
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Output Band
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t_high (charge time)
—
t_low (discharge time)
—
Low (0 V)
—
High (V+)
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
t_high = 0.693 × (R1 + R2) × C
t_low = 0.693 × R2 × C
T = 0.693 × (R1 + 2×R2) × C
f = 1.44 / ((R1 + 2×R2) × C)
D = (R1 + R2) / (R1 + 2×R2) × 100%
Summary
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%.
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.
Frequently Asked Questions
Last updated: 2026-09-25 ·
Reviewed by Nham Vu