555 Timer Calculator
Classic R1-R2-C astable circuit — compute frequency, period, and duty cycle.
What it does: Compute the frequency and duty cycle of the square wave from a 555 astable circuit.
When to use it: When building blinkers, buzzers, clock pulses, or starting out with PWM.
MEANS Outputs a square wave at about — with a duty cycle of — (the longer high time is inherent to the classic circuit).
Common cases pre-computed: browse 555 astable frequency presets →
No history yet. Each calculation is automatically saved to this device.
Read the guide: NE555 pinout, specs & typical circuits
How to use the 555 timer calculator
Enter R1, R2, C.
- 01
Enter R1 and R2
The two timing resistors in the classic astable circuit. Accepts
1k,10k. - 02
Enter timing capacitor C
The capacitor that sets the overall speed. Accepts
100n,1u,10uF. - 03
Read frequency and duty cycle
Get the oscillation frequency f, period T, high/low times, and duty cycle.
Astable formulas
Standard formulas for the classic R1-R2-C circuit (using ln2 ≈ 0.693).
| Quantity | Formula |
|---|---|
| High time t_H | 0.693 × (R1 + R2) × C |
| Low time t_L | 0.693 × R2 × C |
| Period T | 0.693 × (R1 + 2·R2) × C |
| Frequency f | 1 / T |
| Duty cycle D | (R1 + R2) / (R1 + 2·R2) |
Standard NE555 astable formulas (classic datasheet circuit).
Common questions, answered in 3 minutes
Why is the duty cycle always greater than 50%?
In the classic circuit the capacitor charges through R1+R2 but discharges only through R2, so the longer charge path makes the high time longer. To get ≤50% you need a diode to bypass R2, which is a different circuit.
How do I get a square wave close to 50%?
Make R1 much smaller than R2 (e.g. R1 in the 1Ω range, R2 large) so the duty cycle approaches 50%; or use the improved circuit with a diode.
The calculated frequency doesn't match my measurement?
Electrolytic capacitors have wide tolerance (±20% is common) and leakage also affects timing, so the actual frequency will deviate from the theoretical value. For accuracy, use a film capacitor and trim by measurement.
Is this astable or monostable?
This tool computes the astable mode (self-oscillating, continuously outputting a square wave). The monostable mode (one pulse per trigger) uses a different formula and may be added later.
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| Frequency | f = 1.44 / ((R1 + 2·R2)·C) | NE555 astable (datasheet) |
| Duty cycle | D = (R1 + R2)/(R1 + 2·R2) | NE555 astable (datasheet) |
Standard NE555 astable formulas, no external API. The actual frequency is affected by capacitor tolerance/leakage and needs trimming by measurement.