555 Timer Astable Frequency Calculator
The 555 timer in astable mode is a free-running oscillator: it produces a continuous square wave with no external trigger. The frequency and duty cycle are set by two resistors and one capacitor. This calculator applies the standard NE555 astable formulas to return the output frequency, period, duty cycle, and the separate high and low times from R1, R2, and C. It is the everyday tool for designing blinkers, tone generators, clock sources, and PWM bases around the classic 555 chip.
555 astable formula
high time t_high = 0.693 * (R1 + R2) * C
low time t_low = 0.693 * R2 * C
period T = t_high + t_low = 0.693 * (R1 + 2*R2) * C
frequency f = 1.44 / ((R1 + 2*R2) * C)
duty cycle = (R1 + R2) / (R1 + 2*R2)
The constants come from the natural log of 2 (about 0.693) and the 555 thresholds at one-third and two-thirds of the supply voltage. R is in ohms, C in farads.
Design notes
- The standard two-resistor astable always has a duty cycle above 50 percent.
- For a near-50 percent duty cycle, add a diode across R2 or use a different topology.
- Larger C lowers the frequency proportionally; larger resistors do the same.
- 1 microfarad equals 0.000001 farad; convert before entering.
- Exact timing limits are in the specific 555 datasheet for your part.
555 astable: frequently asked questions
What is the 555 astable frequency formula?
For the standard NE555 astable, the output frequency is f = 1.44 / ((R1 + 2*R2) * C), where R1 and R2 are in ohms and C is in farads. The constant 1.44 comes from 1 divided by ln(2) times the (R1 + 2R2)C charge and discharge timing of the threshold and trigger thresholds at one-third and two-thirds of the supply.
How is the duty cycle calculated?
The output is high while the capacitor charges through R1 and R2, and low while it discharges through R2 only. Duty cycle = (R1 + R2) / (R1 + 2*R2). Because the high time always includes R1, a standard 555 astable duty cycle is always above 50 percent.
What are the high and low times?
High time t_high = 0.693 * (R1 + R2) * C and low time t_low = 0.693 * R2 * C, where 0.693 is the natural log of 2. The period is their sum and the frequency is one divided by the period.
Can a 555 astable produce a 50 percent duty cycle?
Not with the basic two-resistor circuit, since the high time always exceeds the low time. A 50 percent duty cycle needs a diode across R2 (so charge and discharge use different paths) or a different topology. This calculator models the standard two-resistor circuit.
What capacitance and resistance ranges work well?
Keep R1 above about 1 kilohm and total timing resistance below a few megohms to limit leakage errors, and use stable capacitors. The exact limits come from the specific 555 datasheet; these formulas hold across the linear timing range it specifies.
Official sources
- NIST: SI units (hertz, ohm, farad, second).
- NASA Glenn Research Center: RC timing and oscillator fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.