555 Timer Astable Calculator
The 555 timer in astable mode produces a continuous square wave with no external trigger. Two resistors (R1 and R2) and one capacitor (C) set the timing as the capacitor charges through R1 and R2 and discharges through R2 alone. Enter the three component values to find the output frequency, period, high time, low time, and duty cycle. Because charging and discharging use different resistor paths, the standard circuit always produces a duty cycle above 50 percent.
555 astable formulas
High time tH = 0.693 * (R1 + R2) * C
Low time tL = 0.693 * R2 * C
Period T = tH + tL = 0.693 * (R1 + 2 R2) * C
Frequency f = 1.44 / ((R1 + 2 R2) * C)
Duty cycle = (R1 + R2) / (R1 + 2 R2)
The constant 0.693 is the natural log of 2, from the capacitor swinging between one third and two thirds of the supply voltage.
Worked example
With R1 = 1,000 ohms, R2 = 10,000 ohms, C = 0.00001 F (10 microfarads): tH = 0.693 * 11,000 * 0.00001 = 0.0762 s. tL = 0.693 * 10,000 * 0.00001 = 0.0693 s. Period = 0.1455 s, frequency = 1 / 0.1455 = 6.87 Hz. Duty cycle = 11,000 / 21,000 = 52.38 percent.
555 astable: frequently asked questions
What is the 555 astable frequency formula?
In standard astable mode the frequency is f = 1.44 / ((R1 + 2 R2) C). The 1.44 factor comes from 1 / (ln(2) times (R1 + 2 R2) C), since the capacitor charges and discharges between one third and two thirds of the supply voltage.
How is the duty cycle set?
Duty cycle = (R1 + R2) / (R1 + 2 R2). Because the capacitor charges through R1 and R2 but discharges only through R2, the standard configuration always gives a duty cycle above 50 percent. A diode across R2 is needed to reach 50 percent or below.
What units should I use?
Enter resistances in ohms and capacitance in farads. For common parts convert first: 10 kilohms is 10,000 ohms and a 10 microfarad capacitor is 0.00001 F.
What are the high and low times?
High time (output on) = 0.693 (R1 + R2) C and low time (output off) = 0.693 R2 C, where 0.693 is the natural log of 2. The period is their sum and the frequency is its reciprocal.
Sources
- Texas Instruments: NE555 precision timer product page and datasheet.
- NIST: SI units (ohm, farad, hertz).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.