Calculate 555 timer astable frequency and duty cycle, or monostable pulse width, from resistor and capacitor values.
Astable: frequency = 1.44 / ((R1 + 2R2) × C); duty cycle = (R1 + R2) / (R1 + 2R2). Monostable: pulse width = 1.1 × R × C. Results use ideal 555 behaviour; real chips vary with supply voltage and tolerances.
Frequency (Hz, astable)
How to use this tool
Astable for a repeating oscillator, monostable for a single timed pulse on trigger.
Use kilohms; in monostable mode R1 is the timing resistor and R2 is ignored.
Use microfarads; larger capacitors give lower frequencies or longer pulses.
Astable duty cycle cannot go below 50% in the standard circuit without an added diode.
Formula & worked example
Astable with R1 = 1 kΩ, R2 = 2 kΩ, C = 10 µF: frequency = 1.44 / (5,000 × 10⁻⁵) = 28.8 Hz, duty cycle 60%.
Limitations — read this before you rely on it
Results assume an ideal 555 and exact component values; real resistors and capacitors carry tolerances (often 5–20%), and frequency drifts with supply voltage and temperature. Always verify timing on the bench for critical applications.
Frequently asked questions
Not without modification — add a diode across R2 to shorten the charge path.
Roughly from well under 1 Hz to a few hundred kHz, depending on component values.
In theory the timing ratios cancel supply voltage, but real chips drift slightly with voltage and temperature.
Ceramic or film for small values; electrolytic for long timings, accepting wider tolerance.
Component tolerances dominate: measure actual resistor and capacitor values for precision work.