555 Timer Monostable Pulse Calculator
In monostable mode the 555 timer is a one-shot: a single trigger produces one output pulse of a fixed width, then the circuit waits for the next trigger. The pulse width depends only on a single timing resistor and capacitor. This calculator returns the pulse width using the standard t = 1.1 * R * C relationship, along with the time constant and helpful unit conversions to milliseconds. Use it for debounce timers, delay stages, missing-pulse detectors, and any place you need a clean, repeatable timed pulse from a 555.
555 monostable formula
time constant tau = R * C
pulse width t = 1.1 * R * C
(the 1.1 factor equals the natural log of 3)
max retrigger rate = 1 / t
R is in ohms and C in farads, giving t in seconds. The output is high for the pulse width after each trigger, then returns low until the next negative-going trigger.
Design notes
- The pulse width is set by one resistor and one capacitor only.
- 1 megohm equals 1,000,000 ohms; 1 microfarad equals 0.000001 farad.
- Trigger with a brief negative-going pulse shorter than the desired output.
- The max retrigger rate is the inverse of the pulse width.
- Use low-leakage capacitors for long, accurate pulses.
555 monostable: frequently asked questions
What is the 555 monostable pulse width formula?
In monostable (one-shot) mode the output pulse width is t = 1.1 * R * C, with R in ohms and C in farads, giving t in seconds. The 1.1 factor is the natural log of 3, since the capacitor charges through R until it reaches two-thirds of the supply voltage.
Why is the constant 1.1 used?
The timing capacitor charges from zero toward the supply voltage and the comparator trips at two-thirds of the supply. Solving the RC charge equation for that level gives a multiplier of ln(3), which is approximately 1.0986, rounded to 1.1 in datasheet practice.
What triggers the 555 monostable?
A negative-going pulse on the trigger pin (pulling it below one-third of the supply) starts the timing cycle. The output goes high for the calculated pulse width, then returns low until the next trigger. Re-triggering during the pulse is ignored in the basic circuit.
What units should I enter?
Enter R in ohms and C in farads to get the pulse width in seconds. For common parts, convert first: 1 megohm is 1,000,000 ohms and 1 microfarad is 0.000001 farad. Large R and C give long pulses up to many seconds.
What limits the maximum pulse length?
Very large timing resistors and electrolytic capacitors introduce leakage that makes long pulses inaccurate. The practical maximum depends on the specific 555 datasheet and capacitor quality, but the t = 1.1 * R * C relationship holds across the rated timing range.
Official sources
- NIST: SI units (second, ohm, farad, hertz).
- NASA Glenn Research Center: RC charge timing fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.