555 Monostable Timer Calculator
In monostable, or one-shot, mode the 555 timer outputs a single pulse of a fixed length each time it is triggered, then settles back to its resting state. The pulse width is set by one resistor and one capacitor through the standard NE555 relation t = 1.1 times R times C. This calculator returns the pulse width in seconds and milliseconds and the equivalent one-shot frequency if the timer were retriggered continuously. Enter R in ohms and C in farads (use the suffix-free value, for example 0.00001 for 10 microfarads).
555 monostable formula
Pulse width t = 1.1 * R * C (seconds)
1.1 = natural log of 3 (approx 1.0986)
R in ohms, C in farads
milliseconds = t * 1000
max retrigger rate = 1 / t (Hz)
The pulse ends when the timing capacitor reaches two-thirds of the supply voltage. Because the threshold and the supply both scale together, the pulse width is independent of supply voltage.
One-shot design context
- The output is one fixed-length pulse per trigger, then a return to rest.
- Pulse width depends only on R and C, not on the supply voltage.
- Keep R between roughly 1 kilohm and a few megohms per the 555 datasheet.
- For long pulses use a low-leakage capacitor to avoid timing drift.
- Common uses include debouncing, timed relays, and pulse stretching.
555 monostable: frequently asked questions
What is the 555 monostable pulse width formula?
For the standard NE555 in monostable (one-shot) mode the output pulse width is t = 1.1 times R times C, with R in ohms and C in farads. The constant 1.1 is the natural log of 3, because the timing capacitor charges from zero toward the supply and the output ends when it crosses two-thirds of the supply.
Where does the constant 1.1 come from?
The capacitor charges exponentially toward the supply voltage Vcc. The pulse ends when it reaches the two-thirds Vcc threshold, which happens after a time of R times C times the natural log of 3. The natural log of 3 is about 1.0986, rounded to 1.1 in datasheets.
How do I pick R and C for a target pulse?
Choose a convenient capacitor, then solve R = t divided by (1.1 times C). Keep R roughly between 1 kilohm and a few megohms and use a low-leakage capacitor for long pulses. The exact recommended range is in the specific 555 datasheet.
Does the supply voltage change the pulse width?
No. Because both the charging target and the two-thirds threshold scale with the supply voltage, they cancel out. The pulse width depends only on R and C, which is one reason the 555 monostable is so stable across supply voltages.
What is monostable mode used for?
A monostable produces one fixed-length output pulse each time it is triggered, then returns to rest. It is used for switch debouncing, timed relays, pulse stretching, missing-pulse detection, and turning a brief button press into a defined on-time.
Official sources
- Texas Instruments: NE555 / LM555 precision timer documentation.
- NIST: SI electrical units and constants.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.