555 Timer Monostable Calculator
A 555 timer wired as a monostable produces one output pulse of fixed duration each time it is triggered, making it a simple one-shot timer. The pulse width is set entirely by one resistor and one capacitor. Enter the timing resistor and capacitor to find the output pulse width in seconds, and the equivalent in milliseconds. Because both the charge rate and the threshold scale with supply voltage, the pulse width is independent of the supply.
555 monostable formula
t = 1.1 * R * C
where R = timing resistance (ohms), C = timing capacitance (F)
The constant 1.1 = -ln(1/3), from charging to two thirds of supply.
The pulse begins on a trigger and ends when the capacitor reaches two thirds of the supply voltage. The width depends only on R and C.
Worked example
With R = 100,000 ohms and C = 0.00001 F (10 microfarads): t = 1.1 * 100,000 * 0.00001 = 1.10 s. That is 1,100.00 ms, a one-second pulse from a single trigger.
555 monostable: frequently asked questions
What is the 555 monostable formula?
The output pulse width is t = 1.1 R C. The 1.1 factor comes from -ln(1/3), the time for the timing capacitor to charge to two thirds of the supply voltage through R.
What does monostable mode do?
In monostable (one-shot) mode the 555 produces a single output pulse of fixed length each time it is triggered. The pulse width depends only on the timing resistor R and capacitor C, not on the supply voltage.
What units should I enter?
Resistance in ohms and capacitance in farads. For typical parts: 100 kilohms is 100,000 ohms and a 10 microfarad capacitor is 0.00001 F. The pulse width is returned in seconds.
Why is the pulse width independent of supply voltage?
Both the charging rate and the two-thirds threshold scale with the supply voltage, so the ratio cancels. This is why a 555 one-shot gives a consistent timing across a wide supply range.
Sources
- Texas Instruments: NE555 precision timer product page and datasheet.
- NIST: SI units (ohm, farad, second).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.