NE555 Monostable Calculator
From one resistor and one capacitor, get the exact length of the single output pulse a 555 timer fires after each trigger.
One trigger, one pulse
Enter the timing resistor and the timing capacitor (in microfarads) and the calculator returns the pulse width in both seconds and milliseconds.
The 1.1 is ln(3), not a fudge factor
The pulse lasts exactly as long as it takes the capacitor to charge to 2/3 of Vcc through R, which is 1.1 × R × C seconds — that 1.1 is the natural log of three.
What is an NE555 monostable calculator?
One resistor and a cap in, one timed pulse out
An NE555 monostable calculator turns two component values — a timing resistor R and a timing capacitor C — into the length of the one-shot pulse a 555 timer produces. In monostable mode the output rests low until a trigger arrives; then it snaps high for a fixed time and falls back, no matter how brief or long the trigger was. That makes the 555 a clean pulse stretcher and timed switch: press a button and a light stays on for exactly five seconds, or debounce a noisy contact into a single clean edge. Pick R and C and the timer holds its pulse for precisely 1.1 × R × C seconds.
Enter the timing resistor and the timing capacitor (in microfarads) to get the pulse width instantly, in seconds and milliseconds.
A single short formula, built from the timing resistor R, the capacitance C (converted from microfarads to farads), and the constant 1.1.
T = 1.1 × R × CWhen the 555 is triggered, the timing capacitor charges through R from zero toward Vcc. The output stays high until the capacitor reaches 2/3 of Vcc, and the time to get there is 1.1 × R × C, where 1.1 is ln(3). Because you enter the capacitance in microfarads, the calculator first multiplies by one-millionth to get farads, then multiplies by R and by 1.1. The result in milliseconds is just the seconds value times 1000.
Suppose you build a one-shot timer with R = 10 kΩ and C = 10 µF.
Convert the capacitance
10 µF × 0.000001 = 0.00001 F — the capacitance in farads.
Apply the formula
1.1 × 10000 × 0.00001 = 0.11 s — the pulse width in seconds.
Read it in milliseconds
0.11 s × 1000 = 110 ms — the same pulse, handy for short timings.
The pulse width is how long the output stays high after a single trigger — here 0.11 s, or 110 ms. That is the whole point of monostable mode: one input event produces one output pulse of a known, repeatable length, regardless of how long the trigger itself lasts. To make the pulse longer, increase R or C; to make it shorter, shrink them. Because the relationship is linear, doubling either value doubles the time, so a 10 µF cap with a 100 kΩ resistor gives 1.1 s, while the same cap with a 1 kΩ resistor gives just 11 ms. Choose the capacitor first from what you have on hand, then pick R to land on the time you need — or add a trimmer pot for R so you can dial the pulse in. Common uses are timed lights, button debouncers, missing-pulse detectors, and turning a momentary press into a fixed delay.
The formula is the standard datasheet approximation, but a couple of practical points are worth keeping in mind.
Real timers, tolerances, and re-triggering
This formula uses the idealised 1.1 (ln 3) timing constant and assumes an ideal 555 with no leakage. Real resistor and capacitor tolerances (often ±5 % to ±20 %, and electrolytics worse) shift the measured pulse from the computed value, so treat the result as a starting point and trim with a trimmer pot if precision matters. Very large resistors make the timing sensitive to leakage, while very small ones draw heavy current. Note too that the standard monostable circuit ignores triggers that arrive while the output is already high — it is not re-triggerable unless you add extra circuitry — and the trigger pulse must be shorter than the output pulse for the timing to hold.