What is the formula for a 555 monostable pulse width?

T = 1.1 × R × C, where R is in ohms and C is in farads. The constant 1.1 comes from -ln(1/3) ≈ 1.0986, which is the time it takes the capacitor to charge from 0 V to 2/3 VCC through R.

Does the pulse width depend on the supply voltage (VCC)?

No. Because the 555 switches at exactly 2/3 VCC, the threshold scales with VCC and the timing is independent of supply voltage. Only R and C set the pulse duration.

What happens if the trigger pulse is longer than the output pulse?

If the trigger pin (Pin 2) remains low longer than T, the output remains forced high. Because the trigger comparator overrides the threshold reset, the output will not drop low until the trigger voltage rises back above 1/3 VCC. For reliable one-shot operation, keep trigger pulses shorter than T or AC-couple the trigger input.

What resistor and capacitor values are practical?

Resistors from about 1 kΩ to 10 MΩ and capacitors from a few pF up to 470 µF give pulse widths spanning nanoseconds to minutes. Keep R above ~1 kΩ to limit discharge current; above 10 MΩ leakage current can affect timing accuracy.

Can I use this calculator for CMOS 555 variants (TLC555, LMC555)?

Yes. The T = 1.1 × R × C formula applies equally to CMOS 555 variants. CMOS versions draw less trigger current and can operate at lower supply voltages, but the timing equation is identical.

555 Monostable Calculator

Enter R and C values to instantly compute the output pulse width for a 555 timer in monostable (one-shot) mode.

Component Values

Timing Resistor (R)

Timing Capacitor (C)

Monostable Formulas

T = 1.1 × R × C

T ≈ -ln(1/3) × R × C

R (given T, C) = T / (1.1 × C)

C (given T, R) = T / (1.1 × R)

T is the output pulse width in seconds, R in ohms, C in farads. The constant 1.1 comes from –ln(1 – 2/3) = ln(3) ≈ 1.0986.

Summary

A 555 timer in monostable mode fires a single output pulse of fixed duration each time the trigger pin is pulled low. The pulse width is set entirely by one resistor (R) and one capacitor (C) via T = 1.1 × R × C. This calculator converts your chosen R and C values into the output pulse width in the most readable unit (ns, µs, ms, or s), and shows the equivalent frequency if the pulse were repeated.

How it works

Enter the timing resistor (R) value and select the unit (Ω or kΩ).
Enter the timing capacitor (C) value and select the unit (µF, nF, or pF).
Click Calculate to see the output pulse width T = 1.1 × R × C.
The result is shown in the most readable time unit (ns, µs, ms, or s).
Use Reset to clear all fields and start a new design.

Use cases

Set a fixed debounce delay for a pushbutton input.
Generate a precise one-shot pulse to latch a downstream circuit.
Size R and C to produce a specific time delay from a trigger signal.
Verify timing component values read from a 555 monostable schematic.
Prototype a missing-pulse detector by comparing measured pulse width.
Teach or learn monostable 555 timer theory with live numeric feedback.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu