555 Timer Calculator (Monostable Mode)

One-shot pulse width using T = 1.1 × R × C. Solve for T, R, or C with unit conversions and Show Work.

How to Use

  1. Pick what you want to solve: Pulse Width (T), Resistor (R), or Capacitor (C).
  2. Enter the other two values and select units.
  3. Results update instantly in-browser (no uploads).
  4. Use Show Work for the formula and steps in base units.
Solve For
Typical: 1kΩ–1MΩ (very small R may overstress discharge current in some designs)
Typical: 1nF–1000µF (electrolytics have tolerance/leakage)
One-shot output high time for a standard 555 monostable
Does not change math; affects practical limits (supply/current/leakage). Used for guidance messaging only.
Pulse Width (T)
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Resistor (R)
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Capacitor (C)
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Equation
T = 1.1 × R × C
Enter values above; results update instantly. Use Share to generate a restore link.
Show Work (step-by-step)
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Work is shown in base units: Ω, F, seconds.

Reference

  • Monostable pulse width: T = 1.1 × R × C
  • Solve for R: R = T / (1.1 × C)
  • Solve for C: C = T / (1.1 × R)

Practical designs may need to account for capacitor tolerance, leakage, and minimum/maximum recommended resistor ranges for your specific 555 variant.

FAQ

Why 1.1?

In the classic 555 monostable, timing ends when the capacitor reaches about 2/3 of VCC. The internal threshold levels yield the common approximation T ≈ 1.1RC.

Does supply voltage change the time?

Ideally, the timing equation stays about the same because both thresholds scale with VCC. In real circuits, leakage and input currents can shift timing—especially at long durations.

Why is my real pulse shorter/longer?

Common causes: capacitor tolerance (±20% electrolytics), leakage, PCB contamination, very large R values, and loading the threshold/discharge nodes.

Tool Info

Last updated:

Updates may include unit expansions, presets, and edge-case handling.