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555 Timer Monostable Calculator

Work out 555 timer monostable instantly with clear inputs, formula shown and shareable results.

About 555 Timer Monostable Calculator

One-shot pulse width is 1.1 RC, the time for the capacitor to charge from zero to two thirds of the supply — that is -ln(1/3) = 1.0986 time constants. Because the threshold is a fixed fraction of the supply, timing is independent of supply voltage.

Formula

555 monostable

t = 1.1 x R x C, from -ln(1 - 2/3) = 1.0986 time constants

Worked example

With the default values below, 555 Timer Monostable Calculator returns Output pulse width of 1.10. Change any input above and every figure here is recalculated in your browser.

Inputs used

Timing resistor
100,000 Ω
Timing capacitor
10 µF
Supply voltage
5 V

Results

Output pulse width
1.10 s
Pulse width
1,100.00 ms
Threshold voltage (2/3 Vcc)
3.33 V
Minimum retrigger interval
1,210.00 ms

Step by step

  1. 1Values used
  2. 2555 monostable
  3. 3Output pulse width= 1.10 s
  4. 4Pulse width= 1,100.00 ms
  5. 5Threshold voltage (2/3 Vcc)= 3.33 V
  6. 6Minimum retrigger interval= 1,210.00 ms

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why is the timing supply independent?

The threshold is derived from an internal divider, so it tracks the supply. The ratio the capacitor must reach stays at two thirds regardless of voltage.

What limits the maximum pulse length?

Capacitor leakage and the timer's own input current. Above roughly 100 seconds a CMOS version and a low-leakage capacitor are needed.

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