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555 Timer Astable Calculator

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

About 555 Timer Astable Calculator

In astable mode the capacitor charges through R1 + R2 and discharges through R2 alone, so high time is 0.693(R1+R2)C and low time is 0.693 R2 C. Frequency is 1.44/((R1+2R2)C), and the duty cycle always exceeds 50% because charging goes through the extra resistor.

Formula

555 astable

f = 1.44 / ((R1 + 2 R2) x C); duty = (R1 + R2) / (R1 + 2 R2)

Worked example

With the default values below, 555 Timer Astable Calculator returns Output frequency of 138.72. Change any input above and every figure here is recalculated in your browser.

Inputs used

R1 (to supply)
10,000 Ω
R2 (to discharge pin)
47,000 Ω
Timing capacitor
100 nF

Results

Output frequency
138.72 Hz
Duty cycle
54.81% %
Output high time
3.95 ms
Output low time
3.26 ms

Step by step

  1. 1Values used
  2. 2555 astable
  3. 3Output frequency= 138.72 Hz
  4. 4Duty cycle= 54.81 %
  5. 5Output high time= 3.95 ms
  6. 6Output low time= 3.26 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 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 can duty cycle not go below 50%?

Because the charge path always includes R1 in addition to R2, so high time always exceeds low time. A diode across R2 is the usual workaround.

Where does 0.693 come from?

It is ln(2), because the capacitor swings between one third and two thirds of the supply — an exponential interval of exactly ln 2 time constants.

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