555 Timer Calculator

Astable 555 frequency, duty cycle and period.

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555 Timer Calculator

How to use this tool

  1. Enter R1 and R2 in ohms and C in microfarads for an astable 555 circuit.
  2. Press Generate. The tool shows frequency, duty cycle and period.
  3. Use Load sample for R1 1,000 ohms, R2 10,000 ohms and C 10 microfarads, or Clear to start again.

Worked example

For R1 = 1,000 ohms, R2 = 10,000 ohms and C = 10 microfarads, the frequency is 1.44 / ((1,000 + 2 x 10,000) x 10 x 10^-6) = 6.86 Hz, the duty cycle is (1,000 + 10,000) / (1,000 + 2 x 10,000) = 52.38%, and the period is 1 / 6.86 = 145.83 ms.

What this tool does not do / Limits

This uses the ideal astable formulas and ignores real 555 tolerances, supply voltage effects and component tolerances, so a built circuit will differ by a few percent. It covers astable mode only, not monostable timing.

Frequently asked questions

Can the duty cycle go below 50%?

Not with the basic two-resistor astable circuit: the formula always gives at least 50%. Below 50% needs an extra diode or a different circuit arrangement.

Which units must I use?

Ohms for both resistors and microfarads for C, exactly as labelled. Mixing units (for example, kilohms with farads) gives a wrong frequency.

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