555 Timer Calculator

Work out the frequency and duty cycle of a 555 oscillator from its two resistors and capacitor, the pulse length of a one-shot, or the resistor needed for a frequency you want.

Frequency
Period
Output high time / low time
Duty cycle

How it works

In the astable circuit the capacitor charges through R1 + R2 and discharges through R2, between 1/3 and 2/3 of the supply. The output is high for tH = 0.693 × (R1 + R2) × C and low for tL = 0.693 × R2 × C, so f = 1.44 ÷ ((R1 + 2 R2) × C) and the duty cycle is (R1 + R2) ÷ (R1 + 2 R2), always above 50 percent. In the monostable circuit one trigger gives a pulse of t = 1.1 × R × C. The 555 is explained in Op-amps and Integrated Circuits.

Worked example

R1 = 1 kΩ, R2 = 10 kΩ and C = 100 nF give f = 1.44 ÷ (21 kΩ × 100 nF) = 685.7 Hz; the output is high for 0.762 ms and low for 0.693 ms, a duty cycle of 52.4 percent. A one-shot with R = 100 kΩ and C = 10 µF gives a pulse of 1.1 × 100 kΩ × 10 µF = 1.1 s. For exactly 1 kHz with R1 = 1 kΩ and C = 100 nF, R2 = (1.44 ÷ (1000 × 100 nF) − 1 kΩ) ÷ 2 = 6.7 kΩ.

Things to know

The formulas are for the bipolar NE555; CMOS versions (TLC555, LMC555) use almost the same constants. Component tolerance and the capacitor’s leakage change the result: use film or C0G capacitors for accuracy.

A 50 percent duty cycle needs a diode across R2, or a flip-flop to divide the output by two.

Keep R1 at 1 kΩ or more and the timing resistors below about 10 MΩ.

The reset, control and threshold pins have tolerances: decouple the supply with 100 nF and the control pin with 10 nF for a stable frequency.

Common questions

Why is the duty cycle never below 50 percent?

The capacitor charges through R1 + R2 but discharges only through R2, so the high time is always longer than the low time.

Can I drive a load directly?

The output sources or sinks up to about 200 mA (bipolar 555): enough for a small LED, buzzer or relay with a flyback diode; use a transistor for more.

How do I make a very slow timer?

Use a large R and a CMOS 555 (small bias current), or a counter chip after the oscillator: see Digital Logic.

Sources

  • Texas Instruments, NE555, SE555 precision timers and LMC555 CMOS timer datasheets.
  • Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press: the 555 timer.

Last reviewed: October 22, 2026