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555 Timer Calculator

Find a 555 timer's frequency and duty cycle.

How it is calculated

For a 555 timer wired as an astable oscillator, the tool computes f = 1.44 / ((R1 + 2·R2) × C) and duty = (R1 + R2) / (R1 + 2·R2) × 100. R1 and R2 are the two timing resistors in ohms, C is the timing capacitor in farads and f is the output frequency in hertz. There is no unit conversion, so a 1 µF capacitor must be typed as 1e-6 (or 0.000001). The duty cycle is the share of each period the output is high, and because the capacitor charges through R1 + R2 but discharges through R2 alone, it is always above 50% and approaches 100% only when R1 is much larger than R2. All three values must be above zero. The constant 1.44 is an approximation of 1/ln 2, and real parts vary with tolerance.

Worked example

Use R1 = 1 kΩ (1000), R2 = 10 kΩ (10000) and C = 1 µF (1e-6 F). First, R1 + 2·R2 = 1000 + 20000 = 21000 Ω. Multiply by the capacitance: 21000 × 0.000001 = 0.021 s. Then f = 1.44 / 0.021 = 68.57 Hz, so about 68 flashes per second at the output, or a period near 14.6 ms. The duty cycle is (1000 + 10000) / 21000 = 0.5238, which is 52.38%: the output is high for just over half of every cycle.

Try this example →

Common mistakes

  • Entering the capacitor in microfarads. Typing 1 for a 1 µF capacitor means 1 farad, and the tool returns 0.00 Hz. Use 1e-6.
  • Entering resistances in kilohms. The fields are in ohms, so a 10 kΩ resistor is 10000, not 10; the frequency would be off by a factor of a thousand.
  • Expecting a 50% duty cycle. In this basic circuit the duty cycle is always above 50%, so a pure square wave needs a different arrangement, such as a diode across R2 or a divide-by-two stage.
  • Applying the result to monostable or bistable circuits. The formula is only for the astable configuration with two resistors and one capacitor.
  • Treating the frequency as exact. The 1.44 factor is rounded and real resistors and capacitors have tolerances, so the actual frequency can differ by several percent.

About this calculator

In astable mode, a 555 timer IC charges and discharges a timing capacitor C through two resistors, R1 and R2, producing a continuous square wave whose frequency follows f = 1.44 / ((R1 + 2·R2) × C), with the output high while the capacitor charges through R1+R2 and low while it discharges through R2 alone — which is also why the duty cycle in this basic configuration is always above 50%.

This exact circuit topology, introduced with the original 555 chip in 1971, is a staple of hobby and educational electronics: it drives LED flashers, tone generators, PWM motor controllers, and clock signals for simple digital logic, and its frequency can be tuned just by swapping resistor or capacitor values.

Hobbyists building their first oscillator circuit, electronics students learning RC timing, and makers prototyping blinking-LED or alarm circuits all use this formula to pick R1, R2 and C for a target frequency before soldering anything down.

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Comments (1)

  • Editorial team

    In this basic astable circuit the output is high longer than it is low, so the duty cycle cannot reach 50% or go below it. To get close to 50%, make R1 small compared with R2, but not tiny, since R1 also limits current through the discharge pin. Real capacitors can be off by 20% or more, so expect the measured frequency to differ. What frequency are you aiming for, and with which capacitor?

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