555 Timer Frequency Calculator
Compute the output frequency, period and duty cycle of a 555 timer in astable mode from R1, R2 and the timing capacitor.
How it works
f = 1.44 ÷ ((R1 + 2·R2)·C) · Duty = (R1 + R2) ÷ (R1 + 2·R2) · t_high = 0.693·(R1+R2)·C · t_low = 0.693·R2·C
In astable mode the 555 charges the capacitor through R1 and R2 and discharges it through R2 alone, producing a continuous square wave. The frequency depends on the two resistors and the capacitor; because the charge path is longer than the discharge path, the duty cycle is always above 50% for a standard 555. Making R1 much smaller than R2 pushes the duty cycle close to (but never reaching) 50%.
Worked example
With R1 = 1 kΩ, R2 = 10 kΩ and C = 0.1 µF: f = 1.44 ÷ ((1000 + 20000) × 0.0000001) = 685.7 Hz. The duty cycle is (1 + 10) ÷ (1 + 20) = 52.4%.
Frequently asked questions
Why can’t a standard 555 reach a 50% duty cycle?
The capacitor charges through R1 + R2 but discharges through R2 only, so the high time is always longer than the low time. You can approach 50% by making R1 tiny, or add a diode across R2 to separate the charge and discharge paths.
What sets the frequency range?
The resistor–capacitor product does. Larger capacitors or resistors give lower frequencies; the practical range spans well under 1 Hz to a few hundred kHz. Keep R1 above about 1 kΩ so the discharge transistor is not overloaded.
Does the supply voltage change the frequency?
No — a classic 555’s timing depends only on R and C, not the supply, because the comparator thresholds scale with the supply voltage. That voltage independence is a big part of its popularity.
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This calculator is for educational purposes. Double-check important results before acting on them.