555 Timer Calculator
Enter your resistor and capacitor values to find the astable 555 timer's output frequency and duty cycle.
This is for the standard astable (free-running oscillator) configuration.
How 555 timer astable frequency is calculated
In the standard astable configuration, frequency is found from f = 1.44 / ((R1 + 2×R2) × C). The capacitor charges through both R1 and R2, then discharges through R2 alone, which is why R2 appears doubled in the charging term but not the overall formula structure.
Duty cycle (the percentage of each cycle spent with the output high) is (R1 + R2) / (R1 + 2×R2) × 100. Because of this circuit topology, the standard astable 555 configuration can never achieve exactly 50% duty cycle — it will always be somewhat above 50%, approaching it only as R1 becomes very small relative to R2.
Frequently asked questions
Why can't the basic astable 555 circuit produce a perfect 50% duty cycle?
Because the capacitor charges through R1+R2 but discharges only through R2, the high portion of the cycle is always somewhat longer than the low portion in the standard configuration. Getting closer to 50% requires making R1 very small relative to R2, or using a modified circuit with a diode across R2.
What happens if I use a very small R1?
A smaller R1 pushes duty cycle closer to 50%, since the formula (R1+R2)/(R1+2R2) approaches 50% as R1 approaches 0. However, R1 should not be set to zero or an extremely small value, since it affects discharge transistor current — check the datasheet's minimum recommended R1.
Can this formula be used for monostable mode too?
No — this calculator is specifically for the astable (continuously oscillating) configuration. Monostable mode (a single timed pulse) uses a different formula based on just one resistor and the capacitor.