555 timer calculator
Calculate the frequency, duty cycle and on/off times of a 555 timer in astable (blinker) mode, or the pulse length in monostable (one-shot) mode. A live LED blinks at the real rate so you can see your timing.
Enter your values
The classic timer chip. Astable mode blinks forever; monostable mode gives one pulse when triggered.
Result
Astable: f = 1.44 ÷ ((R1 + 2·R2) × C)thigh = 0.693 (R1 + R2) C · tlow = 0.693 R2 CDuty = (R1 + R2) ÷ (R1 + 2·R2)Monostable: t = 1.1 × R × CCoro's tip: In astable mode the duty cycle is always above 50%. Want a near-50% square wave? Make R2 much bigger than R1. Real timings can be 10–20% off because capacitors are not exact.
555 timer calculator: common questions
What is the 555 astable frequency formula?
f = 1.44 ÷ ((R1 + 2 × R2) × C). The high time is 0.693 × (R1 + R2) × C and the low time is 0.693 × R2 × C.
How long is a 555 monostable pulse?
t = 1.1 × R × C. For example 100 kΩ and 10 µF give about 1.1 seconds.
Why is the 555 duty cycle always above 50%?
In the standard astable circuit the capacitor charges through R1 + R2 but discharges only through R2, so the high time is always longer. Make R2 much larger than R1 for close to 50%.
For learning and low-voltage hobby circuits (12 V and below). Always check the part's datasheet. Never use these tools to plan mains (230 V) wiring.