555 timer calculator
Calculate the frequency, period, high and low time and duty cycle of the 555 timer in astable mode, the pulse width in monostable mode, and find R1 and R2 for the frequency you want.
555 timer, astable and monostable
Design for a frequency
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555 timer calculator: what it is for
Calculate the 555 timer circuit in its two most common modes. In astable mode, enter R1, R2 and C to see the frequency, f = 1.44 ÷ ((R1 + 2R2) × C), the period, the high and low times, the duty cycle and a drawing of the waveform. In monostable mode, see the pulse width, T = 1.1 × R × C. You can also start from the frequency and duty cycle you want to find R1 and R2, with the nearest standard values.
The calculations run in your browser, nothing is sent to a server. It accepts values with prefixes, like 4k7, 100n and 10u.
How to use
- Choose astable or monostable mode.
- Enter the resistors and the capacitor.
- Or type the frequency you want to calculate the resistors.
When to use it
- Build blinkers, clock generators and alarms.
- One-shot timers.
- Basic electronics classes and projects.
Frequently asked questions
What is the 555 astable formula?
In astable mode, the high time is 0.693 × (R1 + R2) × C, the low time is 0.693 × R2 × C and the frequency is f = 1.44 ÷ ((R1 + 2 × R2) × C). The duty cycle is (R1 + R2) ÷ (R1 + 2 × R2), always above 50% in the basic circuit.
And in monostable mode?
In monostable mode, the 555 makes a single pulse each time it is triggered, lasting T = 1.1 × R × C. With R = 100 kΩ and C = 10 µF, the pulse lasts about 1.1 seconds.
How do I get a duty cycle below 50%?
It is not possible in the basic astable circuit, because the capacitor charges through R1 + R2 and discharges only through R2. A common fix is a diode in parallel with R2, so charging goes through R1 only.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.