About the 555 Timer Calculator
The 555 timer is one of the most used chips in electronics, and its timing depends on two resistors and a capacitor. This calculator handles the three jobs you meet most often. In astable mode, enter R1, R2 and C to get the output frequency, period, high and low times and the duty cycle. In monostable mode, enter R and C to get the length of the output pulse. In design mode, give a target frequency, R1 and C to find the value of R2. Values are entered in kilohms and your choice of microfarads, nanofarads or picofarads. It runs in your browser.
Updated October 2026
How to use the 555 Timer Calculator
- 1Choose astable, monostable or design.
- 2Enter R1 in kilohms, and R2 for astable mode.
- 3Enter the capacitor and pick its unit.
- 4For design mode enter the frequency you want.
- 5Read the frequency, the times and the duty cycle.
How the circuit keeps time
Inside the chip, two comparators watch the voltage across the timing capacitor. In astable mode the capacitor charges from one third to two thirds of the supply through R1 and R2, then discharges back through R2. The output flips each time, and the time to charge and discharge depends only on the resistors and the capacitor, not on the supply voltage.
In monostable mode a trigger starts the capacitor charging through R. When it reaches two thirds of the supply the output drops again. That makes a clean pulse of fixed length, useful for debouncing a switch or delaying a signal.
Quick examples
- R1 1 kΩ, R2 10 kΩ, C 100 nF: about 687 Hz.
- R1 1 kΩ, R2 100 kΩ, C 10 µF: about 0.7 Hz, a slow blinking LED.
- R 100 kΩ, C 10 µF in monostable: about 1.1 seconds.
- For 1 kHz with R1 1 kΩ and C 100 nF, use design mode to find R2.
Related tools
Use the RC Time Constant Calculator to check the charge time of any resistor and capacitor, the Capacitor Calculator for capacitors in series and parallel and the Resistor Calculator for combined resistances. The Frequency Converter turns hertz into kilohertz or megahertz, and the LED Resistor Calculator sizes the resistor for the output LED. Test your circuit on a breadboard before building it.
Features
- Astable, monostable and design modes
- Frequency, period and duty cycle
- High and low times
- Capacitor in µF, nF or pF
- Sensible units on the output
- Error for impossible combinations
- Free and private
Frequently asked questions
What is the astable formula?
The frequency is about 1.44 divided by R1 plus 2 times R2, multiplied by C. The high time is 0.693 times R1 plus R2 times C, and the low time is 0.693 times R2 times C.
What is the monostable formula?
The output pulse lasts 1.1 times R times C seconds after the trigger.
Why is the duty cycle always above 50 percent?
In the basic astable circuit the capacitor charges through R1 and R2 but discharges through R2 only, so the output is high longer than low. A diode across R2 can bring it near 50 percent.
Which resistor values are sensible?
Keep R1 and R2 between about 1 kilohm and 1 megohm. Very small values draw too much current and very large values make the timing sensitive to leakage.
How accurate is the result?
The formulas are exact for an ideal chip, but resistors and capacitors have tolerances of 1 to 20 percent, so real circuits differ a little. Trim with a variable resistor if you need accuracy.
Does it apply to the CMOS 7555?
Yes, the same formulas apply, and the CMOS version works at higher frequencies and lower current.
How do I get a 50 percent duty cycle?
Use a very small R1 compared with R2, or add a diode across R2, or divide the output by two with a flip-flop.