How to use the 555 timer calculator
- Choose astable for a circuit that repeats, or monostable for one pulse per trigger.
- Enter the resistor values and pick the unit for each: ohms, kilohms or megohms.
- Enter the capacitor value and pick picofarads, nanofarads or microfarads.
- Read the frequency, high and low times and duty cycle, or the pulse length.
Formula
astable high time = ln 2 x (R1 + R2) x C
astable low time = ln 2 x R2 x C
frequency = 1 / (high time + low time)
duty cycle = high time / (high time + low time)
monostable pulse = ln 3 x R x C
The well-known shortcuts 1.44 / ((R1 + 2 x R2) x C) and 1.1 x R x C are these formulas with ln 2 and ln 3 rounded. The page uses the exact values.
R1 = 1 kilohm, R2 = 10 kilohms, C = 1 microfarad
The high time is 0.693147 x 11,000 x 0.000001 = 7.6246 ms and the low time is 0.693147 x 10,000 x 0.000001 = 6.9315 ms. The period is 14.556 ms, so the frequency is 68.70 Hz, and the duty cycle is 52.38%. The 1.44 shortcut gives 68.57 Hz, within 0.2% of this.
With R1 = 4.7 kilohms, R2 = 47 kilohms and C = 10 nanofarads the frequency is 1.4617 kHz. A monostable circuit with R = 100 kilohms and C = 1 microfarad gives a 109.86 ms pulse, where the 1.1 shortcut says 110 ms.
Why the duty cycle is above 50 percent
In the standard astable circuit the capacitor charges through R1 and R2 but discharges through R2 only. Charging takes longer than discharging, so the output spends more time high than low. The duty cycle is always above 50 percent, and it approaches 50 percent when R2 is much larger than R1. Reaching a duty cycle under 50 percent needs extra parts, such as a diode, which this page does not model.
Real parts differ from ideal ones. Resistors and capacitors have tolerances, and capacitors change with temperature and age. Expect the real frequency to differ by a few percent or more, and use a trimmer if you need a precise value.
Related electronics tools
Read the markings on small parts with the Capacitor Code Calculator and the SMD Resistor Code Calculator. The Resistor Color Code Calculator decodes through-hole resistors, and the Ohm's Law Calculator helps with currents and voltages.
Assumptions and limits
- The formulas are for the standard 555 circuits with ideal parts.
- The supply voltage does not affect the timing in these formulas.
- Datasheet limits on resistor and capacitor values are not checked, except for a warning about very small resistors.
- Component tolerance and temperature are not modeled.
Frequently asked questions
How do I calculate the frequency of a 555 timer?
Add R1 and twice R2, multiply by the capacitor and by ln 2, and take the reciprocal. For 1 kilohm, 10 kilohms and 1 microfarad that is 68.70 Hz.
What is the duty cycle of a 555 astable circuit?
It is (R1 + R2) / (R1 + 2 x R2). With R1 = 1 kilohm and R2 = 10 kilohms the duty cycle is 52.38%.
How long is a 555 monostable pulse?
The pulse lasts ln 3 times R times C, which is about 1.0986 x R x C. A 100 kilohm resistor with a 1 microfarad capacitor gives 109.86 ms.
Why does my real circuit differ from the result?
Part tolerances, leakage, temperature and wiring all shift the timing. Treat the result as a starting point and tune with a trimmer or by changing parts.