What the 555 Timer Calculator does
This calculator designs a 555 timer circuit three ways: the frequency, high time, low time and duty cycle of an astable from R1, R2 and C; the component values for a frequency and duty cycle you choose; and the pulse width of a monostable one-shot. The output waveform is drawn so the duty-cycle limit of the basic circuit is visible rather than merely stated.
The 555 has been in production since 1972 and is still the fastest way to get a pulse or a slow oscillator without a microcontroller. Its arithmetic rests on one fact: the capacitor charges and discharges between one third and two thirds of the supply, and the time to cross that span is ln(2) times the RC product - the 0.693 in every data sheet.
How to use it
- Choose the mode. Astable free-runs, monostable produces one pulse per trigger, and "astable for a target" works backwards from a frequency and duty cycle.
- For an astable, enter R1 (between the supply and the discharge pin), R2 (between discharge and threshold) and the timing capacitor.
- For a monostable, enter the timing resistor and capacitor; the pulse width is ln(3) x R x C, which data sheets round to 1.1 RC.
- Read the frequency, the high and low times and the duty cycle, and look at the drawn waveform.
- Check the findings. The control-pin capacitor and the minimum value of R1 are the two things most often left out of a working design.
Reading the results
The basic astable is always above 50% duty, because the capacitor charges through R1 + R2 and discharges through R2 alone. Getting to or below 50% needs a diode across R2 so that charging bypasses it, or a different circuit entirely.
Timing does not depend on the supply voltage. The thresholds are a fixed fraction of the supply, so the charging voltage and the threshold scale together and cancel - which is why a 555 timer built for 5 V works identically at 12 V.
The accuracy you get is the accuracy of your capacitor. A 20% electrolytic gives 20% timing error; the resistors are usually the smaller contributor.
Worked example: R1 = 1 kOhm, R2 = 10 kOhm, C = 100 nF
The output is high while C charges through R1 + R2: t_high = ln(2) x 11,000 x 100e-9 = 762.5 us. It is low while C discharges through R2 alone: t_low = ln(2) x 10,000 x 100e-9 = 693.1 us.
The period is 1.4556 ms, so the frequency is 687 Hz, and the duty cycle is 762.5 / 1455.6 = 52.4%. Making R2 much larger than R1 pushes the duty closer to 50% without ever reaching it; making R1 larger pushes it towards 100%.
Swapping to the monostable mode with 100 kOhm and 10 uF gives a pulse of ln(3) x 100,000 x 10e-6 = 1.0986 seconds - the familiar "1.1 RC" from the data sheet, which is just ln(3) rounded.
Formulas and scoring rules
- Astable high time
t_high = ln(2) x (R1 + R2) x Cln(2) = 0.6931; data sheets print 0.693.- Astable low time
t_low = ln(2) x R2 x C- Astable frequency
f = 1 / (ln(2) x (R1 + 2 R2) x C) = 1.4427 / ((R1 + 2 R2) C)Often quoted as 1.44 / ((R1 + 2R2) C).- Duty cycle
D = (R1 + R2) / (R1 + 2 R2)Always above 50% in the basic circuit.- Monostable pulse
t = ln(3) x R x C = 1.0986 R CRounded to 1.1 RC in every data sheet.- Solving for components
R2 = t_low / (ln(2) C), R1 = t_high / (ln(2) C) - R2
Getting a 50% or lower duty cycle
The standard trick is a diode across R2, cathode towards the discharge pin, so the capacitor charges through R1 only and discharges through R2 only. Then t_high = ln(2) R1 C and t_low = ln(2) R2 C, and any duty cycle is possible - at the cost of the diode's forward drop, which makes the high time slightly longer than the formula says.
The cleaner alternative is to run the 555 at twice the frequency and divide by two with a flip-flop, which gives an exact 50% duty cycle regardless of component tolerance. When the duty cycle has to be precise, that is the approach to take.
Choosing components that will actually work
Keep R1 at or above about 1 kOhm: the discharge transistor sinks the capacitor current through it, and below that the transistor's saturation voltage distorts the timing as well as running hot. Keep the total resistance below about 1 MOhm on a bipolar 555 and 10 MOhm on a CMOS one, beyond which the input bias currents and leakage compete with the timing current.
Choose the capacitor for stability, not just value. A Class 2 ceramic can lose a large fraction of its capacitance under DC bias, so a 555 built with one runs fast; film and C0G parts are stable. Electrolytics are fine for slow flashers where 20% error does not matter, and poor everywhere else. Finally, fit the 10 nF capacitor from pin 5 to ground - without it the internal thresholds follow supply noise and the output jitters.
Limitations: what the result does not prove
- This is the ideal timing model. Real devices add a propagation delay of a few hundred nanoseconds, which is negligible at 1 kHz and dominant above a few hundred kilohertz.
- The discharge transistor is treated as a perfect switch. Its saturation voltage adds to the low time, especially with small R1.
- The bipolar NE555 draws a supply current spike at every transition; without proper decoupling it can reset itself or upset the rest of the board. CMOS versions are far better behaved.
- The result is indicative and must be checked against the data sheet for the part you actually fit. Timing accuracy in a product should be verified by measurement, not by calculation.
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Standards and sources
- Texas Instruments NE555 precision timer datasheet - checked 19 Sep 2026
- Texas Instruments - LMC555 CMOS timer data sheet
- Texas Instruments - Designing with the 555 timer (application report)
Frequently asked questions
How do I calculate 555 astable frequency?
f = 1.4427 / ((R1 + 2 R2) x C), which comes from t_high = ln(2)(R1+R2)C and t_low = ln(2) R2 C. With 1 kOhm, 10 kOhm and 100 nF that is 687 Hz. Data sheets usually round the constant to 1.44.
Why can't a 555 astable reach a 50% duty cycle?
Because the capacitor charges through R1 + R2 and discharges through R2 alone, so the high time is always longer than the low time. The duty cycle approaches 50% as R2 grows relative to R1 but never reaches it. A diode across R2, or dividing a doubled frequency by two, gives 50% or less.
What is the formula for a 555 monostable?
t = 1.1 x R x C, which is really ln(3) x R x C = 1.0986 R C. With 100 kOhm and 10 uF the pulse is 1.1 seconds. The trigger pulse must be shorter than the output pulse, or the output stays high until the trigger is released.
Does the supply voltage change 555 timing?
No, and this is the chip's most useful property. The thresholds are set at one third and two thirds of the supply by an internal divider, so the charging voltage and the thresholds scale together and cancel out of the equations. The same circuit times identically at 5 V and 15 V.
What is pin 5 for and do I need a capacitor on it?
Pin 5 is the control voltage - direct access to the upper threshold. Driving it changes the timing, which is how a 555 is made into a simple voltage-controlled oscillator or a PWM modulator. If you are not using it, fit a 10 nF capacitor from it to ground: without one, supply noise reaches the thresholds and the timing jitters.
What is the highest frequency a 555 can produce?
A bipolar NE555 is reliable to a few hundred kilohertz; beyond that its propagation delay makes the output times deviate from the formula. CMOS versions such as the TLC555 and LMC555 reach into the megahertz and draw far less current. For anything faster, use a logic oscillator or a microcontroller timer.
Why is my 555 running at the wrong frequency?
Usually the capacitor. Class 2 ceramics lose capacitance under DC bias and electrolytics have 20% tolerance and drift with age and temperature, so the timing follows. Check the value with an LCR meter at the working voltage, and use a film or C0G part where accuracy matters.
What is the smallest resistor I should use with a 555?
About 1 kOhm for R1. The discharge transistor sinks the timing capacitor's current through R1, so smaller values raise its dissipation and let its saturation voltage distort the low time. At the other end, keep the total above the leakage currents - under about 1 MOhm for a bipolar part.
Last reviewed by the A2Z.Tools team against the sources listed above.
