555 Timer Calculator
Work out 555 frequency, duty cycle and high and low times, or a one-shot pulse width, then design RA, RB and C for a target frequency with standard values.
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About this toolWhat is the 555 Timer Calculator?
The 555 is the chip people reach for when they need a clock, a blinking light, a tone or a delay, and the first question is always the same: what frequency will these resistors and this capacitor give? This 555 timer calculator answers it for the astable (free-running oscillator) circuit, showing the frequency, the period, the duty cycle and the separate high and low times, and for the monostable (one-shot) circuit, showing the pulse width.
It also works backwards. Enter the frequency and duty cycle you need, pick a timing capacitor (or press Use it to accept a suggested one), and the calculator returns the exact RA and RB, the nearest E12, E24 or E96 resistors, and the frequency and duty those stock parts really produce. A diode option covers the common trick for getting a duty cycle of 50% or less, and a two-cycle sketch shows the capacitor ramping between one third and two thirds of the supply alongside the square-wave output.
- Astable and monostable modes, analyse or design
- Steering diode option for duty cycles of 50% or less
- Exact resistors plus nearest E12, E24 or E96 values and the resulting error
- Suggested timing capacitor that keeps resistors between 1 kΩ and 1 MΩ
- Waveform sketch of the capacitor voltage and the output
- Warnings for small RA, very large resistors, tiny capacitors and high frequency
How to use the 555 Timer Calculator
- Choose Astable for a continuous square wave or Monostable for a single pulse after a trigger.
- Pick From parts to analyse an existing circuit, or From target frequency (or width) to design one.
- Enter RA, RB and C with their units, or the target frequency and duty cycle. Tick the diode option for a duty cycle of 50% or less.
- In design mode choose the resistor series you stock, then read the exact and standard values and the frequency error the standard parts introduce.
- Check the waveform sketch and the warnings, then copy the results or share a link with your inputs.
Worked examples
Datasheet-style oscillator
Input
Astable, RA 1 kΩ, RB 10 kΩ, C 10 nF
Output
6.87 kHz, duty 52.38%, tH 76.25 µs, tL 69.31 µs
With RB ten times RA the duty cycle is close to 50% but can never reach it, because the capacitor still charges through the extra 1 kΩ.
One-hertz LED flasher
Input
Astable, RA 10 kΩ, RB 68 kΩ, C 10 µF
Output
0.9881 Hz, duty 53.42%, on for 0.541 s and off for 0.471 s
A common hobby value set. A ±20% electrolytic moves the rate by the same 20%, so trim RB with a potentiometer if the blink rate matters.
Designing a 1 kHz clock at 60% duty
Input
Target 1 kHz, 60%, C 10 nF, E24
Output
Exact RA 28.85 kΩ and RB 57.71 kΩ; 30 kΩ and 56 kΩ give 1.016 kHz at 60.6%
Rounding both resistors to E24 costs 1.6% in frequency, well inside the tolerance of a typical film capacitor.
25% duty with the steering diode
Input
Target 1 kHz, 25%, C 10 nF, diode on
Output
RA 36.07 kΩ, RB 108.2 kΩ; E24 36 kΩ and 110 kΩ give 988.1 Hz at 24.7%
Without the diode the calculator refuses this target and explains why, since the standard circuit cannot go below 50%.
Background and insight
Inside the 555 two comparators watch the timing capacitor. When its voltage climbs to 2/3 Vcc the output goes low and the discharge transistor on pin 7 starts emptying the capacitor; when it falls to 1/3 Vcc the output goes high again and charging resumes. Charging an RC circuit from 1/3 to 2/3 of the way to its final voltage takes ln 2 time constants, which is where the constant 0.693 comes from. In the standard astable circuit the capacitor charges through RA + RB and discharges through RB alone, so tH = 0.693 (RA + RB) C, tL = 0.693 RB C and f = 1.44 ÷ ((RA + 2RB) C). These are the expressions printed in the Texas Instruments NE555 datasheet, and they hold whatever the supply voltage because both thresholds scale with Vcc.
Because RA always adds to the charging path, the high time is always longer than the low time and the duty cycle stays above 50%. Putting a diode across RB, anode toward pin 7, lets the capacitor charge through RA only, so tH = 0.693 RA C and any duty cycle becomes possible; equal RA and RB then give a square wave. The diode's forward drop lengthens the high time a little at low supply voltages, which the ideal formula ignores. In the monostable circuit the capacitor starts empty and charges to 2/3 Vcc, which takes ln 3 time constants, giving the familiar t = 1.1 RC.
The classic bipolar NE555 runs from 4.5 to 16 V. Keep RA at 1 kΩ or more so the discharge transistor does not short the supply through a small resistor, keep the timing resistors below a few megohms so threshold current and capacitor leakage do not distort the timing, and use a CMOS version such as the TLC555 or LMC555 for low supply voltages, low current or frequencies above about 100 kHz.
Common use cases
Blinkers and indicators
Set a flash rate for an LED or relay from a few parts and see the on and off times separately.
Clock and tone generators
Design a square wave for logic, a buzzer or an audio test tone and pick resistors you already have.
Delays and debouncing
Size a monostable for a switch debounce, a timed relay or a pulse stretcher.
PWM and motor speed control
Use the diode circuit to vary duty cycle with a potentiometer split between RA and RB at nearly constant frequency.
Tips and limitations
- Timing accuracy is set by the capacitor. Film (polyester or polypropylene) and C0G ceramic parts hold their value; electrolytics can be 20% off and drift with age and temperature.
- Decouple the supply with 100 nF close to pins 8 and 1, and tie pin 5 (control voltage) to ground through 10 nF on bipolar 555s to keep the thresholds steady.
- For a 50% square wave without the diode trick, divide the output by two with a flip-flop, or use a CMOS 555 with the output driving the timing resistor.
- In a monostable keep the trigger pulse shorter than the output pulse; if pin 2 is still low when the timing ends, the output stays high.
- Above roughly 100 kHz the internal propagation delay of the bipolar NE555 becomes a noticeable fraction of the period and the real frequency comes out lower than calculated.
Frequently asked questions
What is the formula for 555 astable frequency?
f = 1.44 ÷ ((RA + 2RB) × C). The 1.44 is 1 ÷ ln 2 rounded, because the capacitor charges and discharges between 1/3 and 2/3 of the supply each half cycle. The high time is 0.693 (RA + RB) C and the low time is 0.693 RB C.
Why can a 555 astable not reach 50% duty cycle?
In the standard circuit the capacitor charges through RA and RB in series but discharges through RB only, so the high time is always longer than the low time. Adding a diode across RB makes charging go through RA alone, and then any duty cycle is possible.
How do I calculate the 555 monostable pulse width?
Pulse width t = 1.1 × R × C, more precisely ln 3 × RC = 1.0986 RC. 100 kΩ with 10 µF gives about 1.1 seconds.
Does the 555 frequency change with supply voltage?
In theory no, because both thresholds are fixed fractions of Vcc, so the timing cancels out. In practice it moves by a fraction of a percent per volt on bipolar parts; a stable supply and a 10 nF capacitor on pin 5 help.
What resistor values should I use with a 555?
Keep RA at 1 kΩ or more and both resistors below about 1 MΩ, then choose C to suit. The calculator suggests a capacitor that lands the resistors in that range and greys out combinations that fall outside it.
What is the maximum frequency of a 555 timer?
The bipolar NE555 is reliable up to around 100 kHz; above that its internal delays shorten the useful period and the formula overestimates the frequency. CMOS versions such as the TLC555 and LMC555 work into the megahertz range.
Can I use an electrolytic capacitor for 555 timing?
Yes for long delays and blink rates, where large values are needed, but expect ±20% tolerance and leakage that lengthens the timing. For accurate frequencies use film or C0G ceramic capacitors and trim with a potentiometer.
Is there an in-depth guide to the 555 Timer Calculator?
Yes. “555 Timer Frequency and Duty Cycle: Where 1.44/((RA+2RB)C) Comes From” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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