555 Timer Calculator


Fill in one of the values below and press ‘go’ and the calculator will give you a range of options for R12 & C1.

How often do you want the timer to fire? seconds
What frequency do you want the timer to fire at? Hertz

About the calculator

There are other calculators on the web, but they tend to approach it from the opposite direction — given R1, R2, and C1, what timing will result? But of course that’s not what you are usually looking for. You have a timing in mind, and you want to know what parts to use to achieve that. Your problems are solved! Just plug the timing you are looking to get into the form below, hit the appropriate ‘Go’ button, and you’ll get a list of possible components options.

For the components, I chose those that come with the Radio Shack ‘grab bags’ of resistors (part #271-003) and capacitors (part #272-801 & 272-802). There are, of course, a lot more possibilities than these, but for most hobbyists there’s a decent chance these will be kicking around.

Update 1 – Mar 13, 2013 – I have added the ability to use SI units, so you can specify “10ms” or “4.5KHz” in the text fields.

56 thoughts on “555 Timer Calculator

      • It would be great to be able to use/select SI prefixes (k, M for Hz, and m, u, n for s) and also be able to select a duty cycle beforehand.

        • That’s great input. I have made the change so it supports SI prefixes (5uS, 1.4kHz) if you want to use them. Supporting the selection of duty cycle would be a bigger change so I’ll look at that later.


  1. This is great, I used a CMOS TLC 555 and w/ a 100K POT to fine tune the frequency and it works great. Can you post the calculations used for this circuit?

    • It’s just the standard calculations for an oscillator circuit, where the frequency of oscillation depends on 2 resistors, R1, R2 and one capacitor, C1.

      F = 1/(0.693 × (R1+2R2) × C1)

      • Perfect thanks! Anyway I could get the code you used to for the calculations? I’m trying to put together an easier way to match things up.

  2. Nice work! I’ve written several 555 calculators too using either Visual Basic or Excel but this one is by far the best I’ve seen, especially in the standard value component conversion. I really don’t see anything more you could do to improve it. Well maybe, adding an additional table showing the most extreme variations that would occur with 5% resistors and 10% or 20% capacitors… but that would be rather “nit – picky”. I think you did a dynamite job just as is! Thanks, this is really useful and I’ve been designing (hardware and software) for 25+ years!

  3. Thanks for this…

    The only thing I can think of which might help is a “one shot” calculator as well

    Thanks again

  4. hey I found it a bit confusing to understand what period means and duty cycle etc, because I’m used to the typical criteria of amount of time on, and amount of time off. I know I could search these terms on other sites, but I think it would be nice if you had a little section that explained terms and made it that much easier for noobs like me.

  5. please, friends, I want a 2.4MHz freq. ,is that possible with 555 timer? because some websites mention that the minimum value of capacitor is 0.005uF, and R1 is 1K. so is that correct? thanks for calculator.

    • I restrict the calculations to parts that are easy to find, there might be others that will help you hit that bar. But really, at a frequency like that, it seems like you’d be better off going with a crystal?

  6. Best 555 calculator out there… Many thanks for making this handy script!

    If you got some spare time left, would it be possible to make an input for the duty cycle? That would make this calculator even better. What may be a little bit easier tweak, is writing an resistor input, when using a pot for example.


  7. What a sweet surprise to find this calculator of your’s! I have been searching the web for a couple of days and decided to finely write a program for the job. But then on second thoughts I found the CMOS 555 would give me up to 3 MHz and my previous scheme didn’t allow that high a frequency. So I looked for a calculator once more and ended up on your site. What a pleasure and what a relief that is! Thank you so much! Great job well done!

  8. Hey thanks for creating this, this really helped out. Is there any way to get results that use capacitance above 10uF? (I don’t know if 15hz can support a larger capacitor.)

  9. OMG,

    This is awesome, can’t believe there are so many calculators out there asking you to give them the r and c values ….I need to know which resistors and capacitors to use , Thanks!!!!!

  10. Could someone expain what the duty cycle should mean to me in practical applications as in LED’s and small breadboard mounted motor loads.
    Also how is hz and seconds actually related, do I need to concern myself with the hz?
    I see that some of the configurations give you 99% duty cycle and other lots less, great that your results offer so many options, I’m just not real clear on when I should choose the higher or lower cycles.

    Thanks again!!!!

    • The duty cycle refers to how long the power is high vs. low. With a 50% duty cycle, the power would be on for half the time, off for half the time. With a 90% duty cycle, the power would be on for 90% and off for 10.

      If you want a traditional ‘blinking light’ then you probably want something close to 50%. If you are just triggering something the moment when the circuit goes high, then any old duty cycle value will work for you.

      Hz is the ‘inverse’ of seconds. Hz actually means ‘cycles per second’, so 2 HZ means two times a second, or 1/2 second per cycle. 10Hz is 1/10 seconds per cycle. 100 Hz is 1/100 seconds per cycle.

      Hope that helps!

  11. I’m not sure how to word this next thought but….:

    Do you have a calculator that will calculate r and c’s for: On 3 seconds off 1 minute on 3 seconds off 1 min…etc.

    I guess the values given with this calculator could enable me to set up a motion sensor and buzzer that would bzz and reset along with an LED indicator etc…


    I calculated my circuit’s resistors and capacitor , right from here and gladly, it’s working unbelievable . The best part is that table with the different duty cycles possible

  13. Awsome work big thankyou good sir.
    I saw in an early post you asked if there was anything that could be done to make it better.
    How about being able to input the readings from my parts and use the calculator to find the exact hrz?

  14. Great 555 calculator, the best I’ve seen!
    But I have just one doubt, to change the duty cycle, shoud I use a pot in place of R1 or R2? What would be the best option?

  15. Thanks! I’m a software developer who sometimes needs a little hardware designed as clear and simple as possible. Needed 4 audio signals of different frequencies for a reaction timing app. Your calculator laid out the choices of resistors and capacitors, was able to pick the design with fewest different component values. Much appreciated.

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