RC Filter Calculator

Resistor of 100.0 Ω with capacitor of 4.7 uF

Capacitor (E, 10%)Resistor (E, 1%)Frequency ƒcToleranceTC 63%4TC 98%5TC 100%
4.7 uF100.0 Ω338.6 Hz304.8 Hz – 380.1 Hz470.0 usec1.9 msec2.4 msec
4.7 uF110.0 Ω307.8 Hz277.1 Hz – 345.5 Hz517.0 usec2.1 msec2.6 msec
6.8 uF100.0 Ω234.1 Hz210.7 Hz – 262.7 Hz680.0 usec2.7 msec3.4 msec
6.8 uF110.0 Ω212.8 Hz191.5 Hz – 238.8 Hz748.0 usec3.0 msec3.7 msec

Enter any two values to find the third one:

About this calculator

Note that you can enter values in typical formats. Case is ignored.

  • For frequencies, '10k', '5KHZ','8MHz' and '1M' are all valid.
  • For resistors, '100R', '470r', '1k2','47k', '2M2', and '10M' are all valid.
  • For capacitors, '22p', '330p', '2n2', '10nF', '4.7u', '4u7', and '22uF' are all valid.

Since case is ignored, 'milli' prefixes will be interpreted as 'Mega'. This only really affects frequency.

A '1% resistor' usually means +/-1% tolerance. This calculator works this way. 10% is +/-10%.

Capacitors and Resistors are both limited to a 'typical' range; 10pF to <10000uF for caps, 1Ω to <100MΩ for resistors. Outside this range things will probably turn to nonsense.

10 thoughts on “RC Filter Calculator

  1. The time constants don’t seem to be right. For 1M, 1nF, I’m getting 6.3ms. Should it not be 1ms?

    1. Well spotted, Ernest. Thanks very much for that. It used to be ok, so it looks like it lost a “2 Pi” term in the transfer to the new website.
      Now corrected.

  2. Hi Tom,
    thanks for all the information and tips and stuff on your site. I do not consider you an idiot! (Yeah, I’m a creep). Don’t know if this is the best place to ask this question but since we’re talking about caps and filters, what kind of cap do we use now that polystyrene have gone? I was told (by the tinternet) not to use ceramic as filter caps so what would you recommend? Polyester Film? Polypropylene? Metallised Polyprop? Two bits of tinfoil and damp toilet paper? I scored six 330pF polystyrene caps for the Thomas Henry VCF-1 but further searches have not found any more of any value that I can afford. I initially built the VCF-1 (on +/-5V ’cause I’m a masochist) with ceramics as the integrator caps and it wasn’t what I wanted. Played it to a musician friend who said he liked it but I think he was being nice rather than truthful. So what type of caps would you recommend?



    1. I use polypropylene for filter caps, mostly. I like the Wima box caps. They’re not cheap, but they’re available in tight tolerances and they’re good quality. Polyprop vs metallized polyprop is mostly a question of what values you need.
      For SMD parts, NP0/C0G is highly recommended.
      There are plenty of good modern film caps, so I wouldn’t worry too much. For good resonance, it helps to have close values, so tolerance is important. Or you could *could* match them for a one-off, I suppose.

      1. Thanks Tom. I shall buy accordingly though SMD are off the menu unless the NHS perfect new eyeballs and fingers. Thanks for the tip about tolerances and matching for resonance. I have my Atlas passives meter to hand.

  3. Hi!
    Im looking for passive “T” filter calculator, for audio experimentation.
    Somebody can help me?
    Thanks and cheers!

Leave a Reply

Your email address will not be published. Required fields are marked *