A capacitor marked 0.1 MFD is not a 0.1 farad part. It is 0.1 microfarad, which is 100 nanofarad, which is the same thing as the 104 sitting next to it in the drawer. The marking is older than the SI prefix, and that is the whole trap.

Three marking systems cover almost every capacitor on a hobby bench. Two are arithmetic. One is a fossil.

The three digit code

Three digits: the first two are the value in picofarads, the third is how many zeros to add.

  • 104 is 10 followed by four zeros: 100,000 pF = 100 nF = 0.1 µF.
  • 223 is 22,000 pF = 22 nF.
  • 471 is 470 pF.

The result is always picofarads. Divide by 1,000 for nanofarads, again by 1,000 for microfarads (or by 1,000,000 once). Most people learn 104 = 0.1 µF by repetition and never touch the arithmetic again.

R notation for values below 100 pF

When the multiplier would be zero or negative, the letter takes the decimal point's place:

  • 4n7 is 4.7 nF.
  • 2u2 is 2.2 µF.
  • R47 is 0.47 pF, not 0.47 µF.
  • 1p0 is 1 pF.

The unit letter is the decimal point. 4n7 is not "four n seven"; it is 4.7 nanofarad.

MFD and MMFD, the markings that look wrong

MFD means microfarad. MMFD means micro-microfarad, which is a picofarad. Both predate the modern µF/pF notation, and both still appear on surplus parts, old schematics, and equipment manuals.

  • 0.1 MFD = 0.1 µF = 100 nF = 104.
  • 0.001 MFD = 1 nF = 102.
  • MMFD on its own, or .001 MMFD, is a picofarad-scale part.
  • 100 MMFD = 100 pF.

The failure mode is not subtle. Read 0.1 MFD as 0.1 farad and you have a component three orders of magnitude larger than the one the circuit wants. Read MMFD as microfarad and the error goes the other way. Either way the board does not work, and the part usually looks fine.

The traps that are not about the code

  • Voltage rating is separate. A 104 can be 50 V, 100 V, or 1 kV. The code says nothing about it.
  • Tolerance changes the part you can substitute. A 104 in a timing circuit may be a 5% or 10% part; a 1% part is a different purchase.
  • Dielectric matters more than the number. A 100 nF ceramic and a 100 nF film are not interchangeable in filters, oscillators, or anything with a signal across it.
  • In circuit, expect the neighbours. A capacitor in parallel with a resistor or another capacitor reads differently. Lift one leg before you believe the meter.
  • Old parts drift. Electrolytics age; a reading far below the marking is information, not proof that the marking is wrong.

The tool

I keep a free reader at tinkernews.com/tools/capacitor-code-calculator that handles the numeric codes, R notation, and legacy MFD/MMFD values, and shows the nearest standard value. It is free, with no signup.

If you would rather call it from a script, the same arithmetic lives in the small library at github.com/tinkernews/electronics-tools.

Next time a drawer label says MFD, read it as microfarad first. The arithmetic is easy; the fossil is the part that trips people.