A Capacitor Marked 220 Is Twenty-Two Picofarads

The letter after the digits is the tolerance

A marking of 104K is a hundred nanofarads to within ten per cent; J is five per cent and M is twenty. The letters are worth learning because they decide whether a part belongs in a timing circuit or only in a supply rail, and because a bag of parts that all read 104 may hold two quite different components. Separately, the three-character code beside the value — C0G, X7R, Y5V and their relatives — describes how the value behaves across temperature, and it varies far more than the tolerance letter suggests. The first is stable enough for a filter; the last can lose a large fraction of its capacitance at the ends of its range.

Class-2 ceramics lose capacitance under voltage

The number printed on the part is measured with almost no voltage across it, and the common high-capacitance ceramic types lose a substantial part of that value once a direct voltage is applied — an effect the marking does not mention and the datasheet shows as a curve rather than a figure. A part chosen as a 10 µF decoupling capacitor can be delivering a fraction of that at the rail voltage it is actually sitting at, which is why supplies sometimes behave worse than the bill of materials promises. Where the value matters, read the derating curve for the specific part and voltage, and prefer a stable dielectric or a larger case size rather than a larger printed number.

Frequently asked questions

What if the marking has only two digits?

Then it is the value in picofarads with no multiplier: 22 is 22 pF and 47 is 47 pF. Small ceramic parts often carry this form because there is no room for more, and the absence of a third digit is itself the signal that no zeros are to be added. A part whose value cannot be expressed this way — 4.7 pF, say — is usually marked 4R7, with the letter standing in for the decimal point in the same way resistors use R. When a marking cannot be read with confidence, a capacitance meter settles it in seconds and a wrong guess can take an hour to find.

Why is 100 nF the value on almost every board?

Because it is the conventional decoupling value: one next to every integrated circuit's supply pin, holding a small local reserve of charge so that a sudden demand is met from a centimetre away rather than from the power supply at the other end of the board. The value is a compromise rather than a calculation — large enough to supply a switching transient, small enough that the part itself stays fast — and it has been the default long enough that 104 is the most common marking in most parts drawers. The number that actually matters for that job is the inductance of the path to the chip, which is why the capacitor goes as close to the pin as the layout allows.