Capacitors are marked in three different ways depending on the type, which is why a drawer of them looks like three unrelated systems. It is really one idea printed at three sizes.
| Unit | In farads | Typical use |
|---|---|---|
| 1 pF (picofarad) | 0.000 000 000 001 F | Crystal load caps, RF trimming |
| 1 nF (nanofarad) | 0.000 000 001 F | Filters, timing |
| 1 uF (microfarad) | 0.000 001 F | Coupling, supply decoupling |
| 1 mF, written as 1000 uF | 0.001 F | Power supply smoothing |
1 uF = 1 000 nF = 1 000 000 pF. Three zeros a step. Nearly every conversion mistake is one step of this ladder.
Ceramic discs: the three-digit code
A small ceramic capacitor is marked with three digits and the value is in picofarads. Same rule as a resistor: the first two digits are the value, the third is the number of zeros after it.
| Printed | In picofarads | Usually written as |
|---|---|---|
22 | 22 pF | 22p — two digits only, so no multiplier |
101 | 100 pF | 100p |
221 | 220 pF | 220p |
102 | 1 000 pF | 1n |
103 | 10 000 pF | 10n |
104 | 100 000 pF | 100n, or 0.1 uF — the decoupling cap |
105 | 1 000 000 pF | 1u |
4n7 or 472 | 4 700 pF | 4n7 |
If there are only one or two digits and no third figure, the number is the value in pF as it stands.
Capacitor calculator
Open on its own pageType the code off the part — 104, 22, 4n7 — and read the value.
The voltage printed on a capacitor is a maximum, not an operating point. Run an electrolytic at no more than about two thirds of its rating — a 12 V rail wants a 25 V part. And check the dielectric code: an X7R or Y5V ceramic loses a large part of its value with temperature and applied voltage, so a timing circuit built with one will not match this arithmetic.
The letter after the number
A letter following the digits is the tolerance. On a timing circuit it matters; on a decoupling capacitor it does not.
| Letter | Tolerance |
|---|---|
J | +/- 5 % |
K | +/- 10 % |
M | +/- 20 % |
Z | +80 % / -20 % |
F | +/- 1 % |
G | +/- 2 % |
So 104K is 100 nF at +/- 10 %. Z looks alarming and is normal for a bulk ceramic — it just means nobody is promising you much.
The three-character code is the dielectric
X7R, C0G, Y5V and friends describe what the ceramic is made of and therefore how much the value moves with temperature and applied voltage. This catches people out badly in timing circuits.
| Code | Stability | Use it for |
|---|---|---|
C0G / NP0 | Excellent, near zero drift | Oscillators, filters, anything where the value is part of a calculation |
X7R | Good — around +/- 15 % over temperature | General purpose, decoupling, coupling |
Y5V / Z5U | Poor — can lose most of its value when hot or at rated voltage | Bulk decoupling only, never timing |
Electrolytics: printed in full, and polarised
An aluminium electrolytic is big enough to print on, so it says what it is: 10uF 25V, 1000uF 16V. There is nothing to decode. What there is, is a direction.
- The stripe down the side marks the negative lead, usually with a row of minus signs.
- The longer lead is positive on a new, untrimmed part.
- On a PCB, the silkscreen outline is usually shaded on the negative side, and the square pad is often positive.
The voltage on the body is a maximum, not an operating point. Run an electrolytic at no more than about two thirds of its rating: a 12 V rail wants a 25 V part, not a 16 V one. Life roughly doubles for every 10 degrees C cooler it runs, so the derating buys real years.
The old capacitor colour code
You will meet this on vintage equipment and in older textbooks. It uses the same colour-to-digit table as a resistor and the answer is in picofarads: the first two bands are digits, the third is the multiplier, then tolerance and working voltage.
Brown, Black, Orange reads 1, 0, x 1 000 = 10 000 pF = 10 nF. Nothing new — but note the unit, because reading it as ohms or microfarads gives an answer that is out by a factor of a million.
Modern parts print digits instead, and you are unlikely to see this on anything bought new.
Values you will actually use
| Value | Marked | What it is doing |
|---|---|---|
| 22 pF | 22 | Either side of a crystal, setting its load |
| 100 nF | 104 | Decoupling, one per IC, as close to the power pin as it will go |
| 1 uF | 105 or printed | Coupling audio, reset timing |
| 10 uF | printed | Local bulk storage near a regulator |
| 100 uF | printed | Regulator output, smoothing a small supply |
| 1000 uF | printed | Smoothing after a bridge rectifier |
Testing one
- 1
Discharge it first
A large capacitor out of a power supply can hold a painful charge for a long time after the mains is off. Short it through a resistor of a few hundred ohms, not through a screwdriver — a direct short welds the tip and can rupture the can.
- 2
Use the capacitance range if your meter has one
Read it out of circuit. A part that reads well under its marking, or refuses to settle, is dried out.
- 3
If it has no capacitance range, use resistance
A healthy electrolytic on the ohms range shows a low reading that climbs steadily towards open as the meter charges it. A dead short both ways means it has failed short; an instant open on a large value means it has failed open or lost its electrolyte.
- 4
Look at it
A bulged top, a lifted rubber plug or crust around the leads means it is finished, whatever it measures. On old equipment this is the first thing to check and often the whole fault.