123 capacitor value
12 nF
12,000 pF · 12 nF · 0.012 µF
| Marked code | 123 |
| Picofarads | 12,000 pF |
| Nanofarads | 12 nF |
| Microfarads | 0.012 µF |
| Farads | 1.20e-8 F |
Reading the code
The three digits are not a part number. The first two are the value in picofarads and the third says what to do with them:
12 × 103 = 12,000 pF
So 123 is 12 followed by 3 zeros: 12,000 pF. The code is always in picofarads even when the part is sold in microfarads, which is the whole difficulty with it — a capacitor marked 123 is stocked as 0.012 µF and the two numbers look nothing alike.
What 12 nF does at frequency
A capacitor's opposition to a signal is its reactance, 1 ÷ (2πfC), and it falls as the frequency rises. Over the range of frequencies a circuit can put across it, a 12 nF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 265.26 kΩ |
| 1 kHz — audio | 13.26 kΩ |
| 100 kHz — switching | 132.63 Ω |
| 10 MHz — RF | 1.33 Ω |
Ten times the frequency is a tenth of the reactance, every time — which is why choosing a decoupling capacitor is really choosing the frequency band you want it to be low-impedance across, and why boards carry several sizes in parallel rather than one large one.
The letter after the number
A code like 123K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 12 nF means |
|---|---|---|
123J | ±5% | 11.4 nF to 12.6 nF |
123K | ±10% | 10.8 nF to 13.2 nF |
123M | ±20% | 9.6 nF to 14.4 nF |
123Z | +80% / −20% | 9.6 nF to 21.6 nF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 123Z can legitimately be anything from 9.6 nF to 21.6 nF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 12 nF turns up
Tens of nanofarads is coupling between audio stages and the smaller decoupling capacitors on a logic board. A 10 nF part alongside a 100 nF one is a common pairing, the smaller of the two handling the frequencies the larger one is too slow for.
Frequently asked questions
What is the value of a 123 capacitor?
12 nF — 12,000 pF · 12 nF · 0.012 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 123 is 12 followed by 3 zeros = 12,000 pF.
What is a 123 capacitor in µF?
0.012 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 12,000 pF, 12 nF and 0.012 µF are the same capacitor written three ways.
What does the letter after 123 mean?
Tolerance. 123K is ±10%, 123J is ±5% and 123M is ±20% — so a 123K is anywhere from 10.8 nF to 13.2 nF and still in specification. A Z means +80%/−20%, which is not a mistake: some ceramic dielectrics really are that loose.
What is the impedance of a 123 capacitor at 1 kHz?
13.26 kΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 132.63 Ω at 100 kHz and 1.33 Ω at 10 MHz.
Nearby codes
- 153 capacitor — 15 nF
- 183 capacitor — 18 nF
- 223 capacitor — 22 nF
- 273 capacitor — 27 nF
- 333 capacitor — 33 nF
- 393 capacitor — 39 nF
- 473 capacitor — 47 nF
- 563 capacitor — 56 nF
- 683 capacitor — 68 nF
- 823 capacitor — 82 nF