129 capacitor value
1.2 pF
1.2 pF · 0.0012 nF · 0.000001 µF
| Marked code | 129 |
| Picofarads | 1.2 pF |
| Nanofarads | 0.0012 nF |
| Microfarads | 0.000001 µF |
| Farads | 1.20e-12 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 × 0.1 = 1.2 pF
The last digit here is 9, which is one of the two exceptions. Eight and nine do not mean 10⁸ and 10⁹ — they mean × 0.01 and × 0.1, and they exist so that values below ten picofarads can be written in the same three characters. Read literally, 129 would be 12 × 109 = 12,000 µF, which is not a ceramic capacitor and not what is in your hand.
What 1.2 pF 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 1.2 pF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 2.65 GΩ |
| 1 kHz — audio | 132.63 MΩ |
| 100 kHz — switching | 1.33 MΩ |
| 10 MHz — RF | 13.26 kΩ |
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 129K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 1.2 pF means |
|---|---|---|
129J | ±5% | 1.14 pF to 1.26 pF |
129K | ±10% | 1.08 pF to 1.32 pF |
129M | ±20% | 0.96 pF to 1.44 pF |
129Z | +80% / −20% | 0.96 pF to 2.16 pF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 129Z can legitimately be anything from 0.96 pF to 2.16 pF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 1.2 pF turns up
Values of a few picofarads are trimming and compensation parts — tuning a crystal oscillator onto frequency, flattening the response of a probe, neutralising stray capacitance in an RF stage. At this size the capacitance of the board traces either side of the part is a serious fraction of the part itself.
Frequently asked questions
What is the value of a 129 capacitor?
1.2 pF — 1.2 pF · 0.0012 nF · 0.000001 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 129 is 12 followed by a divide rather than a multiply: 12 × 0.1 = 1.2 pF.
What is a 129 capacitor in µF?
0.000001 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 1.2 pF, 0.0012 nF and 0.000001 µF are the same capacitor written three ways.
What does the letter after 129 mean?
Tolerance. 129K is ±10%, 129J is ±5% and 129M is ±20% — so a 129K is anywhere from 1.08 pF to 1.32 pF 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 129 capacitor at 1 kHz?
132.63 MΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 1.33 MΩ at 100 kHz and 13.26 kΩ at 10 MHz.
Nearby codes
- 159 capacitor — 1.5 pF
- 189 capacitor — 1.8 pF
- 229 capacitor — 2.2 pF
- 279 capacitor — 2.7 pF
- 339 capacitor — 3.3 pF
- 399 capacitor — 3.9 pF
- 479 capacitor — 4.7 pF
- 569 capacitor — 5.6 pF
- 689 capacitor — 6.8 pF
- 829 capacitor — 8.2 pF