399 capacitor value
3.9 pF
3.9 pF · 0.0039 nF · 0.000004 µF
| Marked code | 399 |
| Picofarads | 3.9 pF |
| Nanofarads | 0.0039 nF |
| Microfarads | 0.000004 µF |
| Farads | 3.90e-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:
39 × 0.1 = 3.9 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, 399 would be 39 × 109 = 39,000 µF, which is not a ceramic capacitor and not what is in your hand.
What 3.9 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 3.9 pF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 816.18 MΩ |
| 1 kHz — audio | 40.81 MΩ |
| 100 kHz — switching | 408.09 kΩ |
| 10 MHz — RF | 4.08 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 399K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 3.9 pF means |
|---|---|---|
399J | ±5% | 3.71 pF to 4.1 pF |
399K | ±10% | 3.51 pF to 4.29 pF |
399M | ±20% | 3.12 pF to 4.68 pF |
399Z | +80% / −20% | 3.12 pF to 7.02 pF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 399Z can legitimately be anything from 3.12 pF to 7.02 pF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 3.9 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 399 capacitor?
3.9 pF — 3.9 pF · 0.0039 nF · 0.000004 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 399 is 39 followed by a divide rather than a multiply: 39 × 0.1 = 3.9 pF.
What is a 399 capacitor in µF?
0.000004 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 3.9 pF, 0.0039 nF and 0.000004 µF are the same capacitor written three ways.
What does the letter after 399 mean?
Tolerance. 399K is ±10%, 399J is ±5% and 399M is ±20% — so a 399K is anywhere from 3.51 pF to 4.29 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 399 capacitor at 1 kHz?
40.81 MΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 408.09 kΩ at 100 kHz and 4.08 kΩ at 10 MHz.
Nearby codes
- 479 capacitor — 4.7 pF
- 569 capacitor — 5.6 pF
- 689 capacitor — 6.8 pF
- 829 capacitor — 8.2 pF
- 100 capacitor — 10 pF
- 120 capacitor — 12 pF
- 150 capacitor — 15 pF
- 180 capacitor — 18 pF
- 220 capacitor — 22 pF
- 270 capacitor — 27 pF