821 capacitor value
820 pF
820 pF · 0.82 nF · 0.00082 µF
| Marked code | 821 |
| Picofarads | 820 pF |
| Nanofarads | 0.82 nF |
| Microfarads | 0.00082 µF |
| Farads | 8.20e-10 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:
82 × 101 = 820 pF
So 821 is 82 followed by 1 zero: 820 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 821 is stocked as 0.00082 µF and the two numbers look nothing alike.
What 820 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 820 pF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 3.88 MΩ |
| 1 kHz — audio | 194.09 kΩ |
| 100 kHz — switching | 1.94 kΩ |
| 10 MHz — RF | 19.41 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 821K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 820 pF means |
|---|---|---|
821J | ±5% | 779 pF to 861 pF |
821K | ±10% | 738 pF to 902 pF |
821M | ±20% | 656 pF to 984 pF |
821Z | +80% / −20% | 656 pF to 1.476 nF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 821Z can legitimately be anything from 656 pF to 1.476 nF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 820 pF turns up
Hundreds of picofarads is filtering at radio frequencies and the timing element in fast oscillators. Below a nanofarad, a ceramic capacitor is close to ideal — very little loss, very little drift — which is why this range is used where accuracy matters.
Frequently asked questions
What is the value of a 821 capacitor?
820 pF — 820 pF · 0.82 nF · 0.00082 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 821 is 82 followed by 1 zero = 820 pF.
What is a 821 capacitor in µF?
0.00082 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 820 pF, 0.82 nF and 0.00082 µF are the same capacitor written three ways.
What does the letter after 821 mean?
Tolerance. 821K is ±10%, 821J is ±5% and 821M is ±20% — so a 821K is anywhere from 738 pF to 902 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 821 capacitor at 1 kHz?
194.09 kΩ. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 1.94 kΩ at 100 kHz and 19.41 Ω at 10 MHz.
Nearby codes
- 102 capacitor — 1 nF
- 122 capacitor — 1.2 nF
- 152 capacitor — 1.5 nF
- 182 capacitor — 1.8 nF
- 222 capacitor — 2.2 nF
- 272 capacitor — 2.7 nF
- 332 capacitor — 3.3 nF
- 392 capacitor — 3.9 nF
- 472 capacitor — 4.7 nF
- 562 capacitor — 5.6 nF