125 capacitor value
1.2 µF
1,200,000 pF · 1200 nF · 1.2 µF
| Marked code | 125 |
| Picofarads | 1,200,000 pF |
| Nanofarads | 1200 nF |
| Microfarads | 1.2 µF |
| Farads | 1.20e-6 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 × 105 = 1,200,000 pF
So 125 is 12 followed by 5 zeros: 1,200,000 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 125 is stocked as 1.2 µF and the two numbers look nothing alike.
What 1.2 µF 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 µF part covers several orders of magnitude:
| Frequency | Reactance |
|---|---|
| 50 Hz — mains | 2.65 kΩ |
| 1 kHz — audio | 132.63 Ω |
| 100 kHz — switching | 1.33 Ω |
| 10 MHz — RF | 13.3 mΩ |
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 125K carries a tolerance letter. It is not part of the value:
| Letter | Tolerance | 1.2 µF means |
|---|---|---|
125J | ±5% | 1.14 µF to 1.26 µF |
125K | ±10% | 1.08 µF to 1.32 µF |
125M | ±20% | 960 nF to 1.44 µF |
125Z | +80% / −20% | 960 nF to 2.16 µF |
A Z is worth knowing about. +80%/−20% is a real tolerance on cheap high-permittivity ceramics, and it means a part marked 125Z can legitimately be anything from 960 nF to 2.16 µF. Those go in decoupling positions where only the rough size matters, never in a timing or filter position.
Where 1.2 µF turns up
Hundreds of nanofarads up to a microfarad is bulk decoupling and audio coupling where the low end has to survive. Ceramics this large have moved to multilayer construction, and the capacitance falls off sharply with applied voltage — a part marked 1 µF can measure half that with its rated voltage across it.
Frequently asked questions
What is the value of a 125 capacitor?
1.2 µF — 1,200,000 pF · 1200 nF · 1.2 µF. The first two digits are the value in picofarads and the third is how many zeros follow, so 125 is 12 followed by 5 zeros = 1,200,000 pF.
What is a 125 capacitor in µF?
1.2 µF. Going from picofarads to microfarads is six decimal places, which is where most of the confusion with these codes comes from — 1,200,000 pF, 1200 nF and 1.2 µF are the same capacitor written three ways.
What does the letter after 125 mean?
Tolerance. 125K is ±10%, 125J is ±5% and 125M is ±20% — so a 125K is anywhere from 1.08 µF to 1.32 µF 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 125 capacitor at 1 kHz?
132.63 Ω. Reactance is 1 ÷ (2πfC), so it falls as the frequency rises: the same part is 1.33 Ω at 100 kHz and 13.3 mΩ at 10 MHz.
Nearby codes
- 155 capacitor — 1.5 µF
- 185 capacitor — 1.8 µF
- 225 capacitor — 2.2 µF
- 275 capacitor — 2.7 µF
- 335 capacitor — 3.3 µF
- 395 capacitor — 3.9 µF
- 475 capacitor — 4.7 µF
- 565 capacitor — 5.6 µF
- 685 capacitor — 6.8 µF
- 825 capacitor — 8.2 µF