Capacitors, farads and prefixes
Three prefixes: uF, nF, pF
The unit for capacitance is the farad, F. One farad is a lot of capacitance, so the labeling of real capacitors (with a few exceptions) is always done with one of three prefixes: microfarad (µF or uF), nanofarad (nF) or picofarad (pF). The prefixes works just like when we write mm for a thousandth of a meter (millimeter) or kV for a thousand volts (one kilovolt).
The prefixes are common SI prefixes, and are a thousand times larger/smaller than the ”neighbouring” prefixes. An nF is the same as one thousand pF, and the same as a thousandth of a uF.
| Prefix | Symbol | Power | Equals | Part of 1 F |
|---|---|---|---|---|
| (inget) | F | 100 | one whole | |
| mikro | µF / uF | 10-6 | 1000nF | one millionth |
| nano | nF | 10-9 | 1000pF / 0,001uF | en billionth |
| pico | pF | 10-12 | 0.001 / 0.001nF | en trillionth |
A microfarad is a thousandth of a thousandth of a farad, or a millionth of a farad. It is also equal to 1000nF. An nF is a thousandth of a uF, or 1000 pF. One pF is one thousandth nF, and one millionth uF. It's also a trillionth danger.
Converting between prefixes
Because the unit farad is so very large (compared to real capacitors), the prefixes become very small. In addition, there is a very large range of values that are completely normal: it is no wonder that a same circuit has capacitors with 1000uF, 100nF and 10pF.
If you calculate an optimal capacitance for e.g. a filter, you can get the result with an inappropriate prefix: 0.00001uF instead of 10pF. It quickly becomes many zeros and difficult to read.
There are calculators on the internet, and a table further down, but it's relatively easy to learn to recalculate between different ways of writing. The prefixes are a thousand times larger or smaller than each other, so just count the numbers, and you will find which prefix is appropriate.
For example, if it says 2200pF, we quickly see that it is four digits (thousands). If we divide the value by a thousand, we get the result in nF, which means that it can be written as 2.2nF. With decimals, it's possibly a bit more difficult, but still easy. You just have to think a little backwards:
0.001 of the unit with a prefix (pF or nF) corresponds to 1 with the prefix above (nF and uF, respectively). If you keep the four numbers in mind, you can easily rethink and calculate that 0.001 becomes 1 "backwards", and if we jump one step to the right (times ten), we see that 0.010 becomes 10, and 0.100 becomes 100. After that comes 1000nF, i.e. 1000pF.
Thus: 0.47uF = 470nF, and 0.015nF = 15pF, etc.
Writing conventions
Small capacitors are typically specified in pF, ranging from 1 pF to just under 1000 pF. Sometimes values above 1 nF are written with ‘pico’ as a prefix, e.g. 2200 pF or 1000 pF. This mainly applies to capacitors with high precision and stability, much like how exact measurements are sometimes written with decimals: 1.000 mm. However, it is most common to write 1 nF.
From 1 nF, the scale continues up to 1000 nF, which is virtually never written out in full. Instead, 1 μF is used. Values below 1 μF are also often written with the prefix ‘micro’, e.g. 0.68, 0.47 or even 0.1 μF. And from 1uF upwards, the prefix ‘micro’ is used, with the exception of supercapacitors, which are specified directly in F – 1F, 0.2F, etc.
Exactly which prefix is used depends on various factors, such as the type of capacitor. The value of electrolytic capacitors, for example, is almost always written with uF.
Millifarads?
An electrolytic capacitor with 1000uF capacitance is nothing unusual. 1000uF could also be written as 1mF – one millifarad – just like 1000nF capacitors are more often called 1uF capacitors. However, this is not done today, even for very large capcitors with e.g. 33000uF capacitance. There is a good reason for this: in the past, people often wrote MFD or mF for microfarad, (with the m indicating micro, not milli) rather than using u or the Greek letter µ as we do today. Using millifarads is also so uncommon today that there is a high risk of people misreading it as something else – either nF or uF.
Conversion table between different capacitance prefixes
This table will allow you to convert between different ways of writing E-12 series capacitor values using the three prefixes. The values written in italics are only rarely encountered.
| Picofarad | Nanofarad | Microfarad | ||
|---|---|---|---|---|
| 100pF | = | 0.1nF | ||
| 120pF | = | 0.12nF | ||
| 150pF | = | 0.15nF | ||
| 180pF | = | 0.18nF | ||
| 220pF | = | 0.22nF | ||
| 270pF | = | 0.27nF | ||
| 330pF | = | 0.33nF | ||
| 390pF | = | 0.39nF | ||
| 470pF | = | 0.47nF | ||
| 560pF | = | 0.56nF | ||
| 680pF | = | 0.68nF | ||
| 820pF | = | 0.82nF | ||
| 1000pF | = | 1nF | ||
| 1200pF | = | 1.2nF | ||
| 1500pF | = | 1.5nF | ||
| 1800pF | = | 1.8nF | ||
| 2200pF | = | 2.2nF | ||
| 2700pF | = | 2.7nF | ||
| 3300pF | = | 3.3nF | ||
| 3900pF | = | 3.9nF | ||
| 4700pF | = | 4.7nF | ||
| 5600pF | = | 5.6nF | ||
| 6800pF | = | 6.8nF | ||
| 8200pF | = | 8.2nF | ||
| 10000pF | = | 10nF | ||
| 12000pF | = | 12nF | ||
| 15000pF | = | 15nF | ||
| 18000pF | = | 18nF | ||
| 22000pF | = | 22nF | ||
| 27000pF | = | 27nF | ||
| 33000pF | = | 33nF | ||
| 39000pF | = | 39nF | ||
| 47000pF | = | 47nF | ||
| 56000pF | = | 56nF | ||
| 68000pF | = | 68nF | ||
| 82000pF | = | 82nF | ||
| 100nF | = | 0.1uF | ||
| 120nF | = | 0.12uF | ||
| 150nF | = | 0.15uF | ||
| 180nF | = | 0.18uF | ||
| 220nF | = | 0.22uF | ||
| 270nF | = | 0.27uF | ||
| 330nF | = | 0.33uF | ||
| 390nF | = | 0.39uF | ||
| 470nF | = | 0.47uF | ||
| 560nF | = | 0.56uF | ||
| 680nF | = | 0.68uF | ||
| 820nF | = | 0.82uF | ||
| 1000nF | = | 1uF |