Capacitance is defined in units called Farads. Farads are a relatively large measure of capacitance for capacitors which typically store a small amount of power inside of various electronic devices. For this reason rather than writing Farads (F) smaller units such as pF, nF, uF, and mF are used.
The same capacitance value can be written in a number of different ways. For instance, 0.1uF is the same as writing 100nF. Both are ways of writing the same number and both are technically correct. The difference is that uF represents microfarads while nF represents nanofarads. Using a different unit puts the unit described in more relatable terms. By using the correct unit the capacitance value can be known at a quick glance rather than by counting the number of zeros before or after a decimal point.
In the example above 0.1uF and 100nF were mentioned. These are both the same capacitance value described in different unit terms. The best unit to use in this scenario is nanofarads (nF) because it eliminates the need for a decimal point to be used. Whenever possible it’s best to use a unit which describes capacitance without using either a decimal point or a number ranging into the thousands. When a number is below 1 it’s best to use a smaller unit of measure rather than decimals. When a number is above 999 it’s best to use a larger measurement units. You wouldn’t measure the weight of a car in grams. You would use kilograms for something that heavy. It’s the same concept but in reverse since we’re dealing with increasingly smaller units. If the capacitance value stays between 1 and 999.99 you know that you’re most likely using the right unit of measurement.
Each smaller unit is a 1000x change (3 decimal points) from the next larger unit. This is analogous to going from gram to milligram, milligram to microgram, and so on.
millifarad (mF): 1,000
microfarad (uF): 1,000,000
nanofarad (nF): 1,000,000,000
picofarad (pF): 1,000,000,000,000
By saying 220pF you’re saying 220-trillionth of a farad. If you begin to think in those terms you begin to develop a complete understanding of electronics.
Converting to The Next Unit
Converting between the next unit is relatively straightforward. If you’re going from a larger unit to smaller unit you multiply by 1000.
Example 1: 1F x 1000 = 1000mF
Example 2: 0.1nF x 1000 = 100pF
If you’re going from smaller to larger divide by 1000.
Example 1: 2000pF/1000 = 2nF
Example 2: 20000nF/1000 = 20uF
The main thing is to memorize the order of descending units if you plan on working with capacitors often. By knowing which comes first it’s possible to know easily which units above and below a given stated value are available for conversion.
While the numbers can be large, it’s important to note that it’s always simply a decimal point movement of 3 places in either direction when converting between neighboring units. Putting the conversion down on paper in terms of an equation like I did above makes the process look more difficult than it really is. All you’re doing is adding or subtracting 3 0’s before or after the original number.
Converting Between Distant Units
This isn’t common. Typically a unit is written in a relatively close unit to the one you’re converting. It’s common to see units written as something like 0.2uF. You don’t usually see something written like 0.0002uF. If you do see something like this it’s possible to simply move the decimal point 3 places and then 3 places again.
Step 1: 0.0002uF -> 0.2nF
Step 2: 0.2nF -> 20pF
Converting between distant units like this can be very useful if you’re working on a project that contains many capacitance values. Sometimes it’s easier to think of all capacitors in terms of the same measurement unit if you need to compare their capacitance values against one another during the project. 10,000,000pF may be the best value for the needs of your own project even if it isn’t the best way to write it or search for that particular part.