Units of measurement

A Sanwa YX360TRF analog multimeter.

Units of measurement commonly used in electronics can be very large or very small. Instead of using a lot of zeros the quantity is given a metric prefix.

Prefixes for multiples and submultiples

Common metric prefixes used in electronics.

Prefix		Base 10	Decimal	English word
Name	Symbol
giga	G	1 x 109	1,000,000,000	billion
mega	M	1 x 106	1,000,000	million
kilo	k	1 x 103	1,000	thousand
hecto	h	1 x 102	100	hundred
deca	da	1 x 101	10	ten
		1 x 100	1	one
deci	d	1 x 10-1	0.1	tenth
centi	c	1 x 10-2	0.01	hundredth
milli	m	1 x 10-3	0.001	thousandth
micro	µ	1 x 10-6	0.000 001	millionth
nano	n	1 x 10-9	0.000 000 001	billionth
pico	p	1 x 10-12	0.000 000 000 001	trillionth

The most common SI units in electronics

There are other SI units in electronics, this is a list only of those most likely to come up in practical experiments. The units with capitalised symbols are named after significant people.

The metre (m)

The unit of length. In the USA and Phillipines spelt meter. Usually in use as decimal fractions of a metre, as centimetres (cm) or as millimetre (mm) and fractions of mm e.g. stripboard holes are spaced at 2.54 mm intervals.

The ampere or amp (A)

The unit of electric current. Usually used in amps or milliamp (mA) e.g. a 780x voltage regulator rated at one amp.

The volt (V)

The unit for electric potential. Measure differently for DC or AC but in the range of volts and millivolts (mV) e.g. a 9 V PP3 battery.

Ohms (Ω)

The unit of electrical resistance. In electronics also with the symbol R. As resistors usually encountered in ohms through kilohms to megaohms, often with the unit instead of the decimal point. e.g. 4R7 for 4.7 ohm or 1k5 for 1.5 kilohm.

The watt (W)

A unit of power. Most often measures of milliwatt e.g. a laser pointer outputs about five milliwatts of light power. Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.

Farad (F)

The unit of electrical capacitance. Capacitors range in value from a few picofarad (pF) through nanofarads (nF) to microfarads (μF) and sometimes the same value can be specified in one multiple or another e.g. 0.1 nF or 100 pF for the same value ceramic decoupling capacitor.

Henry (H)

The unit of electrical inductance. For coils usually as microhenries (μH) e.g. a radio tuning coil might have an inductance of a few tens of microhenries.

Hertz (Hz)

The unit of frequency. From DC which is 0 Hz through to to microwaves which are in the gigahertz range e.g. the audio range is ideally from about 20 Hz to 20 kHz.

Imperial and metric

Depending on who you speak to PCB designs are specified with "thou" or "mil", e.g. “ten thou spacing” or “a 25 mil grid”. A thou or mil is thousandth of an inch. To avoid confusing mil with millimetre avoid using the term mil and use thou. PCB design uses both imperial inches and the metric millimeter. As a general rule thous are used for tracks, pads, spacings and grids, while mm are for mechanical requirements like hole sizes and board dimensions. Most through hole components have a tenth of an inch (aka 0.1") pin spacing, that's 100 thou or 2.54 mm.

See also

• Decibel

External links

• SI derived unit, Wikipedia
• Metric system, Wikipedia