Units and metric prefixes

One unit per quantity

Electronics measures a small set of quantities, and each has exactly one unit. Units named after people use a capital letter for the symbol (V, A, W, Hz, F, H, Ω) and a lower-case name when spelled out: volt, ampere, watt, hertz.

Real values range from a few picofarads to gigahertz, so each unit comes with prefixes that scale it by powers of ten. Radio spans more orders of magnitude than almost any field, so fluency here saves real mistakes.

The prefixes

Prefix	Symbol	Factor	Ham example
giga	G	10⁹	2.4 GHz, a microwave band
mega	M	10⁶	14.2 MHz; 1 MΩ
kilo	k	10³	4.7 kΩ; 3.5 MHz = 3500 kHz
(none)		1	13.8 V; 5 A
milli	m	10⁻³	20 mA; 150 mW
micro	µ	10⁻⁶	10 µF; 500 µV signal
nano	n	10⁻⁹	100 nF bypass capacitor
pico	p	10⁻¹²	22 pF capacitor

Each step is a factor of 1000. Other prefixes such as centi (10⁻²) turn up in lengths, but rarely in electronics.

Converting without mistakes

Moving one prefix is three decimal places. Going to a smaller unit makes the number larger; going to a larger unit makes it smaller. It helps to ask whether the answer is sensible.

From	To	Working
0.0047 A	mA	0.0047 × 1000 = 4.7 mA
14 200 000 Hz	MHz	14 200 000 ÷ 1 000 000 = 14.2 MHz
2200 pF	nF	2200 ÷ 1000 = 2.2 nF
150 mW	W	150 ÷ 1000 = 0.15 W
4.7 kΩ	Ω	4.7 × 1000 = 4700 Ω

In formulas, the safest habit is to convert everything to base units (volts, amperes, ohms, hertz, farads), calculate, then convert back. A useful shortcut: volts ÷ kilohms = milliamperes, so 5 V across 10 kΩ gives 0.5 mA.

Mistakes to avoid

• m versus M. 1 mW is a thousandth of a watt; 1 MW is a million watts. Case matters: k (kilo) is lower-case, M (mega) and G are upper-case.
• µ versus m. 1 µF is a thousand times smaller than 1 mF. Where µ cannot be typed, an "u" stands in (uF), and a part marked 4k7 means 4.7 kΩ.
• Mixing prefixes in a formula. Using mA with volts and ohms gives a thousand-fold error. Convert first.
• Dropping the unit. "Ten" means nothing; 10 µF, 10 mF and 10 pF are very different parts.

Wavelength and frequency in metric units are covered in Frequency and wavelength, and ratios in Decibels, which compare two quantities rather than measure one.

Related

• Voltage, current and resistanceThe three quantities every circuit is about.
• Ohm's lawThe relationship that links them, and how to use it.
• Frequency and wavelengthTwo ways to describe the same wave, tied together by the speed of light.
• DecibelsThe logarithmic scale for gain, loss and signal strength.
• ResistorsTypes, ratings, tolerance and the colour code.
• CapacitorsCeramic, electrolytic, film and variable: choosing the right one.