Hintlink - Units of measure

Units of measure¹ - now let's try to get this right

Specifying a quantity is a three-part 'thing' - Number, Prefix (or multiplier) PLUS the unit being used

First, let's deal with the prefixes
IMPORTANT! Many prefixes start in lower case, working from the smallest upwards. When the lower case version has been used, THEN the upper-case version is used.
In practice, this involves y/Y, z/Z, p/P and m/M. The ones to watch out for are pico/peta and milli/mega as they are the most in use. However, pico and peta are never (?) used with the same base or derived unit [e.g. pF and PB*], whereas mHz and MHz are used and are much closer because of the smaller difference between 10^-3 and 10⁶.
*PB is used in computing to mean peta Bytes, an increasingly used term.

yotta	Y	10²⁴	Extremely large
zetta	Z	10²¹
exa	E	10¹⁸
peta	P	10¹⁵
tera	T	10¹²
giga	G	10⁹
mega	M	10⁶
kilo	k	10³
hecto	h	10²
deca	da	10¹
-	-	10⁰ (=1)	-
deci	d	10^-1
centi	c	10^-2
milli	m	10^-3
micro	µ	10^-6
nano	n	10^-9
pico	p	10^-12
femto	f	10^-15
atto	a	10^-18
zepto	z	10^-21
yocto	y	10^-24	Extremely small

Hintlink - Units of measure

Now the fundemental (base) units upon which everything is built
IMPORTANT! Units which are named after real scientists begin in UPPER CASE, all others are in lower case.

Name	Symbol	Measure	Named After
metre	m	length
kilogram^[*]	kg	mass
second	s	time
ampere	A	electric current	Ampère
kelvin	K	thermodynamic temperature	Kelvin
mole	mol	amount of substance
candela	cd	luminous intensity

* The kilogram is an unusual exception as it is based on the mass of a small squat cylinder of ~47 cubic centimetres of platinum-iridium alloy kept in a laboratory in France (near Paris). The gram is far too small to reliably measure, so the kilogram is used. The standard is rarely examined and cannot be touched or knocked, so as not to change the mass.
New definitions come into use on 2019-05-20. These change the world's definition of the kilogram, the ampere, the kelvin and the mole.

Hintlink - Units of measure

Some of the derived units
IMPORTANT! Units which are named after real scientists begin in UPPER CASE, all others are in lower case.

Name	Symbol	Quantity	Named After
hertz	Hz	frequency	Hertz
radian	rad	angle (dimensionless)
steradian	sr	solid angle (dimensionless)
newton	N	force, weight	Newton
pascal	Pa	pressure, stress	Pascal
joule	J	energy, work, heat	Joule
watt	W	power, radiant flux	Watt
coulomb	C	electric charge or quantity of electricity	Coulomb
volt	V	voltage, electrical potential difference, electromotive force	Volta
farad	F	electrical capacitance	Faraday
ohm^[*]	Ω	electrical resistance, impedance, reactance	Ohm
siemens	S	electrical conductance	Siemens
weber	Wb	magnetic flux	Weber
tesla	T	magnetic field strength, magnetic flux density	Tesla
henry	H	electrical inductance	Henry
degree Celsius	°C	temperature relative to 273.15 K	Celsius
lumen	lm	luminous flux
lux	lx	illuminance
becquerel	Bq	radioactivity (decays per unit time)	Becquerel
gray	Gy	absorbed dose (of ionizing radiation)	Gray
sievert	Sv	equivalent dose (of ionizing radiation)	Sievert
katal	kat	catalytic activity
bel	B	ratio of values	Bell

* The ohm is an exception, being abbreviated to Ω rather than O (to save confusion with 0).
Dr. Georg Simon Ohm did, of course, exist, as in Ohm's Law.

Hintlink - Units of measure

Common mistakes

KHz	You mean kHz
khz	You mean kHz
meter	You mean metre
liter*	You mean litre
mhz	You mean MHz
mL	You mean ml (there was no Mr./Dr. Litre). In Europe mℓ is printed on products / packaging
MS	You mean ms, most likely, for milliseconds. megasiemens would be unusual
MG	You mean mg, for milligrams
MCG	You mean µg
mil	You mean a millimetre, not 1/1000"
M	You mean m, metres. As in 20m or 160m (amateur radio bands)
CM	You mean cm, centimetres. As in 70cm or 23cm (amateur radio bands)
MF	You mean µF
MMF	You mean pF
db or DB	You mean dB
	undoubtedly more to follow :-)

*The litre is not actually a unit recognised under SI definitions, but is in common usage.

Hintlink - Units of measure

Common uses:
mil = millimetre
Angstrom (Å), 10^-10m (8Å is about the size of an atom) - Å is used so as not to be confused with the ampere (A)
micron = 1µm [FYI, resolution of best optical microscope ~2 microns]
mho, reciprocal of ohm. Same as siemen.
Resistance values (often) use the multiplier instead of the decimal point e.g. 1k2, 4M7 (often without the Ω symbol)
[this has the added advantage of removing the decimal point '.', which in Continental Europe is a ',']
Capacitor values (often) use the last figure to indicate the 'power' of the value above 10pF e.g. 104 = 10(0000) = 100,000pF = 100nF = 0.1uF
dB = decibel. Have never seen bel used. Often used against a reference level dBm, dBV [an oddity, dBm refers to 'relative to 1mW' and should really be 'dBmW']
fF do exist and I have made them using quartz pillared capacitors [low, sub pF capacitors can be made by using PTFE covered wire twisted together]

Hintlink - Units of measure

Personal dislikes:
centimetre - an abomination of a measurement unit.
Monetary system disobeys all reason ($1m, $1b, $1bn, $1T, etc.)

Hintlink - Units of measure

Oddities:
VU (volume unit) is a completely arbitrary audio 'standard'.
Peak reading meters are much to be preferred. As used by the Nakamichi 482 cassette deck and other fine audio equipmet.

Further information, in detail, can be found at the SI Reference or these Wikipedia links:
Prefixes Base Units Derived Units

Hintlink - Units of measure

Examples
1) I was finally driven to write this page by a catalog from a well known US vendor with 13 electronics outlets, no less!
On the front page, in glorious color, was printed: FT-891 HF + 50Mhz All Mode Mobile Transceiver with 100w Output.
Reading the rest of the catalog of 124 pages revealed hundreds of typographical and unit errors!
2) Advertising on well-known web page:

3) Please let manufacturers (and writers) know, so that they can correct these errors....