یکاهای استاندارد SI

Second - time
kilogram - mass
degrees kelvin - temperature
meter - distance

candle (candela) - light

ampere - current and resistance

mole - amount of substance

MECHANICS

Parameter:	Metric Unit	English Units
time duration, delay	second * s	second, minute hour, day
frequency	Hertz 1/s	cycle per second
length distance, displacement	meter 1/299 792 458 light sec m	inch, foot, yard rod, chain, furlong, mile
velocity, speed rate of change of position	meter per second m/s	foot per second mile per hour, knot
acceleration rate of change of velocity	meter per second squared m/s2	foot per second squared
mass quantity of material	kilogram * kg	ounce, pound, ton
force push, pull, or weight	newton kg m/s2	pound-force
impulse force times time	newton second kg m/s	pound-force second
momentum mass times velocity	kilogram meter per second kg m/s	pound foot per second
work, energy force times distance	joule (pronounced "jewel") = one newton meter kg m2/s2	foot pound-force
power rate of doing work	watt = one joule per second kg m2/s3	foot pound-force per second horsepower
area size of a surface	square meter m2	square foot, square yard acre, square mile
pressure force per unit area	pascal newton per square meter kg/m s2	pound-force per square inch
volume, capacity	cubic meter (stere) m3	pint, quart, gallon cubic foot, cubic yard
density, heaviness	grams per cubic centimeter g/cm3	pounds per cubic foot

HEAT

Parameter:	Metric Heat Unit	English Heat Unit
heat energy	joule, calorie, Calorie 1 cal = 4.186 joules 1 Cal = 4186 joules	British Thermal Unit
temperature change	kelvin *	Fahrenheit degree
temperature	kelvin	degrees Fahrenheit

LIGHT

Parameter:	Metric Light Unit
luminous intensity	candle *
luminous flux	lumen one candle produces 4 pi lumens
illumination	lux lumen per square meter
focus	diopter reciprocal meters

ELECTRICITY & MAGNETISM (There are no English electrical units)

Parameter:	Metric Electrical Unit
electric charge	coulomb 96,500 coul = 1 faraday faraday = 1 mole of electrons
electromotive force (EMF)	volt (joule per coulomb)
capacitance	farad (coulomb per volt)
electric current	ampere (coulomb per second)
electric energy	joule (watt second) (newton meter) KWH = 3,600,000 joules
electric power	watt (joule per second)
electrical resistance	ohm (volt per ampere)
electrical conductivity	siemens (coulomb per joule) (ampere per volt)
electrical field strength	volts per meter
electromagnetic inductance	henry (pl. henrys)
magnetic field intensity	oersted (formerly the gauss)
magnetic flux	weber (108 maxwells)
magnetic flux density	tesla weber per square meter
magnetomotive force	gilbert

Each physical quantity (length, mass, volume, etc.) is represented by a specific SI unit. That unit is made larger or smaller by addition of a prefix to the stem unit.

Commonly used metric system units and symbols:

Type of Measurement	Unit Name	Symbol
length, width, distance, thickness, girth, etc.	meter	m
mass (often called weight)	kilogram*	kg
mass (larger)	metric ton	t
time	second	s
temperature	degree Celsius**	°C
area	square meter	m2
area (land)	hectare	ha
volume (liquid or other)	liter	L***
volume (larger)	cubic meter	m3
density	kilogram per cubic meter	kg/m3
velocity	meter per second	m/s
velocity (autos)	kilometer per hour	km/h
force	newton	N
pressure, stress	kilopascal	kPa
energy	kilojoule	kJ
power	watt	W

*The gram (g) is the stem unit to which other prefixes are added.
**The kelvin (K) is the SI base unit of thermodynamic temperature.
***The capital el (L) is preferred as the symbol for liter in the USA; however the lower case el (l) also is correct and is used in many metric countries.

Derived Units

Force	newton	N	kg m s-2
Energy	joule	J	kg m2 s-2
Power	watt	W	kg m2 s-3
Frequency	hertz	Hz	s-1
Charge	coulomb	C	A s
Capacitance	farad	F	C2 s2 kg-1 m-2
Magnetic Induction	tesla	T	kg A-1 s-2

The Metric Prefixes

Prefix:	Symbol:	Magnitude:	Meaning (multiply by):
Yotta-	Y	1024	1 000 000 000 000 000 000 000 000
Zetta-	Z	1021	1 000 000 000 000 000 000 000
Exa-	E	1018	1 000 000 000 000 000 000
Peta-	P	1015	1 000 000 000 000 000
Tera-	T	1012	1 000 000 000 000
Giga-	G	109	1 000 000 000
Mega-	M	106	1 000 000
myria-	my	104	10 000 (this is now obsolete)
kilo-	k	103	1000
hecto-	h	102	100
deka-	da	10	10
-	-	-	-
deci-	d	10-1	0.1
centi-	c	10-2	0.01
milli-	m	10-3	0.001
micro-	u (mu)	10-6	0.000 001
nano-	n	10-9	0.000 000 001
pico-	p	10-12	0.000 000 000 001
femto-	f	10-15	0.000 000 000 000 001
atto-	a	10-18	0.000 000 000 000 000 001
zepto-	z	10-21	0.000 000 000 000 000 000 001
yocto-	y	10-24	0.000 000 000 000 000 000 000 001

Commonly used metric prefixes

Prefix Name	Prefix Symbol	Prefix Value
giga	G	1 000 000 000	109
mega	M	1 million or 1 000 000	106
kilo	k	1 thousand or 1000	103
hecto	h	100	102
deka	da	10	10

deci	d	1/10 or 0.1	10-1
centi	c	1/100 or 0.01	10-2
milli	m	1/1000 or 0.001	10-3
micro	µ	1/1 000 000 or 0.000 001	10-6
nano	N	1/1 000 000 000 or 0.000 000 001	10-9

Some special relationships:

1 milliliter = 1 cubic centimeter
1 milliliter of water has a mass of approximately 1 gram
1 liter of water has a mass of approximately 1 kilogram
1 cubic meter of water has a mass of approximately 1 metric ton

Legal/official (exact) definitions of inch-pound units as set by U.S. law:

1 inch = 25.4 millimeters
1 pound = 453.592 37 grams (453.6 g)
1 gallon = 3.785 411 784 liters (3.79 L)

Note: In Canada the inch and the pound are defined identically, but 1 Canadian gallon = 4.546 09 liters.

Approximate conversion factors between inch-pound units and the International System of Units (SI):

Multiply inches by 2.54 to get centimeters (this conversion factor is exact)
Multiply feet by 0.305 to get meters
Multiply miles by 1.6 to get kilometers
Divide pounds by 2.2 to get kilograms
Multiply ounces by 28 to get grams
Multiply fluid ounces by 30 to get milliliters
Multiply gallons by 3.8 to get liters

Some baseline temperatures in the three temperature scales

temperature	kelvins	degrees celsius	degrees fahrenheit
symbol	°K	°C	°F
boiling point of water	373.15	100.	212.
average human body temperature		37.	98.6
average room temperature		20. to 25.	68. to 77.
freezing point of water	273.15	0.	32.
absolute zero	0.	-273.15	-459.67

Temperature conversions between the three temperature scales:

kelvin / degree celsius conversions (exact):

kelvins = degrees celsius + 273.15
degrees celsius = kelvins - 273.15

degree Fahrenheit / degree celsius conversions (exact):

degrees F = degrees C x 1.8 + 32.
degrees C = (degrees F - 32.) / 1.8

A degree celsius memory device:

There are several memory aids that can be used to help the novice understand the degree Celsius temperature scale. One such nemoic is:

When it's zero it's freezing,
when it's 10 it's not,
when it's 20 it's warm,
when it's 30 it's hot!

Or, another one to remember:

30's hot
20's nice
10's cold
zero's ice

Examples of everyday item equivalences for metric units of length, mass, volume, and temperature

(Drawings excerpted from the book, Quick Guide to the Metric System, by Valerie Antoine, USMA Executive Director)

Length: 1 meter (1 m)
1 centimeter (1 cm)
1 kilometer (1 km)
Mass: 1 kilogram (1 kg)
1 gram (1 g)
Volume: 1 liter (1 L or 1 l)
1 milliliter (1 mL or 1 ml)
Temperature: degree celsius (°C)