SI Units|International System Of Units

The SI Units or International System of Units is the modern form of the metric system and the world’s most widely used system of measurement, Coordinated by the International Bureau of Weights and Measures.

The SI comprises a coherent system of units of measurement starting with seven base units, and are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity).

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Contents

Basic rules of SI Units

SI units is not different from the metric system, it is just a selection of certain units.
Multiples of a SI unit should increase or decrease by a factor of 1000
Write units in full or denoted by correct symbols
Except for Celsius, unit names are never capitalized
Prefixes and all symbols that indicate ≤ 103 are written in lower case except for proper names
Separate a numerical value and its symbol (e.g., 400 W not 400W)
When a numerical value is used as an adjective, use a hyphen between the value and its symbols (e.g., a 400-W lamp not a 400 W lamp)
Symbols are not changed in the plural form
Unit names follow normal grammatical rules for plural forms;lux, hertz, and siemens are never changed to a plural form
Use a space between units, e.g., km h^-1 not kmh^-1
Division of one unit can be expressed by a solidus or a negative index; only one solidus may be used
Only the numerator should be changed in multiples of 1000; the denominator must remain as the base value; the one exception is kg

Read Also: Physical Quantities and Units

SI Units table

SI Base units table

Symbol	Name	Quantity
s	second	time
m	metre	length
kg	kilogram	mass
A	ampere	electric current
K	kelvin	thermodynamic temperature
mol	mole	amount of substance
cd	candela	luminous intensity

SI units table

Supplementary SI units table

Physical quantity	Unit	Symbol
Plane angle	radian	rad
Solid angle	steradian	sr

Supplementary SI units table

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Derived SI units with special names

Physical quantity	Unit	Symbol	Derivation
Absorbed dose	gray	Gy	J kg^-1
Capacitance	farad	F	A s V^-1
Conductance	siemens	S	AV^-1
Disintegration rate	becquerel	Bq	l s^-1
Electrical charge	coulomb	C	A s
Electrical potential	volt	V	W A^-1
Energy	joule	J	N m
Force	newton	N	kg m s^-2
Illumination	lux	lx	lm m^-2
Inductance	henry	H	V s A^-1
Luminous flux	lumen	lm	cd sr
Magnetic flux	weber	Wb	V s
Magnetic flux density	tesla	T	Wb m^-2
Pressure	pascal	Pa	N m^-2
Power	watt	W	Js^-1
Resistance	ohm	Ω	V A^-1
Volume	liter	L	dm³

Derived SI units with special names

Non-SI units accepted for use with SI

Name	Symbol	Value
Nautical mile	n m-1	1852 m
Knot	kn	1.852 km h^-1
Hectare	ha	10⁴ m²
Millibar	mbar	10² Pa
Curie	Ci	37 GBq
Roentgen	R	2.58 X 10^-4 C kg^-1
Ton	t	10³ kg

Non-SI units accepted for use with SI

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Units prohibited by SI rules

There are some units that are prohibited by SI Rules. Those are given in the Table below.

Physical quantity	Unit	SI value
Energy	calorie, gram	4.184 J
Energy	Btu	1054.35 J
Energy	erg	10^-7J
Force	dyne	10^-5N
Magnetic flux	maxwell	10^-8 Wb
Length	micron	1 µm
Length	millimicron	1 nm
Length	Angstrom	0.1 nm
Luminance	stilb	10⁴ cd m^-2
Conductance	mho	1 S
Photo flux density	Einstein	1 mol

Units prohibited by SI rules

Multiplication Factors for SI units

Note: * Prefixes of 100, 10, 0.1, and 0.01 are not formal SI units

Multiplication Prefix	Symbol	factor
10¹⁸	exa	E
10¹⁵	peta	P
10¹²	tera	T
10⁹	giga	G
10⁶	mega	M
10³	kilo	k
10² *	hecto	h
10¹ *	deka	da
10^-1 *	deci	d
10^-2 *	centi	c
10^-3	milli	m
10^-6	micro	µ
10^-9	nano	n
10^-12	pico	p
10^-15	femto	f
10^-18	atto	a