# Difference between revisions of "Systems of units"

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## Systems of units

The International System of Units (SI, from the French Système international (d'unités)) is the most widely used system of measurement. It has now been adopted accross the world, except in Liberia, Myanmar and the United States. The US is the only industrialized nation that does not mainly use the SI in its commercial and standards activitiesWikipedia, International System of Units. The SI is available here, free of charge. (The English version follows the French version.)

In the SI, units are classified as follows (with reference to the table in the SI):

• Base units (Table 1)
• Coherent derived units (Table 2)
• Coherent derived units with special names and symbols (Table 3)

Note that coherent derived units can include coherent derived units with special names and symbols (Table 4)

• Non-SI units accepted for use with the SI (Table 6)
• Non-SI units whose values in SI units must be obtained experimentally (Table 7)
• Other non-SI units (Table 8)
• Non-SI units associated with the CGS and the CGS-Gaussian system of units (Table 9)
• Other non-SI units not recommended of use (including the British imperial system and the United States customary system of units)

Conversion factors for the later units are available from the US National Institute of Standards and Technology (NIST):

### SI base units

There are seven base units in the SI:

Name Symbol Formulas question
input
metre m m
kilogram kg kg
second s s
ampere A A
kelvin K K
mole mol mol
candela cd cd

### Coherent derived units

Coherent derived units are products of powers of base units, without a numerical factor. Some examples of coherent derived units are as follows:

Name Symbol Formulas question
Input from either column is acceptable
square meter m2 m^2
cubic meter m3 m^2
meter per second m/s m/s m s^(-1)
meter per second squared m/s2 m/s^2 m s^(-2)
reciprocal metre m-1 m^(-1)
kilogram per cubic metre kg/m3 kg/m^3 kg m^(-3)
kilogram per square metre kg/m2 kg/m^2 kg m^(-2)
cubic metre per kilogram m3/kg m^3/kg m^3 kg^(-1)
ampere per square metre A/m2 A/m^2 A m^(-2)
etc.

### Coherent derived units with special names and symbols

Some coherent derived units have special names and symbols:

Name Symbol Formulas question
Input from either column is acceptable
hertz Hz Hz s^(-1)
newton N N m kg s^(-2)
pascal Pa Pa N/m^2 m^(-1) kg s^(-2)
joule J J N m m^2 kg s^(-2)
watt W W J/s m^2 kg s^(-3)
coulomb C C s A
volt V V W/A m^2 kg s^(-3) A^(-1)
ohm Ω
siemens S S
weber Wb Wb
tesla T T
henry H H
degree Celcius
lumen lm lm
lux lx lx
becquerel Bq
gray Gy
sievert Sv
katal kat

The base and coherent derived units of the SI form a coherent set, designated the set of coherent SI units. There are also units outside the SI.

### Non-SI units accepted for use with the SI

Non-SI units accepted for use with the SI are:

Name Symbol Formulas question
input
minute min min
hour h h
day d d
degree (plan angle) °
minute (plan angle)
second (plan angle)
hectare ha ha
litre L, l L or l
tonne t t

## SI Prefixes

The SI and Formulas question prefixes are given in the table below. The only difference between the two systems is that the prefix micro is written μ in the SI but entered as u in the Formulas question.

Factor Name SI symbol Formulas question
input
Factor Name SI symbol Formulas question
input
101 deca da da 10-1 deci d d
102 hecto h h 10-2 centi c c
103 kilo k k 10-3 milli m m
106 mega M M 10-6 micro μ u
109 giga G G 10-9 nano n n
1012 tera T T 10-12 pico p p
1015 peta P P 10-15 femto f f
1018 exa E E 10-18 atto a a
1021 zetta Z Z 10-21 zepto z z
1024 yotta Y Y 10-24 yocto y y

## Formulas question system of units

The system of units of the Formulas question type complies with the SI. Formulas question units can be any of all the units mentioned above, i.e. SI units and non-SI units.

Formulas question units can be either single or combined.
Formulas question
unit
Type
Examples
Name of unit SI symbol Formulas question
input
Single (i.e. individual) unit SI base unit meter m m
kilogram kg kg
second s s
Coherent derived SI units with special names and symbols hertz Hz Hz
newton N N
pascal Pa Pa
Non-SI units accepted for use with the SI minute min min
hour h h
day d d
Non-SI units whose values in SI units must be obtained experimentally electronvolt eV eV
dalton Da Da
unified atomic mass unit u u
Other non-SI units bar bar bar
millimetre of mercury mmHg mmHg
ångström Å Å (Ascii 143)
Non-SI units associated with the CGS and the CGS-Gaussian system of units erg erg erg
dyne dyn dyn
poise P P
Other non-SI units not recommended for use barrel of oil bbl bbl
inch inch in
foot ft ft
Combined unit (any combination of single units) All types square meter m2 m^2
meter per second m/s m/s
meter per second squared m/s2 m/s^2
hertz expressed in terms of SI base units s-1 s^(-1)
newton expressed in terms of SI base units m kg s-2 m kg s^(-2)
pascal expressed in terms of SI base units m-1 kg s-2 m^(-1) kg s^(-2)
pascal second Pa s Pa s