The Metric System -

An easy guide

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The SI system of measurement

SI (which is the abbreviation in many languages for Systeme International d'Unites) is an extension and refinement of the traditional metric system. It has features which make it logically superior to any other system as well as practically more convenient; it is rational, coherent and comprehensive.

The main features of SI are as follow:

1. There are six basic units (see below), the metre and kilogramme taking the place of the centimetre and gramme of the old metric system.

2. The unit of force, the newton (kg m s-2) is independent of the Earth's gravitation, and the often confusing introduction, in some branches of science and technology, of g into equations is no longer necessary.

3. The unit of energy in all forms is the joule (newton x metre), and of power the joule per second (watt); thus the variously defined calories, together with the kilowatt hour, the Btu and the horsepower are all superseded.

4. 'Electrostatic' and 'electromagnetic' units are replaced by SI electrical units.

5. Multiples of units are normally to be restricted to steps of a thousand and similarly fractions to steps of a thousandth.

Basic SI units

 

physical quantity

name of unit

symbol for unit

length

metre

m

mass

kilogramme

kg

time

second

s

electric current

ampere

A

thermodynamic temperature

degree Kelvin

K

luminous intensity

candela

cd

Symbols for units do NOT take a plural form

   Supplementary units (These units are dimensionless)

physical quantity

name of unit

symbol for unit

plane angle

radian

rad

solid angle

steradian

sr

Derived units with special names

physical quantity

name of unit

symbol for unit

definition of unit

energy

joule

J

kg m2/s2

force

newton

N

kg m/s2 = J/m

power

watt

W

kg m2 /s3 = J/s

electric charge

coulomb

C

A s

electric potential difference

volt

V

kg m2/s3 A = J/As

electric resistance

ohm

W

W kgm2/s3A2 = V/A

electric capacitance

farad

F

A2 s4 1kg m2 = A s/V

magnetic flux

weber

Wb

kgm2/s2A = Vs

inductance

henry

H

kg m2 /s2 A2 = V s/A

magnetic flux density

tesla

T

kg/s2 A2= V s/m2

luminous flux

lumen

lm

cd sr

illumination

lux

lx

cd sr/m2

frequency

hertz

Hz

1/second

customary temperature

degree Celsius

0C

t/0C = T/K-273.15

conductance

siemen

S

A/V

Examples of other derived units

physical quantity

name of unit

symbol for unit

area

square metre

m2

volume

cubic metre

m3

density

kilogramme per cubic metre

kg/m3

velocity

metre per second

m/s

angular velocity

radian per second

rad/s

acceleration

metre per second squared

m/s2

pressure

newton per square metre

N/m2

dynamic viscosity

newton second per square metre

N s/m2

electric field strength

volt per metre

V/m

magnetic field strength

ampere per metre

A/m

luminance

candela per square metre

cd/m2

The Metric system

In the metric system of measurements, the principal unit for length is the metre; the principal unit for capacity, the litre; and the principal unit for weight, the gramme. The following prefixes are used for fractions and multiples:

 

 

multiple fraction

prefix

symbol

1012

tera

T

109

giga

G

106

mega

M

103

kilo

k

102

hecto

h*

10

deka

da*

10-1

deci

d

10-2

centi

c

10-3

milli

m

10-6

micro

m

10-9

nano

n

10-12

pico

p

10-15

femto

f

10-18

atto

a

*To be restricted to instances where there is a strongly felt need.

 

All the multiples and subdivisions are not used commercially. Those used ordinarily are:

Measures of length

10 millimetres (mm) = 1 centimetre (cm)

10 centimetres = 1 decimetre (dm)

10 decimetres = 1 metre (m)

1000 metres = 1 kilometre (km)

Square measure

100 square millimetres (mm2 ) = 1 square centimetre (cm2)

100 square centimetres = 1 square decimetre (dm2)

100 square decimetres = 1 square metre (m2)

  Surveyor's square measure

100 square metres (m2) = 1 are (ar)

100 ares = 1 hectare (har)

100 hectares = 1 square kilometre (km2)

  Cubic measure

1000 cubic millimetres (mm3) = 1 cubic centimetre (cm3)

1000 cubic centimetres = 1 cubic decimetre (dm3)

1000 cubic decimetres = 1 cubic metre (m3)

  Dry and liquid measure

10 millilitres (ml) = 1 centilitre (cl)

10 centilitres = 1 decilitre (dl)

10 decilitres = 1 litre (l)

100 litres = 1 hectolitre (hl)

1 litre = 1 cubic decimetre = the volume of 1 kilogramme of pure water at a temperature of 39.20F.

  Measures of weight

10 milligrammes (mg) = 1 centigramme (cg)

10 centigrammes = 1 decigramme (dg)

10 decigrammes = 1 gramme (g)

1000 kilogrammes (kg) = 1 (metric) tonne (t)

Metric and Imperial conversion tables

Linear measure

1 kilometre = 0.621371 mile

1 metre = 3.28084 feet = 39.3701 inches = 1.09361 yards

1 centimetre = 0.393701 inch = 0.032808 feet

1 millimetre = 0.0393701 inch

1 micron (m) = (0.001 millimetre) = 0.00004 inch

 

1 mile = 1.609344 kilometres

1 yard = 0.9144 metre

1 foot = 0.3048 metre

1 foot = 304.8 millimetres

1 inch = 2.54 centimetres

1 inch = 25.4 millimetres

1 micro-inch = 0.025m

Square measure

1 square kilometre = 0.386102 square miles = 247.1 acres

1 hectare = 2.471 acres = 107,640 square feet

1 are = 0.0247 acre = 1076.4 square feet

1 square metre = 1550.00 square inches = 10.7639 square feet = 1.19599 square yards

1 square centimetre = 0.155000 square inch = 0.00107639 square feet

1 square millimetre = 0.00155000 square inch

 

1 square mile = 2.58999 square kilometres

1 acre = 0.4047 hectare = 40.47 ares

1 square yard = 0.836127 square metres

1 square foot = 0.0929030 square metre = 929.030 square centimetres

1 square inch = 6.4516 square centimetres = 645.16 square millimetres

  Cubic measure

1 cubic metre = 35.315 cubic feet = 1.308 cubic yards

1 cubic metre = 264.2 US gallons = 219.969 Imperial gallons

1 cubic metre = 6.10237x10-4 cubic inch = 35.3147 cubic feet = 1.30795 cubic yards

1 cubic centimetre = 0.0610237 cubic inch = 3.53147x10-5 cubic foot

1 litre (cubic decimetre) = 0.0353157 cubic foot = 61.0255 cubic in.

1 litre = 0.2642 US gallon = 1.0567 US quarts = 0.2200 Imperial gallon

 

1 cubic yard = 0.764555 cubic metre

1 cubic foot = 0.02832 cubic metre = 28.317 litres

1 cubic inch = 16.38706 cubic centimetres = 16.3866 millilitres

1 Imperial gallon = 4.54609 litres

1 Imperial quart = 1.136 litres

1 US gallon = 3.785 litres

1 US quart = 0.946 litre

  Weight

1 metric tonne = 0.9842 ton (of 2240 pounds) = 2204.6 pounds

1 kilogramme = 2.2046 pounds = 35.274 ounces avoirdupois

1 gramme = 0.03215 ounce troy = 0.03527 ounce avoirdupois

1 gramme = 15.432 grains

 

1 ton (of 2240 pounds) = 1.01605 metric tonnes = 1016.05 kilogrammes

1 pound = 0.4536 kilogramme = 453.6 gramme

1 ounce avoirdupois = 28.3495 grammes

1 ounce troy = 31.1035 gramme

1 grain = 0.0648 gramme

Various

1 kilogramme per square millimetre = 1422.32 pounds per sq.in.

1 kilogramme per square centimetre 14.223 pounds per sq. in.

1 kilogramme/metre = 7.233 foot/pounds

 

1 pound per square inch = 0.0703 kilogramme per square centimetre

1 calorie (kilogramme calorie) = 3.968 Btu (British thermal units)

1 kilojoule = 0.948 Btu

1 kilopond (kp) = 1 kilogramme

 

Last edited: 11/03/10

Tangram Technology Ltd. 1999

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