Metric System (SI)

### 1. Introduction

The Système International d'Unités (SI), the modern form of the metric system, is the most widely used system of units and measures around the world.

The SI comprises units and SI prefixes. The prefixes are used to form decimal multiples or sub-multiples of the units.

### 2. Base and Supplementary SI units the ones we use in grade 6 are hi-lited.

The seven Base SI units
Physical quantity Base unit Symbol
length metre m
time second s
mass kilogram kg
electric current ampere A
thermodynamic temperature kelvin K
luminous intensity candela cd
amount of substance mole mol

3. SI Compounds, these are the ones we use in grade 6

Physical quantity Name of SI unit Symbol
area square metre `m²`
volume cubic metre `m³`

### 4. SI Prefixes the ones we use in grade 6 are hi-lited

Prefix multipliers
Prefix Symbol Multiplier Expodential
exa- E 1,000,000,000,000,000,000 1018
peta- P 1,000,000,000,000,000 1015
tera- T 1,000,000,000,000 1012
giga- G 1,000,000,000 109
mega- M 1,000,000 106
kilo- k 1,000 103
hecto- h 100 102
deca- da 10 101
metre m 0 to 9 unit not prefix 100
deci- d 0.1 10-1
centi- c 0.01 10-2
milli- m 0.001 10-3
micro- µ 0.000 001 10-6
nano- n 0.000 000 001 10-9
pico- p 0.000 000 000 001 10-12
femto- f 0.000 000 000 000 001 10-15
atto- a 0.000 000 000 000 000 001 10-18

### 5. Units that may be used with the SI in grade 6

Quantity Name Symbol
time minute min
time hour h
time day d
plane angle degree °
mass metric ton t
volume litre l
speed kilometre per hour km/h
area hectare ha
temperature degree Celsius °C
rotational frequency revolution per minute r/min

### 6. Correct usage of the SI symbols

The SI unitary system differs from some of the older systems in that it has definite rules governing the way the units and symbols are used.

• The name of a unit always starts with a lower case letter, unless it is the first word in a sentence, where normal grammatical rules apply.
• The case of the letters in a symbol is important. For example m and M mean distinctly different things. Mm is megametre (million metre) while mm is millimetre (thousandths of a metre).
• The unit of measure is the metre, not meter. The latter is a device used for measurement. (Unless you live in the USA - in which case you will just have to live with the ambiguity.)