SI, Metric and Derived Unit

metricsScientists use standards for the basic units of measurement. These standards allow scientists to make accurate and precise measurements that are reproducible and unchanging.

Measurements in the scientific world are expressed by the metric system or SI system. The metric system is common system based on multiples of ten.

The SI system is just a modernized version of the metric system adopted in 1964 by the IUPAC and is based on fundamental units. All the other units are derived from the fundamentals.

People often misuse the terms accuracy and precision and it is very important the you understand what each term means.

Accuracy is how close the data matches the true value.

Precision is how well the data can be reproduced, that is repeated.

Believe it or not it is very possible to be precise, yet still be wrong. If your data is repeatable but not in line with the true value you are precise but not accurate. Many times a scientist will be wrong because the tools he is using are defective or imperfect. It is this very reason that many scientists work to improve scientific tools as much as the work on phenomena.

The SI System

The SI system (Systeme International d'Unites) seven fundamental units and 22 derived units. It also allows for a few other units such as time but the cgs system is nit allowed.

 Property  Unit  Symbol
 length meter  m
 mass kilogram  kg
 time  second  s
 temperature  Kelvin  K
 electric current Ampere  A
 luminous intensity  candela  cd
 amount of a substance  mole mol 


The Metric System

The metric system is a common measuring system based on powers of ten. Its fundamental units are the gram, meter and liter. In 1875 the worldwide scientific community accepted the metric system as the standard for science.


 Property  Unit  Symbol
 length  meter  m
 mass  gram g
 volume  liter  L
 time  second  s
 temperature  Celsius  oC


Both the metric and the SI systems are based on the decimal system (powers of ten) and make use of prefixes to indicate fractions and multiples of ten. The same prefixes are used in both systems.

  Prefixes for Large
 Prefix  symbol meaning  value
 giga  G  billion  109
 mega  M  million  106
 kilo  k  thousand  103


  Prefixes for Small
 Prefix  symbol meaning  value
 deci  d  tenths  10-1
 centi  c  hundredths  10-2
 milli  m  thousandths  10-3
micro  µ  millionths  10-6
 nano  n billionths  10-9

Conversion Rule:

  • large unit to small unit = multiply by the conversion

      move the decimal to the right

  • small unit to large unit = divide by the conversion

      move the decimal to the left