Beginner · 8 min
Measurements and SI units
Choose the right units and convert lengths and times before calculating.
A number needs a unit
A measurement tells us how much of a quantity we have. Its unit tells us what the number means: 5 metres is a length, while 5 seconds is a time. Physicists use a shared system called SI. The metre (m), kilogram (kg), and second (s) are SI units for length, mass, and time. Write the unit next to every measurement, and check that an answer has the unit the question asks for.
Worked example
A desk is 1.2 m long. A bag has a mass of 3 kg. A journey takes 40 s.
Convert before calculating
Use the same units for quantities you compare or combine. One kilometre is 1,000 metres, one metre is 100 centimetres, and one minute is 60 seconds. Multiply when converting to a smaller unit because you need more of those units. Divide when converting to a larger unit. The length or time itself does not change.
Worked example
0.8 km = 0.8 × 1,000 = 800 m 120 s = 120 ÷ 60 = 2 min
Make a measurement record
- Choose a length you can measure with a ruler. Record it in centimetres and convert it to metres.
- Choose a duration in minutes and convert it to seconds. Explain why the number becomes larger.