Using units

Standard units

Scientists use the International System of Units - SI units. The table shows some common units.

QuantityUnit nameUnit symbol
Lengthmetrem
Masskilogramkg
Timeseconds
Temperaturedegree Celsius °C
Pressurepascal Pa
Energyjoule J
Currentamp A

Prefixes

Scientists measure a wide range of quantities, some very large and some very small. Measurements for different orders of magnitude have specific prefixes:

Prefix namePrefix symbolFactorPower
tera T1,000,000,000,000 ×1012
giga G1,000,000,000 ×109
mega M1,000,000 ×106
kilo k 1,000 ×103
centi c0.01×10-2
milli m0.001×10-3
micro μ 0.000 001 ×10-6
nano n 0.000 000 001 ×10-9

Converting units

To convert between a unit with difference prefixes, multiplication or division of the right factor of ten (10, 100, 1,000) is used.

Always do a sense check when multiplying or dividing and ask, “Is the answer expected to be larger or smaller?”

Question

Convert 5 cm into mm.

1 cm = 10 mm so the conversion is ×10

5 × 10 = 50 mm

This is what would be expected because millimetres are smaller than centimetres.

Time is dealt with differently if it is recorded in minutes, as this is not an SI unit:

  • 1 minute = 60 s

Worked example

Convert 2 minutes 40 s into s

2 minutes = 2 × 60 = 120 s

2 minutes 40 s = 120 + 40

= 160 s