SI units and measurement

Scientists everywhere use the same units (SI units) so results can be shared and compared. Prefixes like kilo and milli make very big and very small values easier to write.

Key facts

  • Base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol).
  • Derived units: newton (N), joule (J), watt (W), volt (V), pascal (Pa).
  • Prefixes: mega (M) 10⁶, kilo (k) 10³, centi (c) 10⁻², milli (m) 10⁻³, micro (µ) 10⁻⁶, nano (n) 10⁻⁹.
  • Give answers to a sensible number of significant figures (match the data).

Worked examples

Example 1

Write 4.7 km in metres and 350 mA in amps.

  1. kilo = × 1000: 4.7 × 1000 = 4700 m
  2. milli = ÷ 1000: 350 ÷ 1000 = 0.35 A

Answer: 4700 m and 0.35 A

Example 2

Write 0.000 52 m in standard form and in µm.

  1. 5.2 × 10⁻⁴ m
  2. 1 µm = 10⁻⁶ m, so divide by 10⁻⁶. 520 µm

Answer: 5.2 × 10⁻⁴ m = 520 µm