Common Unit Conversions for IGCSE Physics

Many IGCSE Physics questions give values in non-SI units. Converting before substituting into an equation prevents errors.

Length

• 1 km = 1000 m
• 1 cm = 0.01 m (divide by 100)
• 1 mm = 0.001 m (divide by 1000)

Example: 45 cm = 45 / 100 = 0.45 m.

Mass

• 1 kg = 1000 g
• To convert grams to kilograms: divide by 1000

Example: 250 g = 250 / 1000 = 0.25 kg.

Time

• 1 minute = 60 s
• 1 hour = 3600 s
• To convert minutes to seconds: multiply by 60

Example: 2.5 hours = 2.5 x 3600 = 9000 s.

Area

• 1 cm^2 = 1 x 10^-4 m^2 (divide by 10000)
• 1 mm^2 = 1 x 10^-6 m^2

Example: 5.0 cm^2 = 5.0 x 10^-4 m^2.

Volume

• 1 cm^3 = 1 x 10^-6 m^3
• 1 litre = 1 x 10^-3 m^3 = 1000 cm^3

Energy

• 1 kJ = 1000 J
• 1 MJ = 1 000 000 J = 1 x 10^6 J
• 1 kWh = 3.6 x 10^6 J = 3600 kJ

Power

• 1 kW = 1000 W
• 1 MW = 1 x 10^6 W

Pressure

• 1 kPa = 1000 Pa

Temperature

• Temperature in kelvin = temperature in Celsius + 273

Example: 25 degrees C = 25 + 273 = 298 K.

General rules

1. Always convert to SI base units before substituting into equations.
2. Show the conversion step in your working; it earns method marks even if the final answer is wrong.
3. When a question asks for an answer in a specific unit, convert at the end.