How to Convert Units in IGCSE Physics

Written by IGCSEPhysics Content Team · Physics subject adviser: K. S. Tan, 15+ years teaching IGCSE Physics · Checked against the Cambridge IGCSE Physics (0625) 2026 to 2028 syllabus

Use this guide to build a reliable unit conversion habit for Cambridge IGCSE Physics 0625.

The essential conversions

Length

From	To	Multiply by
km	m	1000
cm	m	0.01
mm	m	0.001
nm	m	$10^{-9}$

Mass

From	To	Multiply by
g	kg	0.001
mg	kg	$10^{-6}$

Time

From	To	Multiply by
min	s	60
h	s	3600

Area and volume

Area: if 1 m = 100 cm, then 1 m $^2$ = 10 000 cm $^2$ ( $100^2$ ).

Volume: 1 m $^3$ = $10^6$ cm $^3$ ( $100^3$ ).

This is the most commonly failed conversion. To convert cm $^2$ to m $^2$ , divide by $10^4$ . To convert cm $^3$ to m $^3$ , divide by $10^6$ .

Speed

km/h to m/s: divide by 3.6.

m/s to km/h: multiply by 3.6.

The conversion method

Write the value with its original unit.
Multiply by the conversion factor as a fraction: $\frac{\text{new unit}}{\text{old unit}}$ .
Cancel units and calculate.

Example: Convert 750 g to kg.

$750 \text{ g} \times \frac{1 \text{ kg}}{1000 \text{ g}} = 0.75 \text{ kg}$

Compound unit conversions

For density in kg/m $^3$ from g/cm $^3$ :

$\rho \text{ (kg/m}^3\text{)} = \rho \text{ (g/cm}^3\text{)} \times 1000$

Example: Water density = 1.0 g/cm $^3$ = 1000 kg/m $^3$ .

Common errors and how to correct them

Dividing instead of multiplying (or vice versa). Check: converting to a smaller unit should give a larger number.
Forgetting area/volume scaling. $5 \text{ cm}^2 \neq 0.05 \text{ m}^2$ . It equals $5 \times 10^{-4}$ m $^2$ .
Leaving mixed units in an equation. If pressure is in Pa and you need volume in m $^3$ , do not use cm $^3$ .

How to apply this in an exam

Convert ALL values to SI before substituting. Write the conversion step explicitly. This earns method marks even if the final answer is wrong.

Frequently Asked Questions

When should I convert units?

Convert all values to SI base units before substituting into an equation, unless the question specifically asks for a different unit. This prevents most unit errors.

What are the most common unit conversion mistakes?

The three most common are: not converting cm to m (divide by 100), not converting g to kg (divide by 1000), and not converting minutes to seconds (multiply by 60). Area and volume conversions (cm squared to m squared) also cause frequent errors.

Can I use non-SI units if they are consistent?

In some equations, paired units can be retained consistently (e.g. kPa and litres in gas law calculations, or g and cm cubed for density if the answer is needed in g/cm cubed). But SI units are always safe.

Need Help Applying This?

A 0625 specialist can work through the student's current paper or question and help identify whether the main difficulty is content, mathematics, practical reasoning or exam technique.

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