IGCSE Units, Prefixes and Significant Figures
Unit conversion errors lose more marks across the whole paper than any single topic. Areas and volumes are the worst offenders, because a square metre is ten thousand square centimetres, not one hundred.
Every subject penalises the same handful of unit and rounding errors. This sheet gathers the conversions and rules in one place so they can be revised once rather than rediscovered in each topic.
Unit prefixes
| Prefix | Symbol | Multiplier |
|---|---|---|
| nano | n | 10-9 |
| micro | the Greek letter mu | 10-6 |
| milli | m | 10-3 |
| centi | c | 10-2 |
| kilo | k | 103 |
| mega | M | 106 |
| giga | G | 109 |
Conversions that cost marks
| Conversion | Factor | Why it catches people |
|---|---|---|
| cm to m | divide by 100 | Straightforward, but easy to forget entirely |
| cm2 to m2 | divide by 10000 | A square metre is 100 by 100 centimetres |
| cm3 to m3 | divide by 1000000 | A cubic metre is 100 by 100 by 100 centimetres |
| cm3 to litres | divide by 1000 | 1000 cm3 equals 1 litre |
| g to kg | divide by 1000 | Needed before every W = mg calculation |
| g/cm3 to kg/m3 | multiply by 1000 | The direction surprises people |
| minutes to seconds | multiply by 60 | The ampere is defined per second |
| km/h to m/s | divide by 3.6 | Both parts of the compound unit must convert |
| hours and minutes to decimal hours | divide the minutes by 60 | 2 hours 45 minutes is 2.75, not 2.45 |
Significant figures and rounding
| Rule | What it means | Example |
|---|---|---|
| Counting starts at the first non-zero digit | Leading zeros are placeholders and never significant | 0.004683 to 2 s.f. is 0.0047 |
| Zeros between digits count | They are significant wherever they sit inside the number | 2005 has 4 significant figures |
| Decimal places count from the point | Including any zeros after it | 0.004683 to 3 d.p. is 0.005 |
| Round only at the end | Keep full accuracy on the calculator throughout | Rounding early shifts the third figure |
| Match the data | Give the answer to the same accuracy as the values provided | Data to 3 s.f. means an answer to 3 s.f. |
| Default in IGCSE | 3 significant figures unless told otherwise, angles to 1 decimal place | Stated on the front of most papers |
How to use this sheet
- Convert to SI units before substituting into any formula, not afterwards.
- For areas, square the conversion factor. For volumes, cube it.
- Write the unit next to every answer. Unit marks are awarded and lost routinely.
- Keep intermediate values in the calculator memory rather than rewriting rounded numbers.
- If an answer looks absurd, check the units first. Most impossible answers are conversion errors rather than method errors.
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Frequently asked questions
How do I convert square centimetres to square metres?
Divide by 10000, not by 100. A square metre measures 100 centimetres by 100 centimetres, so it contains 100 multiplied by 100, which is 10000 square centimetres. For cubic units the factor becomes 1000000.
Which zeros count as significant figures?
Zeros before the first non-zero digit are placeholders and never significant, so 0.004683 has four significant figures starting at the 4. Zeros between digits always count, and trailing zeros after a decimal point count as significant.
When should I round my answer?
Only at the very end. Keep full accuracy on the calculator throughout the working, using the memory or answer key rather than rewriting rounded values. Rounding at an intermediate step frequently shifts the third significant figure of the final answer.
How many figures should I give?
Three significant figures unless the question says otherwise, with angles given to one decimal place. Where the data in the question is given to fewer figures, match it, since an answer cannot be more accurate than the measurements it came from.
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Other reference sheets
Written to the published Cambridge IGCSE syllabuses. Always check the formula list and data sheet issued with your own paper, since the material provided differs between subjects and syllabus versions. Last reviewed 2026-08-12.