SI units and prefixes look simple until they start costing marks. A calculation can be perfectly set up and still go wrong because milliamps were treated like amps, or a nanometre reading was copied as metres without converting. This guide gives you a practical reference for physics SI units, the prefixes you meet most often, and the conversion habits that make answers safer in GCSE and A-level questions. It is designed to be something you can return to whenever unit conversions in physics start to feel unreliable.
Overview
The SI system is the standard way of expressing physical quantities. In exam questions, it does two jobs at once: it keeps measurements consistent, and it makes calculations manageable across very large and very small scales.
In physics, many mistakes happen not because the idea is difficult, but because the units were not noticed early enough. A student may know that potential difference equals current multiplied by resistance, or that wave speed equals frequency multiplied by wavelength, but still lose marks through a unit mismatch. That is why prefixes matter. They are not decoration attached to a unit; they change the size of the quantity itself.
The prefixes you are most likely to meet regularly are:
- kilo = 103 = 1,000
- mega = 106 = 1,000,000
- milli = 10-3 = 0.001
- micro = 10-6 = 0.000001
- nano = 10-9 = 0.000000001
You will see them attached to common units such as metres, seconds, volts, amps, watts and hertz. For example:
- 1 km = 1,000 m
- 1 mm = 0.001 m
- 1 mA = 0.001 A
- 1 MHz = 1,000,000 Hz
- 1 ns = 0.000000001 s
At GCSE, this often appears in electricity, waves, motion and practical work. At A-level, it becomes even more important in electronics, particle physics, materials, quantum ideas and data analysis. If you want a broader view of specification differences, see AQA vs Edexcel vs OCR Physics: Key Differences in GCSE Topics, Exams and Formula Use and AQA vs Edexcel vs OCR A-Level Physics: Specification and Assessment Comparison.
The useful mindset is this: before you calculate anything, ask whether the units belong together. That one habit prevents a surprising number of avoidable errors.
Core framework
Here is the core method to use whenever you are revising physics prefixes or solving a calculation question under time pressure.
1. Separate the unit from the prefix
Take a quantity like 5 mA. The base unit is ampere, A. The prefix is milli, meaning 10-3. So:
5 mA = 5 × 10-3 A = 0.005 A
Do the same mentally for every quantity you read. This makes it easier to compare values that were originally written in different forms.
2. Know the direction of the conversion
A reliable rule is:
- Going from a large-prefixed unit to the base unit means the number gets bigger.
- Going from a small-prefixed unit to the base unit means the number gets smaller.
Examples:
- 3 km to m: the number gets bigger, so 3,000 m
- 3 mm to m: the number gets smaller, so 0.003 m
This sounds obvious, but it is a useful sense-check. If your answer says 2 mm = 2,000 m, you know immediately that the decimal has gone the wrong way.
3. Use powers of ten instead of memorising decimals
For quick physics units revision, powers of ten are often safer than strings of zeros.
- kilo = 103
- mega = 106
- milli = 10-3
- micro = 10-6
- nano = 10-9
Then a conversion becomes a change in index rather than a guess with decimal places.
For instance:
250 nm = 250 × 10-9 m = 2.5 × 10-7 m
This style is especially useful at A-level, where standard form appears frequently.
4. Convert before substituting if units differ
If a formula expects standard SI units, convert first. This is usually the safest route in exam conditions.
Suppose:
- Current = 250 mA
- Resistance = 12 Ω
To find potential difference using V = IR:
Convert current first:
250 mA = 0.250 A
Then calculate:
V = 0.250 × 12 = 3.0 V
If you use 250 directly as though it were amps, the answer becomes wildly unrealistic.
5. Keep a short prefix table in memory
You do not need every possible SI prefix for most school physics questions. A compact working set is enough:
| Prefix | Symbol | Power of ten |
|---|---|---|
| kilo | k | 103 |
| mega | M | 106 |
| milli | m | 10-3 |
| micro | µ | 10-6 |
| nano | n | 10-9 |
Notice the symbols carefully. M for mega is not the same as m for milli. That single letter changes a quantity by a factor of one billion between 106 and 10-3.
6. Watch for common base units in physics
Students often revise prefixes in isolation, but it helps more to attach them to topics where they appear:
- Electricity: mA, kΩ, kWh
- Waves: nm, Hz, kHz, MHz
- Particle and nuclear ideas: nm and smaller scales in context
- Mechanics: mm, cm, km, ms
- Practical work: cm converted to m, g converted to kg, ms converted to s
If you are building a larger revision routine, topic-based practice is usually more effective than learning prefix facts on their own. These collections can help: GCSE Physics Topic Questions by Topic and A-Level Physics Topic Questions by Topic.
Practical examples
The fastest way to get confident is to see the same idea working in different contexts. Below are examples that reflect the sort of conversions students meet in exam questions and practical write-ups.
Example 1: milliamps to amps
A circuit has a current of 35 mA. Express this in amps.
milli means 10-3, so:
35 mA = 35 × 10-3 A = 0.035 A
Answer: 0.035 A
Example 2: kilometres to metres
A runner travels 2.4 km. Express this distance in metres.
kilo means 103, so:
2.4 km = 2.4 × 103 m = 2400 m
Answer: 2400 m
Example 3: nanometres in wave questions
A wavelength is 650 nm. Express this in metres.
nano means 10-9, so:
650 nm = 650 × 10-9 m = 6.50 × 10-7 m
Answer: 6.50 × 10-7 m
This kind of conversion appears often in light and electromagnetic spectrum work.
Example 4: frequency in megahertz
A radio transmission has frequency 95.8 MHz. Express this in hertz.
mega means 106, so:
95.8 MHz = 95.8 × 106 Hz = 9.58 × 107 Hz
Answer: 9.58 × 107 Hz
Example 5: using converted units in a formula
A resistor of 4.7 kΩ carries a current of 2.0 mA. Find the potential difference.
Use V = IR.
First convert both quantities:
- 4.7 kΩ = 4.7 × 103 Ω = 4700 Ω
- 2.0 mA = 2.0 × 10-3 A = 0.0020 A
Now calculate:
V = 4700 × 0.0020 = 9.4 V
Answer: 9.4 V
Notice how neat the answer becomes once the units are standardised.
Example 6: milliseconds to seconds in motion
A sensor records a time interval of 25 ms. Express it in seconds.
milli means 10-3, so:
25 ms = 25 × 10-3 s = 0.025 s
Answer: 0.025 s
Example 7: micro as a special case
A capacitor stores charge for 220 µs. Express this in seconds.
micro means 10-6, so:
220 µs = 220 × 10-6 s = 2.20 × 10-4 s
Answer: 2.20 × 10-4 s
Some students find micro harder because the symbol µ is less familiar than m or k. It is worth revisiting regularly until it feels normal.
A quick exam routine for worked calculations
- Underline every unit in the question.
- Convert unusual prefixes into base units before substitution.
- Write the converted values clearly, with units.
- Substitute into the formula.
- Check whether the final unit makes sense.
This routine supports good physics exam technique as much as it supports recall. If you struggle to turn understanding into marks, it also helps to review Physics Command Words Explained and GCSE Physics Formula Triangle Alternatives.
Common mistakes
Most errors with kilo mega milli micro nano in physics are consistent, which is good news: once you know the patterns, you can catch them early.
Confusing the symbol with the unit
In 3 mA, the m is not metres. It is the prefix milli attached to amps. Always read the whole symbol together, not letter by letter.
Moving the decimal in the wrong direction
This is probably the most common mistake. If you convert mm to m, the number should get smaller. If you convert km to m, the number should get bigger. Build in that reasonableness check every time.
Ignoring unit conversion because the formula looks familiar
Students often rush into a well-known equation and only notice later that one value was in milliseconds or kilohms. The safer habit is to pause before substitution, even for easy formulas.
Mixing uppercase and lowercase symbols
Uppercase and lowercase matter:
- M = mega = 106
- m = milli = 10-3
A wrong capital letter creates a huge error, not a small slip.
Forgetting standard form skills
At A-level, some conversions are easiest in standard form. If 500 nm becomes 500 × 10-9 m, it is often cleaner to rewrite it as 5.0 × 10-7 m. That reduces calculator mistakes and matches the style of many mark schemes.
Using mixed units inside one answer
For example, writing distance in cm and speed in m/s without converting the distance first. Examiners usually reward physics done in consistent units. In practical work, this matters just as much as in theory questions. If you are revising methods alongside calculations, see GCSE Physics Required Practicals and A-Level Physics Required Practicals Revision Guide by Exam Board.
Memorising a list without using it
Prefix knowledge becomes reliable only when it is used in context. A short daily set of conversion drills is more effective than staring at a table once a week.
When to revisit
This topic is worth revisiting any time your marks suggest that calculations are failing for avoidable reasons rather than conceptual ones. SI units and prefixes are not a one-off memory task; they are a maintenance skill.
Come back to this guide when:
- you are starting electricity, waves or practical measurements
- you notice wrong answers that are off by factors of 1,000 or 1,000,000
- you begin using standard form more often
- you move from GCSE to A-level and meet a wider range of scales
- you are practising past-paper calculations and want to reduce dropped marks
A sensible revision plan is to treat units as a layer that sits underneath every topic, not as a separate chapter to finish and forget. If you are organising your revision more broadly, these guides can help you place this skill at the right points in your timetable: Best Order to Revise GCSE Physics Topics Before Mocks and Final Exams and Best Order to Revise A-Level Physics Topics for Year 12 and Year 13.
For a practical next step, do this:
- Write the five key prefixes on one flashcard: kilo, mega, milli, micro, nano.
- Add the symbols and powers of ten.
- Practise ten mixed conversions without a calculator.
- Then solve three formula questions where at least one value must be converted first.
- Mark your work by checking units before checking arithmetic.
If you repeat that routine a few times, physics units revision stops feeling like a memorisation problem and starts becoming an automatic exam habit. That is the real goal: not just knowing what kilo or nano means, but spotting immediately when a question depends on converting them correctly.
