GCSE physics units and prefixes
By Mehmood Kakar · Updated
Quick answer
Convert every value to its standard SI unit before you put it into an equation: mass in kg, distance in m, time in s, energy in J. Then the answer comes out in the standard unit too. Prefixes tell you the multiplier: kilo (k) = × 1000, milli (m) = ÷ 1000, mega (M) = × 1,000,000.
Why units cost marks
Physics equations only work when every value is in the right unit. Put a mass of 500 g into Ek = ½mv² as 500 instead of 0.5 kg and your answer is 1000 times too big. The physics was right; the unit wasn't.
The fix is a habit: before you substitute anything, check each value and convert it to its SI unit. Our solvers do this conversion as a separate line of working, so you can see where each number comes from.
SI units table
These are the standard units for the quantities in GCSE Physics. If every value you substitute is in these units, the answer will be too.
| Quantity | Symbol | Unit | Unit symbol |
|---|---|---|---|
| Distance, displacement, height | s, h | metre | m |
| Mass | m | kilogram | kg |
| Time | t | second | s |
| Speed, velocity | v | metre per second | m/s |
| Acceleration | a | metre per second squared | m/s² |
| Force, weight | F, W | newton | N |
| Energy, work done | E, W | joule | J |
| Power | P | watt | W |
| Gravitational field strength | g | newton per kilogram | N/kg |
| Spring constant | k | newton per metre | N/m |
| Momentum | p | kilogram metre per second | kg m/s |
| Pressure | p | pascal | Pa |
| Density | ρ | kilogram per metre cubed | kg/m³ |
| Current | I | ampere | A |
| Potential difference | V | volt | V |
| Resistance | R | ohm | Ω |
| Charge | Q | coulomb | C |
| Frequency | f | hertz | Hz |
| Wavelength | λ | metre | m |
| Specific heat capacity | c | joule per kilogram degree Celsius | J/kg °C |
| Specific latent heat | L | joule per kilogram | J/kg |
| Magnetic flux density (Higher) | B | tesla | T |
Prefixes from nano to giga
A prefix in front of a unit multiplies it by a power of ten. These are the ones you need for GCSE:
| Prefix | Symbol | Multiply by | Example |
|---|---|---|---|
| giga | G | 10⁹ (1,000,000,000) | 2 GW = 2,000,000,000 W |
| mega | M | 10⁶ (1,000,000) | 3.5 MJ = 3,500,000 J |
| kilo | k | 10³ (1000) | 4.2 kJ = 4200 J |
| centi | c | 10⁻² (0.01) | 65 cm = 0.65 m |
| milli | m | 10⁻³ (0.001) | 250 mA = 0.25 A |
| micro | μ | 10⁻⁶ (0.000001) | 40 μs = 0.00004 s |
| nano | n | 10⁻⁹ (0.000000001) | 500 nm = 0.0000005 m |
Watch the capital letters. A capital M is mega (a million times bigger) and a small m is milli (a thousand times smaller). 5 MW and 5 mW differ by a factor of a billion.
How to convert
One rule covers almost every conversion: going from a big unit to a smaller unit, you multiply; going from a small unit to a bigger unit, you divide.
A kilometre is bigger than a metre, so 3 km becomes a larger number of metres: 3 × 1000 = 3000 m. A gram is smaller than a kilogram, so 60 g becomes a smaller number of kilograms: 60 ÷ 1000 = 0.06 kg. If your converted number moves the wrong way, you multiplied when you should have divided.
Common conversions
| From | To | Do this | Example |
|---|---|---|---|
| g | kg | ÷ 1000 | 500 g = 0.5 kg |
| tonnes | kg | × 1000 | 1.2 t = 1200 kg |
| cm | m | ÷ 100 | 120 cm = 1.2 m |
| mm | m | ÷ 1000 | 35 mm = 0.035 m |
| km | m | × 1000 | 1.7 km = 1700 m |
| minutes | s | × 60 | 5 min = 300 s |
| hours | s | × 3600 | 1.5 h = 5400 s |
| km/h | m/s | ÷ 3.6 | 72 km/h = 20 m/s |
| kJ | J | × 1000 | 125 kJ = 125,000 J |
| kW | W | × 1000 | 2.5 kW = 2500 W |
| mA | A | ÷ 1000 | 300 mA = 0.3 A |
| cm² | m² | ÷ 10,000 | 50 cm² = 0.005 m² |
| cm³ | m³ | ÷ 1,000,000 | 250 cm³ = 0.00025 m³ |
| g/cm³ | kg/m³ | × 1000 | 2.7 g/cm³ = 2700 kg/m³ |
Area and volume traps
Squared and cubed units don't convert by 100. There are 100 cm in a metre, so a square metre is 100 × 100 = 10,000 cm² and a cubic metre is 100 × 100 × 100 = 1,000,000 cm³.
This matters for pressure (p = F ÷ A) and density (ρ = m ÷ V). A force of 20 N on 50 cm² is 20 ÷ 0.005 = 4000 Pa. Dividing 50 by 100 instead gives 0.5 m² and an answer of 40 Pa, a hundred times too small.
Prefixes and standard form
Each prefix matches a power of ten, so you can swap a prefix for standard form straight away. 3.5 MJ is 3.5 × 10⁶ J, 250 mA is 250 × 10⁻³ A (0.25 A), and 500 nm is 5 × 10⁻⁷ m.
On a calculator, use the ×10ˣ or EXP button to enter these, rather than typing × 10 ^ 6. It keeps the power attached to the number and avoids bracket mistakes.
Practice conversions
Convert each value to its SI unit. Click to check.
1. 750 g to kg
0.75 kg
2. 2.4 km to m
2400 m
3. 12 minutes to s
720 s
4. 90 km/h to m/s
25 m/s
5. 4.5 MJ to J (standard form)
4.5 × 10⁶ J
6. 30 cm³ to m³
0.00003 m³ (3 × 10⁻⁵ m³)
Frequently asked questions
What are SI units?
SI units are the standard international units scientists use: metres for length, kilograms for mass, seconds for time, amperes for current and kelvin for temperature. Other units, such as the joule and the newton, are built from these.
Why is the kilogram the SI unit of mass and not the gram?
The kilogram is the base SI unit of mass, so equations like F = ma and Ek = ½mv² only give answers in newtons and joules when mass is in kg. Always convert grams to kilograms by dividing by 1000.
How do I convert km/h to m/s?
Divide by 3.6. One kilometre per hour is 1000 m in 3600 s, which is 1 ÷ 3.6 m/s. So 36 km/h = 10 m/s.
What does the prefix micro mean?
Micro (symbol μ) means one millionth, × 10⁻⁶. A current of 50 μA is 50 × 10⁻⁶ A, or 0.00005 A.