GCSE Physics Calculations: A Step-by-Step Method

Many GCSE physics marks sit in calculation questions, and many of those marks are lost in the same few places: a time left in minutes, a kilojoule never converted, an equation rearranged the wrong way, or an answer written with no unit. None of that is about being "bad at maths". It is about not having a routine.

This guide gives you a five-step method you can use on any calculation, a fully worked example, and a short practice plan you can start tonight.

What the exam expects you to do

The AQA GCSE Physics specification lists the equations students should be able to recall and apply "using standard SI units". Its mathematical requirements include changing the subject of an equation, substituting values using appropriate units, and using an appropriate number of significant figures. If you sit a different exam board, check its own specification, but the skills are very similar.

You may also be given an equation sheet. AQA confirmed in September 2026 that its physics equations sheet will continue to be provided as an insert with the GCSE Physics and Combined Science physics papers in 2027, and that sheets will continue from 2028 onwards. A sheet helps, but it does not do the work for you. You still have to pick the right equation, convert the units, rearrange and round. Those are the steps where marks go missing.

The five-step method

Use the same five steps every time, even on easy questions, so the routine is automatic when the question is hard.

1. List what you know. Write each value with its symbol and unit, for example "E = 180 kJ" and "t = 2 min". Then write what you need to find with a question mark.
2. Convert to SI units. Change kilojoules to joules, minutes to seconds, grams to kilograms, centimetres to metres. Do this before you put anything into an equation.
3. Choose the equation. Find the one equation that links the values you know to the value you need. If it is on the sheet, copy it exactly. If you need to recall it, write it from memory first.
4. Rearrange, then substitute. Make the unknown the subject of the equation using symbols, then put the numbers in. Rearranging first keeps the algebra tidy and makes errors easier to spot.
5. Answer with a unit and a sense check. Round to a sensible number of significant figures, write the unit, and ask whether the size of the answer is believable.

Show each step on the page. Clear working makes your own mistakes far easier to find when you check your paper, and it shows the examiner exactly what you did.

Worked example: a kettle's power

Question: A kettle transfers 180 kJ of energy in 2 minutes. Calculate its power.

Step 1, list what you know: E = 180 kJ, t = 2 min, P = ?

Step 2, convert: 180 kJ = 180 000 J. 2 min = 120 s.

Step 3, choose the equation: power = energy transferred ÷ time, or P = E ÷ t.

Step 4, rearrange and substitute: P is already the subject, so P = 180 000 ÷ 120.

Step 5, answer and check: P = 1500 W. A typical kettle is rated at a few thousand watts, so 1500 W is believable.

Now see what happens if Step 2 is skipped. 180 ÷ 2 = 90, which would give a power of 90 W. That is less than many light bulbs, so the sense check flags it straight away.

A second example: rearranging first

Question: A ball of mass 0.8 kg has 10 J of kinetic energy. Calculate its speed.

The equation is kinetic energy = 0.5 × mass × speed², or Ek = ½ m v². The unknown is v, so rearrange before you substitute:

1. Multiply both sides by 2: 2Ek = m v².
2. Divide both sides by m: v² = 2Ek ÷ m.
3. Take the square root: v = √(2Ek ÷ m).

Now substitute: v = √(2 × 10 ÷ 0.8) = √25 = 5 m/s.

The most common slip here is forgetting the square root and writing 25 m/s. Checking that you have found v, not v², takes two seconds.

The four mistakes to watch for

• Units not converted: kJ, minutes, grams and centimetres are the usual suspects. Circle every unit in the question before you start.
• Wrong equation: two equations can look similar. Check that every quantity in the one you chose matches something in the question.
• Rearranging with numbers: moving numbers around halfway through a calculation invites errors. Rearrange with symbols, then substitute.
• No unit or wrong rounding: write the unit every time, and round only at the end, to a sensible number of significant figures, usually the same as the data in the question.

A practice plan for this week

1. Pick six equations from your specification that you find hardest to use.
2. For each one, write two short questions: one where the unknown is already the subject, and one that needs rearranging.
3. Answer all twelve using the five steps, showing every line.
4. Mark them, and for each error, write which step it happened in. This turns a wrong answer into a specific fix, which is the idea behind our guide on learning from practice-test mistakes.
5. Two days later, answer the same questions again without looking at your first attempt.

After a week, look at which step most of your errors came from. If it is Step 2, spend a session only on unit conversions. If it is Step 4, practise rearranging with symbols until it feels routine.

The short version

Physics calculations reward a habit more than a talent. List, convert, choose, rearrange, check. Use the five steps on every question, keep your working visible, and let the sense check catch mistakes before the examiner does.