How to show your working in GCSE physics calculations
By Mehmood Kakar · Updated
Quick answer
Set out every calculation in four lines: write the equation, convert any units, substitute the numbers, then give the answer with its unit, rounded sensibly. Calculation questions are usually marked in steps, so a correct substitution can still earn marks if your final answer goes wrong.
Why working earns marks
A 3-mark calculation is rarely worth 3 marks for the answer alone. Examiners split the marks across the steps: usually one for a correct substitution, one for rearranging or converting, and one for the final answer. Write only the answer and get it slightly wrong, and you lose all three. Write out your steps and you might keep two.
Working also protects you from yourself. Most wrong answers come from one slip, such as a missed square or a unit left in grams. When every step is on the page, you can spot the slip when you check.
The four-line layout
Use the same four lines for every calculation, so it becomes automatic in the exam:
- Equation. Write the equation you are using, in symbols or words. If you need to rearrange it, write the rearranged version on the next line.
- Convert. Change any value that isn't in its standard unit, and write the conversion down: 500 g = 0.5 kg.
- Substitute. Rewrite the equation with the numbers in place of the letters.
- Answer and unit. Write the answer, rounded sensibly, with its unit.
Every solver on this site sets out its working in this order, so you can compare your layout line by line.
A full worked example
Here is a typical 3-mark question, set out in full:
A 1.2 kW motor runs for 5 minutes. Calculate the energy it transfers.
- Equation: P = E ÷ t, so E = P × t
- Convert: P = 1.2 kW = 1200 W; t = 5 min = 300 s
- Substitute: E = 1200 × 300
- Answer: E = 360,000 J
Notice that the conversions have their own line. If you had used 1.2 and 5 directly, you would get 6 J. An examiner reading the working above can see exactly where every number came from. You can check it with the power calculator.
Rounding and significant figures
Round once, at the end. If you round a value halfway through and then use it again, the error grows and your final answer can drift outside the range the mark scheme accepts.
If the question tells you how many significant figures to give, do exactly that. If it doesn't, match the data: when the values in the question are given to 2 or 3 significant figures, give your answer to the same. A train that travels 120 km in 1.5 hours has an average speed of 22.2222… m/s; write 22.2 m/s, not the whole calculator display.
Keep trailing zeros when they count. To 3 significant figures, 5 is written 5.00.
Multi-step questions
Longer questions chain two equations together, such as finding the energy from ΔE = mcΔθ, then the time from P = E ÷ t. Treat each step as its own four-line calculation, and label what you have found: "Energy needed = 504,000 J". Then use that value in the next part.
Carry the unrounded value into the next step. If your calculator shows 22.2222, use that, not 22.2.
If you got the first part wrong, still use your answer in the second part. Examiners can often give credit for correct later working based on an earlier mistake, but only if they can see it.
How working loses marks
- Working that can't be followed. Numbers scattered across the page with no equation make it hard for an examiner to give method marks.
- Skipping the substitution line. Going straight from the equation to the answer hides the step that often carries a mark.
- Crossing out a correct answer. If you change your mind, cross out neatly with one line, and make sure your final answer is clear.
- Contradicting yourself. Writing two different final answers usually means neither gets the mark.
- Leaving off the unit. Some mark schemes give a separate mark for it.
Practice
Set out each answer in four lines, then click to compare.
1. A 60 kg climber rises 20 m. g = 9.8 N/kg. Calculate the gravitational potential energy gained.
Ep = mgh → Ep = 60 × 9.8 × 20 → Ep = 11,760 J (11,800 J to 3 s.f.)
2. A cyclist travels 2.4 km in 8 minutes. Calculate her average speed in m/s.
v = s ÷ t → s = 2400 m, t = 480 s → v = 2400 ÷ 480 → v = 5.0 m/s
3. 0.50 kg of water is heated from 20 °C to 60 °C. c = 4200 J/kg °C. Calculate the energy transferred.
ΔE = mcΔθ → Δθ = 60 − 20 = 40 °C → ΔE = 0.50 × 4200 × 40 → ΔE = 84,000 J
Frequently asked questions
Do I need to show working in GCSE Physics?
You should always show it. Calculation questions are usually marked in steps, so the substitution and any rearranging can earn marks even if the final answer is wrong. A wrong answer with no working usually scores nothing.
How many significant figures should I give?
Use the number the question asks for. If it doesn’t say, match the data you were given, which is usually 2 or 3 significant figures. Never round in the middle of a calculation.
Do I need to write the equation in words or symbols?
Either is fine. Symbols are quicker, such as P = E ÷ t. What matters is that the examiner can see which equation you used.
What if I get the first part of a question wrong?
Use your answer from the first part in the second part anyway, and show the working. Examiners can often credit correct later working that uses an earlier wrong value.