How to Solve Calculation Questions in Cambridge IGCSE Physics (0625)

Calculation questions in Cambridge IGCSE Physics (0625) are often lost before the calculator is used: the wrong quantity is identified, an equation is chosen too quickly, units are mixed, algebra is reversed or the final answer is rounded badly. A reliable calculation starts by turning the wording into a controlled sequence that you can check at every stage. The current 2026–2028 syllabus makes several expectations explicit. Students should be able to use standard form, substitute values into equations using consistent units, solve simple equations for an unknown term, round only the final answer, use decimal places and significant figures appropriately, and make estimates to judge whether an answer is reasonable. That is a much better foundation than memorising a rule such as "always give three significant figures".

A five-step routine for almost every calculation

A useful routine is: identify the quantities, choose the relationship, make the units consistent, calculate with visible working, then finish with the answer and a reasonableness check. The exact Physics changes from question to question, but this sequence keeps the method stable.

Extract the quantities before touching the calculator

Start by turning the sentence into symbols, values and units. If a question gives a mass of 450 g and a speed of 12 m/s and asks for kinetic energy, write m = 450 g, v = 12 m/s and Eₖ = ?. This immediately exposes the unit problem: the mass is not yet in kilograms. It also stops you from grabbing an equation simply because it contains numbers you recognise. Some information is hidden in the wording rather than printed as a number. "Starts from rest" means the initial velocity is zero. A graph question may require you to obtain a gradient or an area before the calculation can begin. The useful habit is to ask: what quantity does every number represent, and which quantity am I actually trying to find?

Choose the physical relationship before substituting numbers

Writing the equation first is more than presentation. It is a check that you have identified the correct Physics. If a question is about electrical energy, you might need E = VIt. If it is about density, use ρ = m/V. If it asks for pressure, p = F/A may be relevant. The equation should come from the situation, not from whichever formula happens to contain the same symbols. This is especially important in questions that look similar. Speed and acceleration both involve time, but speed uses distance divided by time while acceleration uses change in velocity divided by time. On a speed–time graph, the gradient gives acceleration and the area under the graph gives distance. The calculation becomes much safer once the physical meaning is settled before the arithmetic begins.

Make the units consistent, not automatically "SI everything"

The model response correctly identifies unit conversion as one of the biggest traps, but there is an important refinement: you do not need to convert every value blindly into base SI units. You need units that are consistent with each other and with the relationship you are using. Cambridge explicitly expects students to substitute values using consistent units. For example, using 450 g in Eₖ = ½mv² would be wrong if the answer is required in joules, because the equation expects mass in kilograms. Using 4.0 minutes in E = VIt would also be wrong if current is in amperes and energy is required in joules, because the second is the compatible time unit.

Click to enlarge

Rearrange cleanly and leave enough working to protect the method

You can rearrange before substituting or substitute first and then solve for the unknown. Either approach can work, but the algebra needs to remain visible. If v = fλ and the question asks for wavelength, write λ = v/f before typing the values. If p = F/A and the question asks for force, F = pA. A quick dimensional or common-sense check can often catch a reversed operation. Visible working matters because multi-step Physics mark schemes frequently separate method from the final numerical answer. In a recent Cambridge Paper 4 spring calculation, the mark scheme credited the relationship k = F/x or the gradient of the force–extension graph before the final spring constant. In another momentum calculation, credit was available for setting up conservation of momentum and the substitution before the final mass. If your calculator slips at the last step, a clear method can still show the examiner what Physics you understood.

Use the calculator as the last part of the method

Many "Physics mistakes" are really calculator-entry mistakes. Use brackets when an entire denominator contains more than one factor. If you need 120/(2.5 × 4.0), enter the denominator as a grouped expression rather than typing 120 ÷ 2.5 × 4.0 and hoping the order of operations matches what you intended. For standard form, use your calculator's scientific-notation function consistently. More importantly, keep unrounded intermediate values. The current syllabus explicitly states that only the final answer in a calculation should be rounded. Rounding halfway through a two- or three-stage calculation can move the final answer outside the accepted range even when the method is correct.

Click to enlarge

Do not use a universal "three significant figures" rule

Cambridge expects appropriate decimal places and significant figures, not one fixed precision for every calculation. In practical work, calculated quantities should reflect the precision of the raw data used. In theory questions, the sensible precision depends on the information given and the marking context. An answer such as 4.333333333 is usually not useful simply because the calculator displayed it, but neither should every result be forced to three significant figures.

A better rule is to preserve precision while working, then look at the data and the nature of the answer before rounding once at the end. If the mark scheme or question specifies a precision, follow it. If it does not, use a sensible level of precision consistent with the data.

Sanity-check the answer before moving on

A ten-second check can catch errors that the calculator cannot. Ask whether the unit matches the quantity, whether the sign or direction makes sense, and whether the magnitude is physically plausible. Efficiency should not exceed 100%. A normal car should not have a speed close to the speed of light. A human mass of 0.05 kg should immediately look suspicious. The same check works algebraically. If your rearrangement predicts the opposite trend from the Physics you expect, revisit the equation before moving on.

Different topics create different calculation traps

The routine stays the same, but the danger points change across the syllabus. In motion questions, distinguish velocity from acceleration and remember that graph gradients and areas represent different quantities. In density and pressure questions, unit conversions can become more difficult because areas and volumes square or cube the length conversion. In energy questions, do not lose squared terms such as v² in kinetic energy, and remember that work done uses the distance moved in the direction of the force. In thermal Physics, use the temperature change rather than automatically substituting the final temperature. In waves, make sure v = fλ uses compatible units and remember that echo or sonar distances may involve a return journey. In electricity, check time units before using Q = It or E = VIt.

Common calculation mistakes to diagnose

When you lose a calculation mark, label the reason rather than simply writing "careless". Was the wrong equation chosen? Was a unit conversion missed? Did you square the wrong quantity? Was the equation rearranged backwards? Did you round too early? Did you leave off the unit? These errors need different fixes, and spotting the pattern is much more useful than repeating another full paper immediately. Cambridge examiner commentary has repeatedly highlighted exactly these kinds of issues: unit conversion, equation rearrangement and showing the physical relationship in "show that" calculations rather than manipulating the printed numbers until the target appears. That is why a repeatable written method is worth practising even when you can do the arithmetic mentally.

Use feedback to fix the step that failed

Inside NeuraGeek, you can solve IGCSE Physics past-paper and topical questions directly on screen with Live Solver, write your working, receive AI feedback while you solve and then review the final marking report. The useful question after a wrong calculation is not only "what is the correct number?" It is "which step broke?" If the equation was wrong, go back to the concept. If the unit conversion failed, practise a few focused conversion questions. If the algebra was the issue, redo the rearrangement before using the calculator. If the answer was numerically correct but poorly presented, compare your working with the mark scheme. Topicwise Papers, custom tests and the AI Tutor can then target the exact weakness before you return to another full paper.

The calculation checklist

Before leaving a calculation, ask: Have I identified the correct quantities? Is the equation right for this situation? Are the units consistent? Is my rearrangement correct? Have I kept full precision until the final step? Does the final answer have the right unit and sensible precision? Does the number make physical sense? Once the sequence from quantities to equation to units to working to final check becomes automatic, calculation questions become much easier to control across the (0625) syllabus.