Why the same percentage doesn't always mean the same grade
Students often assume there's a fixed rule, "90% is always a grade 9", "40% is always a C", and then panic when a mock or a past paper's published boundary doesn't match. There isn't a fixed rule. Grade boundaries get set after a paper is sat, based on how every student who took it actually performed. They're not decided in advance.
"Comparable outcomes": the principle exam boards use
UK exam boards set boundaries using a principle called comparable outcomes. The goal is that a student of a given ability gets roughly the same grade regardless of which year they happened to sit the exam, even if that year's paper turned out harder or easier than usual. If a paper is judged tougher than previous years, the raw-mark boundary for each grade drops to compensate. If it was easier, the boundary rises. This is also why a harder paper isn't actually bad news in itself. The boundary moves to account for it.
Who sets boundaries, and when
Senior examiners and statisticians at each exam board review a sample of real student scripts at different mark levels shortly after the series, compare national performance to previous years and other available evidence like prior attainment data, and agree boundaries before results are issued. That's why boundaries for the same subject and board can shift by several marks from one year to the next. It's a deliberate response to that year's paper and that year's national cohort, not an error or inconsistency.
GCSE vs A-Level grading scales
| GCSE | A-Level |
|---|---|
| 9 (highest) down to 1 (lowest) | A* (highest) down to E (lowest passing grade) |
| Grade 4 = "standard pass" | No equivalent pass/fail line — E is the lowest reported grade |
| Grade 5 = "strong pass" | — |
| Set per paper, then combined across papers for the subject | Set per paper, then combined across papers for the subject |
Both scales use the same comparable-outcomes principle. Only the labels and the number of grade points differ. (If you're studying IGCSE, see our IGCSE vs GCSE comparison for how its A*-G scale lines up against the GCSE 9-1 scale.)
What this means for your revision
Don't revise by chasing a fixed percentage target. Chase an understanding of what the mark scheme actually rewards at each level, because that's the thing that stays constant even when the raw-mark boundary moves. A mark scheme written in levels of response (see our guide to how mark schemes work) tells you what separates a Level 2 answer from a Level 3 one regardless of where the boundary lands that year. That's a far more reliable thing to practise toward than a number you won't get confirmation of until after you've already sat the exam.