Why marks cannot just be added
Imagine two students. One gets 90 in Mathematics Extension 2. The other gets 90 in Visual Arts. Both marks are impressive, but they are not equivalent evidence of academic achievement, because the groups taking those courses are different. Mathematics Extension 2 attracts a small cohort of very strong mathematics students; a 90 there is a different position in the cohort.
UAC uses a currency analogy. A mark in one course is like a foreign currency: you cannot add pounds to dollars without an exchange rate. Scaling supplies that rate.
The mechanism, in plain terms
- Standardise. Raw marks in every course are put on a common scale. UAC standardises marks to a mean of 25 and a standard deviation of 12 on a one-unit basis before scaling.
- Estimate the cohort’s overall strength. The scaling model looks at how the students in a course perform across all their other courses. A course full of students who do well everywhere will have a higher scaled mean.
- Preserve rank within the course. Scaling changes the mean, spread and maximum of a course, but not the order of students inside it. The top student in a course remains the top student.
- Add the scaled marks. The aggregate is built from scaled marks, not reported marks.
What it looks like with real numbers
UAC publishes a worked example of students in the 90th percentile of their courses. Read the table carefully: the HSC marks are similar, but the scaled marks are not.
| Course | Scaled mean | HSC mark | Scaled mark |
|---|---|---|---|
| Ancient History | 46.0 | 89.0 | 75.6 |
| Biology | 52.0 | 87.0 | 78.4 |
| Business Studies | 47.2 | 90.0 | 77.4 |
| Society and Culture | 46.6 | 90.0 | 76.2 |
| Physics | 61.8 | 91.0 | 86.8 |
| English Standard | 40.2 | 81.0 | 62.6 |
English Standard has the lowest HSC mark in the table (81) and the lowest scaled mark (62.6). It still counts, because NSW requires 2 units of English. Ancient History is then dropped, even though its HSC mark beats English, because its scaled mark is the lowest of the remaining courses.
Which courses have the highest scaled means?
These are scaled means from the 2025 NSW HSC scaling report, shown out of 50 on a one-unit basis. A higher mean means the course cohort performed strongly across all its courses.
Source: UAC, Report on the Scaling of the 2025 NSW HSC, Table A3. These are cohort statistics, not a recommendation. A student at the top of a lower-scaling course can still earn a high scaled mark.
What actually protects your ATAR
- Doing well in the courses you take. Rank inside the course is what scaling preserves.
- Meeting prerequisites. A high ATAR cannot compensate for a missing subject prerequisite.
- Keeping your options open. English is compulsory for an ATAR in every state; some courses also require mathematics or a science.
- Checking the rules for your state. The number of subjects that count, and how they are weighted, is not the same everywhere.
Three scaling myths
“Scaling is a penalty.”
Scaling preserves your rank within a course. It changes the mark, not your position among the people who took the same course.
“A high-scaling subject guarantees a high ATAR.”
No. A high scaled mean reflects the cohort, not your result. You still need a strong mark within that cohort to earn a high scaled mark.
“The tables are fixed.”
They are rebuilt every year from that year’s candidature. A subject’s scaled mean can move as the group taking it changes.