The ATAR is the Australian Tertiary Admission Rank. It is a number between 0 and 99.95 that tells universities where a student sits relative to their age group. An ATAR of 85 means the student performed better than 85 percent of their cohort. It is not a score out of 100. It is a rank.
Most students and parents find the ATAR calculation confusing because it involves several layers of processing between the raw marks a student receives and the final rank. Here is how it actually works.
Internal and External Assessment
Each HSC subject has two components: school-based assessment (internal) and the HSC exam (external). The school-based marks come from assignments, tests, and tasks throughout Year 12. The HSC exam is the final exam sat in October and November. Both contribute to the final HSC mark for each subject.
The school-based assessments are moderated by NESA using the HSC exam results. This means your school assessment rank matters more than the raw marks your school gave you, because NESA adjusts the marks based on how the school's cohort performed in the exam.
Scaling
Once HSC marks are finalised, they are scaled by the universities through UAC. Scaling adjusts marks to account for the difficulty of different subjects. If students who take a particular subject tend to perform well across all their other subjects, that subject scales up. If they tend to perform poorly across their other subjects, it scales down.
This is why subjects like Maths Extension 1 and Physics tend to have higher scaling than subjects like Visual Arts or Sport, Leisure and Recreation. It is not that one subject is more valuable. It is that the cohort taking harder subjects tends to be stronger academically overall.
The Aggregate and Your Best 10 Units
Your ATAR is calculated from your best 10 units of scaled marks. English (Standard or Advanced) must be included, and it counts as 2 units. The remaining 8 units come from your best-performing subjects. If you studied 12 units, your two weakest units are dropped.
This is why some students take an extra subject in Year 11 and drop their weakest one before Year 12. It gives them a safety net. The aggregate of your best 10 units is what determines your ATAR rank.
What This Means in Practice
The practical takeaway is that your ATAR depends on three things: how well you perform in each subject, how each subject is scaled, and how many units you can count towards your best 10. The single most effective strategy is to do well in the subjects you take, regardless of scaling.
Chasing high-scaling subjects you are not good at is a common mistake. A student who scores 85 in a moderately scaled subject will often end up with a higher ATAR contribution than a student who scores 65 in a high-scaling subject. Do what you are strong in and do it well.