The Truth about ATAR Calculators

A highly debated topic which causes a multitude of emotions. Some happy and some sad. As a result, these recurring emotions and arguments cause major misconceptions about ATAR calculators and the ATAR calculation process as a whole.

Firstly, despite ATAR calculators claiming nearly 100% accuracy it is impossible to give such a guarantee without direct input from UAC, the people who actually calculate your ATAR.

Secondly, chances are you’ve used the ATAR calculator completely wrong due to a lack of understanding, and as such, you shouldn’t take any estimations and calculations as gospel.

Let me expand on that second point a bit more. Currently, you only have access to your internal school marks. However, your ATAR is in fact calculated from both your External mark and your internal mark, both contributing 50%.

So that sounds pretty simple right? 50% of your final subject mark is based on your internal marks and 50% on what you get in the exam. But what you need to realise is that there are a few more calculations that are done before we can take 50% of each mark respectively.

I’ve outlined the process as simply but as thoroughly as i can below. (There is a TL;DR at the bottom)

Internal Marks

You may have heard that your External marks are scaled based on your subject, but what you probably don’t know is that your internal marks are also scaled, or moderated.

Your internal marks are moderated based on your school’s performance in the HSC.

Why does this happen?

Well, each school will obviously have slightly different assessments throughout the year and have various standards of marking. Hence, it would be unfair for a mark of 90% at School A to be better than a mark of 80% at School B. School B could have simply had a harder assessment than School A or were marked a bit more harshly.

Thus, it is necessary to have some sort of system to compare student’s internal marks. As a result, the Board of Studies (BOSTES) moderate your internal marks based on how the school performs as a cohort for that each and every subject in the External HSC exam. This means that if School B does better than School A in the HSC then the internal marks of the students from School B will be moderated upwards in comparison to School A. This is where the importance of ranks appear. The upwards (or downwards) moderation occurs in order of your internal ranking. So if you can 1st, you would be moderated the most, and if you can 2nd you would be moderated the 2nd most and so on.

Just to reiterate, this moderation is reliant upon how the cohort performs in the HSC rather than your individual performance. This means that Schools with very dispersed groups of poor students languishing at the bottom of the rankings will drag the school down and in turn have lower levels of moderation. Essentially, the gaps in marks between each rank is incredibly important when calculating how much to moderate certain cohorts. This is the main reason why selective schools do extremely well in the HSC as the rankings within each subject are closely contested and are often separated based on the second decimal place.

With all this being said, your school can’t give you your moderated marks as they can’t foresee how your grade will perform in the external exam. Thus, any mark your school gives you in your report is likely to have been estimated and thus quite inaccurate. Thus, it is unwise to use such marks whilst calculating and estimating your ATAR.

External Marks

So now that you understand how your internal marks are calculated you may think that you have all the pieces to calculate your HSC marks. However, like your internal marks your external marks are scaled as well (to allow for fair comparison across all subjects) in relation to the whole candidature (everyone in NSW). In addition to being scaled, your External marks are also aligned based on the difficulty of the exam and the option modules (if any) that you choose. Essentially, they take your raw exam mark (a % out of 100) and align it based on the factors above and a couple other things which are fairly irrelevant so I won’t discuss it here. Unfortunately, this is not something you can control, but it is something you should realise before you do your external exam.

Just to clarify this, a quick example would be, say you got a mark of 70% in 4U maths, whilst on face value this may seem not enough to get a band 6, after aligning you will receive an external HSC mark of 90. If you want to see raw marks in comparison to their aligned marks take a look at this database here:

Now to account for scaling. Like mentioned above, your marks for each subject need to be standardised in some way across a common subject. That subject happens to be English as any student doing the HSC will have to do a minimum of 2 units of English. By using English as a benchmark, it is possible to compare cohorts performances in English and any other subject.

Well, that’s kinda confusing, so let me break it down into an example.

Let’s say the 5,000 students doing Chemistry also take Business Studies and Advanced English. Now these 5,000 students perform better than the students who take Business studies and Advanced English, then their Chemistry marks will be scaled. The better they do in comparison to other people in Business Studies and Advanced English will result in their Chemistry marks scaling more.

Essentially, what is important to note is that there is some correlation between the difficulty of the subject and how much it scales. This is due to the fact that students who take difficult subjects like 4U Maths, 4U English, Physics etc tend to do better in all their other subjects as well, resulting in these difficult subjects scaling well.

Now, after all this has been compared and correlated, UAC controls the final scaling process. This process is based on percentiles and how your mark places in comparison to the rest of the cohort doing that subject. This means that your aligned exam mark for the subject is correlated with a scaled mark based on a graph that UAC create. What this means for you is that despite being in the top 20% of a subject (4U maths) this mark would be equivalent to being in the top 5-10% of your physics cohort.

This due to the fact that the graphs UAC make contain a scaled mean and a curve starting from the 50th percentile to the 100th percentile. Since the scaled mean for 4U Maths is higher than the scaled mean for physics, you can afford having a lower percentile in math than in physics and still get the same mark.

Now, here lies the reason why people decide to opt for harder subjects when doing the HSC. Most people choose higher scaling subjects because they believe they can get away with a lower mark in that subject yet still achieve a higher mark overall in comparison to if they had done another lower scaling subject. However, this is a common misconception as chances are that person wouldn’t even achieve the benchmark needing to get a mark equivalent to what they would’ve gotten in another subject that they had more capability in. Thus it is still important to choose subjects that you enjoy and are good at, because you can at least be somewhat certain of a good mark.

Final Calculations:

Ok, back to calculating our ATAR. So we have our moderated internal mark, and we have our scaled external mark. Should be enough to calculate our ATAR right? Not exactly. Once UAC receives the final HSC marks for each student, they then calculate an aggregate for each student. This Aggregate is out of 500 (each unit that you do contributes 50 to that score) From this aggregate, percentiles are then calculated to indicate the ranking of every student in the state and following this your rank in the state is given to you in the form of an ATAR.

Hence, since this process is repeated each year from a clean slate, it is impossible to calculate your ATAR for that year, and so ATAR calculators use previous years scaling and percentile information to calculate your ATAR estimate. Whilst not much changes from year to year, there are a lot of inconsistencies when calculating your ATAR.



  • Currently, you have been using your internal raw marks for ATAR calculation. This mark is NOT what your internal or overall mark in the HSC will be due to moderation.
  • Your internal rank and the gaps between each rank matter immensely. The higher your rank, and the closer you are to the person above you, the better the moderation process is going to treat you.
  • Your external marks go through a scaling and aligning procedure before being used to calculate your ATAR.
  • Scaling is based on the performance of the candidature of a subject in relation to other subjects.
  • There isn’t much you can control in the scaling procedure besides chosing subjects you like and doing well in them.
  • Due to all these factors, ATAR calculators can’t give you accurate estimations of your ATAR as you yourself don’t have access to all the information needed to calculate your ATAR.



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