Almost 55,000 of the state's school leavers will find out their ATAR on Thursday. But it is doubtful many, if any, will truly understand the intricacies of how it was calculated.

An ATAR is not an average of a student's HSC marks. It is a rank that measures their overall performance in relation to other students and is used by universities to select school leavers for courses.

For many students, that single number is the culmination of two years of study. And some were so eager to pre-empt their rank on Wednesday, the top ATAR calculator websites crashed after HSC subject results were released at 6am.

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How important are ATAR scores?

HSC student Kim Ho is thrilled his ATAR score will allow him to attend his chosen course at Melbourne University.

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The calculators, which are commonly developed by tutoring companies, give an estimate of what a student's ATAR will be based on the method used last year.

Talent 100, which hosts one of the most popular calculators, said more than 10,000 students accessed it from 7am to 10.30am.

When students access their ATAR from 9am on Thursday, 48 would be receiving the top ATAR of 99.95. While male students dominated the top performers, taking 32 of the highest possible rank, female students came out on top overall. The median ATAR for female students was 71, while the median for male students was 67.

"One-third of the state's students receive an ATAR of 80 and above, almost two-thirds receive at least 60 and more than 9000 crack 90".

"One-third of the state's students receive an ATAR of 80 and above, almost two-thirds receive at least 60 and more than 9000 crack 90". Photo: Andy Zakeli

One-third of the state's students receive an ATAR of 80 and above, almost two-thirds receive at least 60 and more than 9000 crack 90.

The director of information services at the Universities Admissions Centre, Kim Paino, said the most common query at the ATAR inquiry centre was why a student's ATAR did not align with their HSC marks. But because an ATAR is not an average of a student's marks, there is no reason it should fall in a similar numerical range.

The ATAR was designed as a fair way of comparing students, given there are 28,000 different combinations of subjects that students do. Of those, about 20,000 combinations are taken by only one student.

The process for calculating the ATAR is incredibly complex which is why, Kim Paino said, ''we focus on why we do it rather than the nuts and bolts of what the computer does to spit out the number''.

She said even after working at UAC for 12 years she still learns something new each year. ''Sometimes you'll have very technically minded parents with very detailed questions that even I can't answer.''

She said the biggest myth about the ATAR was that you had to study certain subjects to get a high rank.

''That's plainly not true,'' she said. ''Of the students who got 99.95 this year, one person got it with English as a second language (ESL) and another got it with no maths at all.''

HSC Honour Roll liftout: Merit lists are correct at time of publication and may be subject to change. Current lists are available at the Board of Studies website: www.boardofstudies.nsw.edu.au.