IELTS uses a nine-band score to calculate the score with band score 1 for a non-user of English language to 9 for the expert.
Every section is scored on a nine band score and the cumulative score is calculated to get the overall score.
For Example: if a test taker scores 7.0 in Speaking, 6.5 in Reading, 7.5 in Listening and 5.5 in Writing then the average will be 26.5/4 = 6.625. Hence the overall score is 6.5.
Note: if the average of the 4 section’s band score ends in .25 then, the overall score is rounded up to the next half band and if the average ends in .75 then, the overall score is rounded up to the next full band.