You are assessed on your performance on each task by certificated IELTS examiners according to the IELTS Writing test assessment criteria
Each of the four criteria receives a score from 0 to 9 points. After that, an arithmetic mean is calculated to determine the task’s total score. For example, if Task 1 gets following marks:
then score for IELTS Task 1 is (6.0+7.5+7.0+7.5)/4 =7.0.
Task 2 contributes twice as much as Task 1 to the Writing score, thus, Task 1 is worth 33% of the total mark in the Writing test. So, if you get 6.5 for task 2 and 7.0 for task 1, the total score for IELTS Writing Section is
6.5*(2/3) +(7.0) *1/3 = 6.666.
But scores are reported in whole and half bands. For instance:
Since 6.66 is less than 6.75, the final band score will be rounded to 6.5