There is no definitive IELTS Band Score Calculator that you can apply to every test for the Listening and Reading components.
For the IELTS Speaking and Writing components, your band score will be awarded by an examiner. The examiner awards you a score according to a set of IELTS marking criteria called descriptors. You can find out more about these on this page.
The Reading and Listening components are graded by clerical markers. They will mark your paper from an answer sheet. Your answer must match the answer on their sheet exactly – even the spelling. Sometimes different variations and spellings of an answer are allowed – but the variations must be a form of international English.
Once they have marked your paper, your raw score is entered into a computer. The computer will then calculate your band score according to the difficulty of the test.
If the test if very difficult, you will not need as high a score as with an easier test.
A Very Rough IELTS Band Score Calculator For Bands 6-9
As a very rough rule of thumb, you need around 6 out of 10 to achieve a band 6.0 (so 24 total). Adding on another 6 marks will achieve a 7.0, and another 6 an 8.0. To achieve a 9.0 you should not make more than one mistake.
The tables below give you a better approximation of your score.
The Common European Framework (CEFR)
If you have a score from another test and want to convert that into an equivalent IELTS score, you can use the CEFR.
The CEFR is an international standard for describing language ability. It follows a 6-point scale. All major exams map their equivalent scores to the CEFR.