Find your mean raw scores. Look at the table of mean raw scores for Listening, Academic Reading and General Training Reading on IELTS' Researchers -- Score Processing, Reporting and Interpretation web page, which is linked in the Resources section. Find the band score you received for each section in the table, and then find each band score's associated mean raw score out of 40 listed to its right in the table. For example, if you received a band score of 5 in Listening, then your mean raw score is 16. A 5 in a different category, however, relates to a different mean raw score.
Calculate your overall band score. Add your band scores for reading, writing, listening and speaking. Divide that total by 4. The result is your overall band score. For example, if your band scores total 25, then divide 25 by 4. The result is 6.25, and 6.25 is your overall band score.
Type your mean raw scores into an online IELTS calculator to have your overall band scores figured automatically. That is a quick alternative to calculating IELTS scores manually.