To calculate the SUS usability score:
Add up the total score for all odd-numbered questions, then subtract 5 from the total.
Add up the total score for all even-numbered questions, then subtract that total from 25.
Add up the total score of the new values and multiply by 2.5.
For example, let's say that the total sum of odd-numbered questions is 23, and the total sum of even-numbered ones is 20:
23-5 = 18
25-23 = 2
20x2.5 = 50
Your usability score is 50, which is considered all marginal and is a sure sign the design needs iteration.