In raw math, 1/2 HP is 370W. Less than 2A. At powerup a motor usually draws a surge several times the running current (say, 3x to 6x). Your relay must withstand that amount as it closes. A 10A rating may just about be sufficient.
There may be arcing on release. You should watch for it and try to miminize it. Arcing is undesirable because it can transfer metal between the contacts, and lead to sticking.