Formulas to find your one rep max (or 1rm)

2 reps- 1.06 x weight lifted= 1rm
3 reps- 1.09 x weight lifted= 1rm
4 reps- 1.12 x weight lifted= 1rm
5 reps- 1.15 x weight lifted= 1rm
6 reps- 1.18 x weight lifted= 1rm
7 reps- 1.21 x weight lifted= 1rm
8 reps- 1.24 x weight lifted= 1rm
9 reps- 1.27 x weight lifted= 1rm
10 reps- 1.302 x weight lifted= 1rm

Example on how to use-

I went to the gym today and benched 300 pounds for 5 repetitions so i apply the formula 5 reps- 1.15 x 300 pounds = 345 one rep maximum