The following table is a quick and dirty way to find your one rep max in any one of the three powerlifts. I have found it to be accurate to + - five pounds.

To find your one rep max in a particular lift, multiply the constant that corresponds with the number of repetitions of your best set, by the weight of your best set.

Example: 300 lbs. for five repitions on the bench

300 x 1.15 = 345 lbs. (projected 1 rep max.)

Repetitions	Squat	Benchpress	Deadlift
1	1.0	1.0	1.0
2	1.0475	1.035	1.065
3	1.13	1.08	1.13
4	1.1575	1.115	1.147
5	1.2	1.15	1.164
6	1.242	1.18	1.181
7	1.284	1.22	1.198
8	1.326	1.255	1.232
9	1.368	1.29	1.232
10	1.41	1.325	1.24