Predicting your Max

reps performed	multiplication coefficient
1	1.000
2	1.029
3	1.059
4	1.091
5	1.125
6	1.161
7	1.200
8	1.242
9	1.286
10	1.330

So, as yet again another example, if you could use 200 lbs. for five reps then you would get around 200 x 1.125 = 225 lbs. for one
http://www.weightrainer.net/training/coefficients.html