This is the chart i have used. I got it from bb.com
Reps %1RM
1rep= 100%
2reps= 95%
3reps= 90%
4reps= 88%
5reps= 86%
6reps= 83%
7reps= 80%
8reps= 78%
9reps= 76%
10reps= 75%
11reps= 72%
12reps= 70%
USING THE TABLE: This is how you use the table. Find the number of reps to concentric failure that you can perform with a certain weight. In other words, if you can only do eight reps with a certain weight and could not possible do another full rep, that is your point of failure. Find the percentage associated with that number of repetitions from the table above.
Now, divide the weight that you can do by that percentage using decimals (83 percent equals .83) and that will give you an approximation of your one repetition maximum. For example, if you can perform 10 reps with 175 lbs. in the bench press, that means that 175 lbs. is 75% (0.75) of your one repetition maximum. So you would take 175 divided by .75 and that would equal 233 lbs. You should probably use your calculator, we knew a guy who did it in his head and said he could bench 3000 lbs. Good luck!