It is common to want to know how much weight you can lift once, which is known in gym parlance as one repetition max or 1RM. It can also be a little hazardous to try it, especially without spotters. Never fear; a fairly accurate estimate can be derived from an equation.
Let's use the example of bench pressing. Imagine you're able to bench press 175 pounds (175#) six times in a row or six reps.
1RM = (# of reps / 30) + 1 X amount of weight lifted
So your estimated 1RM = (6 / 30) + 1 X 175 = 210 pounds.
A higher weight, lower number of reps (e.g., 3RM) is generally more efficient for building strength. The equation above also increases in accuracy the lower the number of reps you use. For example, estimating your 1RM from your 3RM is much more accurate than using a 10RM.