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.
This appears on page 206 of Fitness For Geeks. By the way, the term "5RM" for example, means the amount of weight you can lift five times, such as "I did a push press five times at 70 pounds per rep." The 5RM is a very useful measure for building strength; a sweet spot between your 1RM and a 10RM. The latter simply doesn't employ enough weight or "load" to prevent you from plateauing eventually, as you try to add muscle and power.
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.