How to Calculate Bench Press Percentages

Bench press training incorporates numerous percentages. For example, you often hear, "use 70 percent of your one-rep maximum," "use 50 percent of your body weight," or "increase your weight by 10 percent." Most sources mention such percentages but fail to tell their readers how to calculate them. Although confusing at first, these percentages become second nature with a little practice.

One-Rep Max

Many bench press percentages are based on the one-rep max, which is the most weight you can successfully lift with just one repetition. This weight can be determined by physically testing your limits, or you can safely calculate the weight using the Brzycki formula and lighter weights. To do so, multiply the number of repetitions of the lighter weight by 0.0278 and subtract the result from 1.0278. Divide this figure into the weight you lifted to calculate your one-rep max. As an example, if you lifted 180 pounds nine times, multiply 9 times 0.0278 and subtract the result from 1.0278. Dividing the resulting 0.7776 into 180 gives you a one-rep max of 231 pounds.

Starting Weights

Many bench press routines begin at a percentage of your one-rep max. As an example, you might be told to perform as many repetitions as you can at 70 percent of your one-rep max. Dividing the percentage by 100 converts it into the decimal format 0.70, which is multiplied by the one-rep max. Continuing with the prior example, multiplying 0.70 times 231 calculates your starting weight of 162 pounds. This formula can be modified to calculate a percentage of your one-rep max as well. Dividing the weight you use by your one-rep max and multiplying by 100 calculates the percentage. In the example, 162 divided by 231 gives you 0.70, or 70 percent.

Body Weight Percent

Some performance indices express your one-rep max as a percentage of your body weight. This calculation uses the same formula as in the Starting Weight section for calculating percentages. Continuing with the prior example, if you weighed 200 pounds and wanted to express your 231 one-rep max as a percentage of your body weight, divide 231 by 200 to get 1.16. Multiplying by 100 converts this figure into 116 percent. According to the University of Delaware, this is good, but 120 percent would be excellent. Between 79 and 98 percent is considered average.

Percent Improvement

Periodically recalculating your one-rep max gives you an indication of your progression. This progression can also be expressed as a percent increase, which not only plots improvement, but also measures the extent of the improvement. Because you are only measuring the amount of improvement, the formula begins with subtracting the initial one-rep max from the later calculation. You then divide by the initial one-rep max and multiply by 100. As an example, if you increased your 231 one-rep max to 270 pounds, subtracting 231 from 270 shows you an increase of 39 pounds. Dividing 39 by 231 calculates the improvement of 0.169, or 16.9 percent.