The new formula for calculating the target heart rate in women sets a lower maximum limit, researchers say.
Until now doctors may have been overestimating women's peak heart rates during a stress test, it has emerged.
This means a woman could hear a worse prognosis than she actually had.
The problem arose because peak heart rates were based on males only.
"Now we know for the first time what is normal for women, and it's a lower peak heart rate than for men," Discovery News quoted Martha Gulati, assistant professor of medicine and preventive medicine and a cardiologist at Northwestern Medicine, as telling ScienceBlog.
Gulati is the lead author of the study appearing in the American Heart Association's journal Circulation.
The old formula is 220 minus the participant's age. But Gulati recommends 206 minus 88 percent of age, for women.
For a 55-year old woman, this would mean a difference of a heart rate of 165 (old formula) versus about 158.
Gulati said: "Before, many women couldn't meet their target heart rate.
"Now, with the new formula, they are actually meeting their age-defined heart rate."
To get the new number, Gulati and her team analysed data from nearly 5,500 women over the age of 35.
Gulati said: "It's important to not get complacent that we have data on men and assume women must be the same. They're not."