A heart rate monitor is an excellent tool for monitoring exercise intensity—but it won’t do you any good if you don’t know your maximum heart rate. Most people use the standard formula to calculate maximum heart rate—220 minus your age—but it turns out, that formula is usually wrong.

That’s right—that formula that you thought was carved in stone may be wrong. And as a result, you might not be working as hard as you think.

Researchers from Scandinavia used data from the HUNT Fitness Study to examine the relationship between age and maximum heart rate. The study included 3,320 healthy Norwegian adults who participated in a maximal exercise test—where they reached their maximum effort and heart rate. The researchers then measured the effect of age on maximum heart rate—and the results were surprising.

Overall, the standard maximum heart rate formula underestimated maximum heart rate in 77 percent of participants. In participants ages 40 to 60, the formula produced too-low results in 90 percent of cases. The formula was off for older participants, too—in those between the ages of 60 and 69, the formula under-predicted maximum heart rate by as much as 15 beats per minute.

Why does this matter? Knowing your true maximum heart rate is crucial for reaching optimal intensity during exercise. To increase fitness and lose weight, you want to spend the majority of your exercise time between 70 and 85 percent of your maximum heart rate—but if you’ve underestimated your maximum, you might be exercising at 60 percent of your maximum when you think you’re exercising at 70 percent. The result? You’re not working hard enough and you won’t see results.

The formula might be quick and easy—but it’s inaccurate. In fact, the researchers say that there is a standard error of 10.8 beats per minute. That’s a pretty big margin for error when it comes to your health and fitness!

So how do you find your maximum heart rate? Of course, the most accurate way is with professional VO2 Max testing—but this expensive test isn’t really necessary for the average exerciser. Another way is to perform the four-minute interval test. Here’s how:

• Strap on a heart rate monitor and warm up for about 15 minutes.
• Perform a four-minute interval at the highest heart rate you can sustain for the entire four-minute period.
• Recover for two minutes.
• Repeat another four-minute interval.
• Take the average of the two four-minute intervals to estimate your maximum heart rate.

Of course, this isn’t a perfect test and you may find you have to tweak your numbers a little bit—but chances are, you’ll find that the number you reach is higher than the one provided by the standard formula. If you want to see fitness results, that’s the number you want to use!

Reference:
Nes BM, Janszky I, Wisløff U, et al. Age-predicted maximal heart rate in healthy subjects: The HUNT Fitness Study. Scandinavian Journal of Medicine and Science in Sports. 2013; 23(6): 697-704.