Trying to quantify your aerobic fitness is a daunting task. It usually requires access to an exercise-physiology lab. But researchers at the Norwegian University of Science and Technology in Trondheim have developed a remarkably low-tech means of precisely assessing aerobic fitness and estimating your “fitness age,” or how well your body functions physically, relative to how well it should work, given your age.

The researchers evaluated almost 5,000 Norwegians between the ages of 20 and 90, using mobile labs. They took about a dozen measurements, including height, body mass index, resting heart rate, HDL and total cholesterol levels. Each person also filled out a lengthy lifestyle questionnaire. Finally, each volunteer ran to the point of exhaustion on a treadmill to pinpoint his or her peak oxygen intake (VO2 max), or how well the body delivers oxygen to its cells. VO2 max has been shown in large-scale studies to closely correlate with significantly augmented life spans, even among the elderly or overweight. In other words, VO2 max can indicate fitness age.

In order to figure out how to estimate VO2 max without a treadmill, the scientists combed through the results to determine which of the data points were most useful. You might expect that the most taxing physical tests would yield the most reliable results. Instead, the researchers found that putting just five measurements — waist circumference; resting heart rate; frequency and intensity of exercise; age; and sex — into an algorithm allowed them to predict a person’s VO2 max with noteworthy accuracy, according to their study, published in the journal Medicine & Science in Sports & Exercise.

The researchers used the data set to tabulate the typical, desirable VO2 max for a healthy person at every age from 20 to 90, creating specific parameters for fitness age. The concept is simple enough, explains Ulrik Wisloff, the director of the K. G. Jebsen Center of Exercise in Medicine at the Norwegian University and the senior author of the study. “A 70-year-old man or woman who has the peak oxygen uptake of a 20-year-old has a fitness age of 20,” he says. He has seen just this combination during his research.

The researchers have used all of this data to create an online calculator that allows people to determine their VO2 max without going to a lab. You’ll need your waist measurement and your resting heart rate. To determine it, sit quietly for 10 minutes and check your pulse; count for 30 seconds, double the number and you have your resting heart rate. Plug these numbers, along with your age, sex and frequency and intensity of exercise, into the calculator, and you’ll learn your fitness age.

The results can be sobering. A 50-year-old man, for instance, who exercises moderately a few times a week, sports a 36-inch waist and a resting heart rate of 75 — not atypical values for healthy middle-aged men — will have a fitness age of 59. Thankfully, unwanted fitness years, unlike the chronological kind, can be erased, Dr. Wisloff says. Exercise more frequently or more intensely. Then replug your numbers and exult as your “age” declines. A youthful fitness age, Dr. Wisloff says, “is the single best predictor of current and future health.”

This article appears in the Nov. 3, 2013 issue of The New York Times Magazine.