The answer depends on what you’re measuring.
Many data readings on cardio machines aren’t precise, but there are ways to increase their reliability, says Nicole Pinto, a clinical exercise physiologist at the University of California, San Francisco’s Human Performance Center.
Steps, time, and distance are based on the movement of the machine’s gears. These can be accurate, provided the machine is well maintained and has a reliable power source. Data like caloric burn, heart rate, and power output are trickier to measure reliably without specialized equipment. Here’s why:
Calorie counts are generally based on a formula that does not take your unique physiology or metabolism into account; on most machines, the default setting is for a male weighing 150 pounds. As a result, the counts are estimates at best. Increase the odds of getting a somewhat accurate reading by entering your age, gender, and weight into the machine before your workout.
Heart-rate measurements are most accurate when you use a chest strap connected to an app or watch. Due to sweat and movement, handgrips (and even chest straps) can lose connectivity to a machine.
Power-output readings are fairly reliable, but performing something as common as a sprint interval can skew the accuracy dramatically.
All readings on cardio machines are based on the assumption that you’re using the equipment correctly. Leaning excessively on handrails, for example — common on stair climbers and inclined treadmills — causes the machine to overstate metrics like calorie burn and power output, sometimes by a wide margin.
Your best bet is an external monitor, such as a fitness or heart-rate tracker with a chest strap. Keeping the battery fresh will help you avoid accuracy issues of your own.