Good news for treadmill fans: Experts agree that the calorie counter is pretty accurate, especially if you input your weight and don't use the handrails. The problem is that many treadmills don't ask for your weight and use a reference of about 155 lbs, Olson says. That means if you weigh 135 lbs, you're really burning about 15 percent fewer calories than the machine says (300 calories vs. 255 calories, for example).
Relying on the handrails—especially during higher inclines or while running at high speeds—can throw off your reading by as much as 40 percent (that 300 calories burned just became 180). And we're not just talking about the people who pull themselves up a hill for 20 minutes. Placing even just a slight amount of weight on the support rails can decrease the actual calories burned by 20 percent (or more), Olson says.
Boost your burn: To maximize your treadmill time, avoid the handrails and be sure to practice good posture. "Slumping over can have a major impact on your oxygen intake, making your workout harder," Cardiello says.
If you can, take your run outdoors whenever possible. "Running outdoors is more difficult on the body. And you can't equally compare the distance and speed with your treadmill run vs. outdoor running; the calibration and physical stress is less from the treadmill than your outdoor trail run."
Can't make it outside? Go a little longer (or farther) with your indoor session to make up for the terrain difference, or try this fat-blasting interval routine.