The amount of time it takes to burn 300 calories depends on the activity you're doing, the intensity of the activity, and your size. Large people will sizzle 300 calories more quickly than smaller folk, and higher intensity movements will also help you achieve the burn faster. Always talk to your doctor before beginning any new exercise regimen.
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At the Gym
Most gym sessions burn 300 calories in about an hour -- and some even sooner. Take a walk on the treadmill at a pace of 3.5 mph and burn 300 calories in one hour if you weigh 155 pounds. It'll take you a little less time if you weigh more and just over an hour if you weigh less. Burn through those calories in about 30 minutes if you pick up your pace to a run of 5.2 mph; faster speeds will fry the calories in less than 30 minutes. Most cardio machines, including stationary bicycles, elliptical trainers and rowers, enable you burn 300 calories in about 30 minutes if you work at a moderate intensity. Weight lifting takes about 75 to 90 minutes to burn 300 calories, but affords numerous other benefits such as improved muscle mass, coordination and bone health.
The gym isn't the only place you burn calories. Your body is constantly churning to promote basic functions such as pumping blood and breathing -- however, the calories you burn are minimal. A 155-pound person must spend about five hours and 20 minutes to burn 300 calories when just sitting and watching television -- a 185-pound person should settle in for four and a half hours of programming. Get up and play with the kids, do some heavy cleaning or mow the lawn and you'll burn the 300 calories in just under an hour.