Keep in mind that all calorie figures are estimates. Numbers for calories in a given food or calories burned in a workout are not going to be exact. There are differences in an individual’s intensity, weight and body mass that may affect a calorie burn that are not taken into account unless measured in a laboratory metabolic chamber. And, with food, there may be a variety of factors with ingredients, ripeness, portion size and preparation that could affect calorie count. The bottom line is to not get too fixated on exact calorie numbers.

A hundred calories is a standard estimate for the energy required to move a body a mile by walking or running. A heavier body requires more energy (hence, a greater calorie burn) to move. Yet any size body can move with greater intensity to also burn more calories (either by speeding up, walking up a hill that requires more effort, etc.). Terrain and environmental conditions can affect intensity and speed.

The slower one moves, the lower their calorie burn. The flatter the surface (flat road vs. dirt hills) or the less resistance it provides (concrete path vs. sand or wind), the lower the calorie burn.

It generally takes an average person around 15 to 20 minutes to walk a mile, and 7 to 12 minutes to run or jog that distance. Walking a little more slowly, at a 20-minute-per-mile pace, will burn around five calories per minute. Running a mile might burn from 7 or 8 calories per minute up to 10 to 12, depending upon a person’s size and speed.

The classic textbook Exercise Physiology: Energy, Nutrition, and Human Performance provides energy expenditure estimates for a variety of activities based on body weight. According to the estimates, a 150-pound person will burn 3.6 calories per minute when walking 2 mph; 5 calories per minute when walking 3 mph; and 6.6 calories per minute when walking 4 mph. So the slowest walker would burn 108 calories walking a mile (3.6 x 30 minutes); the moderate-paced walker would burn 100 calories (5 x 20 minutes); and the fastest walker would burn around 99 calories (6.6 x 15). These numbers are in the 100-calories per-mile range.

To burn more calories in a workout, what I wrote holds true: “Do more cardio or do your cardio more intensely: walk faster, run, jump and climb.” Doing more cardio means adding 10 or 20 minutes to each workout (walking or running more than one mile), working out on more days per week (walking or running more miles per week), or both.

Working out more intensely burns more calories because you cover more ground in the same amount of time. So if you speed up from walking 2 mph to 4 mph, you go from burning around 3.6 calories per minute to around 6.6 calories per minute. Therefore, in 30 minutes of walking you could double your calorie burn simply by walking faster. Of course, you should be fit enough to safely push yourself to exercise at a higher intensity. Work up to harder and longer workouts gradually so that you don’t put any excess stress on your joints while you’re trying to increase the number of calories you burn.