Running provides an effective and efficient way to burn calories. How many you burn depends on numerous factors, including your speed, terrain, fitness level and body composition. You'll need expensive equipment and a scientific setting to achieve the most accurate count of the calories you expend running for 45 minutes. Since this isn't available to most people, estimate your burn by relying on averages based on people with similar attributes as you.
Size and Body Composition
The more you weigh, the more calories you burn in a 45-minute run. For example, a 125-pound person burns about 450 calories per 45 minutes running at a 6 mph pace, while a 185-pound person could burn as much as 666 calories. When you have a bigger engine, it takes more fuel -- or calories -- to run. The more muscle mass you have, the greater your calorie burn as well.
Terrain and Speed
Hauling your body up hills, whether outside or on a treadmill, will burn more calories than running along a flat course. For example, a 155-pound person running about 4 miles in 45 minutes up a 5 percent incline can burn approximately 587 calories. Do that run on level ground and burn 466 calories. Even higher inclines elicit greater calorie burns, but you might slow down as you go uphill, which reduces the net calories burned in your 45-minute session. The faster you run, the higher your calorie burn. If a 155-pound person speeds up from 6 mph to 6.7 mph, he will burn 55 calories more in the 45-minute session -- 613 calories compared to 558.
An experienced 150-pound runner will likely burn fewer calories than a 150-pound newbie going the same speed over the same terrain. Efficiency of stride and muscle memory kick in for the experienced runner, meaning they don't have to put out as much effort to complete the 45-minute run. A newbie may experience a higher heart rate and fatigue sooner than a fit exerciser, however, so he may not be able to sustain the run for the whole 45 minutes. As a result, he will net an overall lower calorie burn.
You cannot find any technology that will guarantee a 100 percent accurate calorie count for your running workout. Treadmill estimates, especially if you fail to input your weight, can be off by 25 to 30 percent, reported the "Los Angeles Times" in 2010. Your heart-rate monitor can be equally inaccurate. Use the calorie-burn numbers provided by these gadgets as a guide only. You'll know you had an especially hard run if the console tells you a calorie-burn number greater than your average. Calories-burned charts also vary in their reports of how much you burn for a specific weight and speed. Use an average of these to guesstimate your overall burn rate in 45 minutes.
If you use a treadmill to run, take care not to hold the handrails, as this will decrease your overall calorie burn rate. Pumping your arms counts toward your overall calorie burn and keeps you in a safe gait. When you are trying to lose weight, err toward a lower estimation of your calories burned in your 45-minute run. If you underestimate calories burned, you will only end up with a higher calorie deficit than you predicted and a faster weight loss. If you overestimate, you might be tempted to overeat to reward yourself for the calories burned -- impeding your results.