No one-size-fits-all approach applies to calorie burning, so no single answer exists as to how many calories are burned in a 3-mile run. A very rough estimate puts your burn at about 300 calories, but that's a very imprecise calculation that likely won't work for many people. If you're larger than average, you will burn more calories, and if you're smaller, you'll burn fewer calories. Your pace doesn't greatly affect your calorie burn per mile; it just affects how long it takes you to complete the distance. The intensity at which you work to complete the three miles affects whether you burn calories primarily from fat or carbohydrates and any calories you continue to burn post-run as your body recovers.
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More Precise Calorie Burn Estimates
Your size affects how many calories you burn over the course of 3 miles: A larger body simply requires more energy to move. For example, a 125-pound person burns about 288 calories running a 12-minute mile, or a 5-mph pace, while a 185-pound person burns 425 calories going the same distance at the same pace.
If you increase your pace for the three miles to a 10-minute mile, or 6 mph, at 125 pounds, you burn exactly 300 calories, while a 185-pound person burns 444 calories. Up the pace to a 7-minute mile -- running the 3 miles in 21 minutes at 8.6 mph -- and a 125-pound person burns 305 calories, while a 185-pound person burns 450 calories. A fast racer who completes 3 miles in just 18 minutes, going at a pace of 10 mph, burns 297 calories at 125 pounds or 440 calories at 185 pounds.
Running at the faster pace burns more calories per minute, but because it takes less time to complete 3 miles, the calories burned covering the same distance don't vary much overall.
The Calorie Source for Your Run
While your speed might not significantly affect the number of calories you burn during a 3-mile run, it does affect where those calories come from. When you work at a low- to moderate-intensity, defined as about 25 to 60 percent of your maximum heart rate, your body uses more stored fat to fuel the run. Increasing your intensity to 70 percent or higher of your maximum capacity causes your body to use a greater ratio of stored glycogen to fat. Glycogen is easier for your body to mobilize for fuel, so when you're working harder, it's a more easily accessible source of energy.
If the pace you choose to run requires an effort of 70 percent of your max heart rate or greater, then most of the approximate 300 calories you burn comes from glycogen, or carbohydrate stores.
The running pace at which you switch from burning primarily fat to primarily glycogen is different for everyone. An elite runner may run three miles at a 7-minute-mile pace using a moderate effort that burned mostly fat. A recreational runner, whose heart rate rises more easily on a run, could find this same pace takes an all-out effort that burns primarily glycogen.
3-Mile Run Intensity Affects Overall Calorie Burn
The more intense the 3-mile run is for you, the more calories you'll burn post session. It takes a longer time for your body to return to a normal resting state when you're exercising at a vigorous intensity. If the pace you ran is hard, the more excess post-exercise oxygen consumption, or EPOC, you'll experience. EPOC refers to a state after exercise at which your body consumes oxygen at a level higher than resting, and thus burns a greater number of calories.
University of New Mexico researchers Chantal A. Vella, Ph.D. and Len Kravitz, Ph.D. conclude in a research review that exercise intensity directly affects how many calories you burn after a workout. Usually, efforts in excess of 70 percent of maximum produce the greatest EPOC; the exact number of extra calories you burn, though, depends on your size, efficiency and run intensity.
Running for Weight Loss
If you're trying to create a calorie deficit by exercising more and eating less, be most concerned about the actual 300 to 450 calories you burned. A deficit of 500 to 1,000 calories results in a 1- to 2-pound loss per week. Make up the rest of your deficit by trimming calories from what you consume daily.
The American College of Sports Medicine concluded that you likely need at least 250 minutes of moderate-intensity exercise per week for clinically significant weight loss. Because running is a vigorous-intensity exercise, you may be able to get away with less than 250 minutes weekly for weight loss. However, if you can work up to 250 minutes of vigorous-intensity cardio weekly -- from running and other types of cardio, like Zumba or rowing -- and combine it with a low-calorie diet and strength training, you create an environment to bring about notable weight loss results.