Losing weight is a matter of creating a calorie deficit; you need to expend more calories than you consume. The recommended way to do this is through a combination of a low-calorie diet and exercise, of which running can be a vital part. Figuring out how much weight you will lose by running three miles is primarily a matter of considering your basal metabolic rate, calorie consumption, current weight and the rate at which you run.
Basal Metabolic Rate
To find out how much weight you can lose from running, you first need to calculate your basal metabolic rate, or BMR. Your BMR is the number of calories you would burn if you didn't exercise or do any activities at all -- the number of calories required for the body's basic needs. For men, the formula is 66 + (6.23 x weight in pounds) + (12.7 x height in inches) - (6.8 x age in years). For women, the formula is 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years). For example, the BMR for a 35-year-old male who is 6 feet tall and weighs 150 lb. would be 66 + (6.23 x 150) + (12.7 x 72) - (6.8 x 35) = 1,677, which is the number of calories he expends per day if he does nothing.
In addition to the calories you use for your BMR, you need to know how many calories you are consuming and expending. It takes an excess of 3,500 calories to burn 1 lb. of fat. If you are consuming 2,000 calories a day and not exercising and your BMR is 1,700, you have a calorie excess of 300 calories and are gaining weight. If you add exercise, you would need to burn at least 300 calories a day with exercise to keep your same weight, which many people do simply by walking or moving around. This is why the recommended daily intake of calories hovers around 2,000 calories a day. Assuming you are burning as many calories as you are consuming before exercise, you need to burn an additional 500 calories a day through exercise to burn 1 lb. of fat in a week.
The number of calories you burn running depends primarily on your rate of speed and your weight. For example, according to NutriStrategy.com, a 155-lb. person running at 6 mph burns 704 calories in a hour. If he ran for three miles at that speed, he would burn 352 calories. If he consumed the same number of calories as he was expending before exercise, he would burn around 1/9 lb. by running three miles. In other words, it would take him nine days running at the same speed and length of time, and eating the same number calories, to burn 1 lb. of fat, because it would take him that many days to reach the excess of 3,500 calories.
Although it can seem complicated, calculating weight loss works better in the long term than it does calculating one instance of exercise. The key to losing weight is to maintain a healthy diet and exercise, as well as to keep calories expended more than calories consumed. The recommended amount of weight to lose weekly is 1 to 2 lbs. by watching your diet and exercising. Running can be a vital part of losing weight as long as you know how the weight loss process works.