The number of calories you need to eat in a day varies for each person, as it is dependent on your weight, age, gender, and physical activity level.
For instance, if you run 3 miles a day, your caloric needs will depend on how much you weigh. A 120-pound person burns 11 calories/minute while running, which means that a 10-minute mile equals 110 calories; conversely, someone who weighs 180 pounds would burn 17 calories per minute (equaling 170 calories for a 10-minute mile) .
To determine how many calories to eat if you run 3 miles a day, it can be helpful to use a tool such as the National Institute of Health’s Body Weight Planner calculator. This takes into account your height, sex, age, current weight, and activity level to calculate a daily calorie level.