A BMR Calculator can be a very valuable tool for runners. BMR stands for basal metabolic rate and, basically, it is the amount of calories you need to consume each day to maintain your weight, if you were being totally inactive. If you want to gain weight, eat more calories than your BMR. Conversely, if you want to lose weight, eat fewer calories than your BMR and add in exercise to burn more calories. But, in order to make sure that you're eating an appropriate amount of calories for your weight goals, you need to know what your target is.
That's where the BMR Calculator comes in. Most BMR Calculators use the Harris Benedict Equation which takes your height, weight, age, and gender and then performances calculations based on them to return your basal metabolic rate. All four of those factors will play a role in determining what your BMR is.
Your BMR is the amount of calories you've burned if you were completely inactive but, of course, as a runner you are not completely inactive, so you also need to use a Calories Burned Calculator to find out how many calories you're burning while running. You can then add that number to your BMR to get a picture of your total daily caloric requirements. For example, if your BMR is 1500 calories per day and you run enough to burn another 500 calories, you'll need to consume 2,000 calories to maintain your weight (and less than 2,000 calories if you're trying to lose weight).
Of course, different people might have different metabolic rates even though they are the same height, weight, age, and gender. So, as with any tool, you need to use the BMR calculator as a guideline, not an absolute truth. Your exact calorie needs might be a little different than what it returns to you, but it's a good enough guide to get you started.
It's important to note that you actually burn some calories while digesting your food, too, as well as performing other non-running activities such as walking, cleaning, shopping, etc. So, you'll want to take these into account when coming up with your daily calorie take plans.
Figuring out what to eat can be complicated even when using tools like a BMR calculator, so sometimes it's best to reach out to an expert. You can ask your friends for a recommendation of a good nutritionist and give him or her a call to get a tailored nutrition plan set up for you.