You use energy no matter what you're doing, even when sleeping. Your basal metabolic rate (BMR), is the number of calories you'd burn if you stayed in bed all day.

The BMR formula uses the variables of weight, height, and age to calculate the basal metabolic rate (BMR). This is more accurate than calculating calorie needs based on body weight alone. The only factor it omits is lean body mass and thus the ratio of muscle-to-fat a body has. Leaner bodies need more calories than less leaner ones. Therefore, this equation will be very accurate in all but the very muscular (will underestimate calorie needs) and the very fat (will over-estimate calorie needs).

If you've noticed that every year, it becomes harder to eat whatever you want and stay slim, you've also learnt that your BMR decreases as you age. Likewise, depriving yourself of food in hopes of losing weight also decreases your BMR, a foil to your intentions. However, a regular routine of cardiovascular exercise can increase your BMR, improving your health and fitness when your body's ability to burn energy gradually slows down.

Harris benedict equation is a formula that uses your BMR and then applies an activity factor to determine your total daily energy expenditure (calories).

If you want to gain body weight, you need to consume more calories than you burn. One pound of body weight is roughly equivalent to 3,500 calories, so eating an extra 500 calories per day will cause you to gain one pound a week.

Calculate your one rep max, based on your max reps.

Calculate your body mass index, based on your weight and height.

Calculate your basal metabolic rate, based on your weight, height, and age.