From Harvard Medical School

Eat less, exercise more. If only it were that simple! As most dieters know, losing weight can be very challenging. As this report details, a range of influences can affect how people gain and lose weight. But a basic understanding of how to tip your energy balance in favor of weight loss is a good place to start.

Start by determining how many calories you should consume each day. To do so, you need to know how many calories you need to maintain your current weight. Doing this requires a few simple calculations.

First, multiply your current weight by 15 — that’s roughly the number of calories per pound of body weight needed to maintain your current weight if you are moderately active. Moderately active means getting at least 30 minutes of physical activity a day in the form of exercise (walking at a brisk pace, climbing stairs, or active gardening). Let’s say you’re a woman who is 5 feet, 4 inches tall and weighs 155 pounds, and you need to lose about 15 pounds to put you in a healthy weight range. If you multiply 155 by 15, you will get 2,325, which is the number of calories per day that you need in order to maintain your current weight (weight-maintenance calories). To lose weight, you will need to get below that total.

For example, to lose 1 to 2 pounds a week — a rate that experts consider safe — your food consumption should provide 500 to 1,000 calories less than your total weight-maintenance calories. If you need 2,325 calories a day to maintain your current weight, reduce your daily calories to between 1,325 and 1,825. If you are sedentary, you will also need to build more activity into your day. In order to lose at least a pound a week, try to do at least 30 minutes of physical activity on most days, and reduce your daily calorie intake by at least 500 calories. However, calorie intake should not fall below 1,200 a day in women or 1,500 a day in men, except under the supervision of a health professional. Eating too few calories can endanger your health by depriving you of needed nutrients.