Every 3,500 calories one burns is equal to one pound, so to lose 30 pounds, an individual needs to burn about 105,000 calories. According to About, a mathematical formula called the Harris-Benedict principle allows individuals to calculate their basal metabolic rate, and the number of calories needed simply to function at their current weight. Knowing their BMR, they can determine how many calories to eat to lose weight.
There are a number of different tools online to help calculate basal metabolic rate, but it's also relatively simple to calculate with a pen and paper. For adult women, use the following formula: 655 + (4.3 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years). For adult men, use this formula: 66 + (6.3 x weight in pounds) + (12.9 x height in inches) - (6.8 x age in years).
Next, calculate how many calories you already burn during the day. If you are mostly sedentary, add 20 percent to your basal metabolic rate. Add 30 percent if you are lightly active and 40 percent if you are moderately active. Once you know how many calories you burn, you can create a calorie deficit, either by eating less or by becoming more active.