The first step is to calculate your basal metabolic rate (BMR), which is the amount of calories you burn in a day just resting, then either add or subtract 500 calories, depending on whether you are trying to lose or gain weight. This number will be how much you’ll want to eat in a day to lose or gain about 1 lb a week (a 3500 calorie deficit equates to about 1 lb of fat loss). There are many different ways to calculate BMR, but if you’re not severely overweight or underweight I recommend just multiplying your current weight by 16. This is roughly how many calories you eat in a day. For example, I currently weigh about 215 lbs, so my BMR is about 3440 calories (215 x 16), and I would need to eat 3940 calories a day to gain 1 lb a week, or eat 2940 calories a day to lose 1 lb a week.