Calculate your BMR (basal metabolic rate). Your BMR is the amount of calories your body needs to maintain basic bodily functions like breathing and digestion. This is the minimum number of calories you need to eat each day. Keep in mind that no calculator will be 100% accurate, so you may need to adjust these numbers as you learn more about your own metabolism.
Calculate your activity level. For a week or so, keep an activity journal and use a calorie calculator to figure out how many calories you burn while sitting, standing, exercising, lifting weights, etc. throughout the day. Another, easier option is to wear a heart rate monitor that calculates calories burned. After a week, add your totals for each day and average them out to get a general idea of how many calories you burn each day.
Keep track of how many calories you eat. For at least a week, enter and track your calories online (e.g., with Calorie Count) or use a food journal to write down what you eat and drink each day. Be as accurate as possible, measuring when you need to or looking up nutritional information for restaurants, if you eat out. After a week, add your totals for each day and average them out to get a general idea of how many calories you eat each day.
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