The number of calories you need to maintain your current weight is equal to the number of calories you burn. You can calculate your daily calorie expenditure from your activity level and your basal metabolic rate, or BMR. This will allow you to limit your weight loss rate to a safe level.
Weight Loss Rate
The National Institutes of Health recommends a maximum weight loss rate of 2 lb. per week. A pound of fat has about 3,500 calories, so 2 lb. is equal to about 7,000 calories. You should therefore ensure that your calorie deficit is no more than 1,000 calories per day when dieting.
Basal Metabolic Rate
Your BMR is your calorie expenditure rate when you're at rest. It does not include any calories you need to digest food or regulate your body temperature. You'll typically express your BMR in terms of calories per day.
Activity
An accurate estimate of your calorie expenditure requires you to estimate your activity factor. Your activity factor is about 1.2 if you have a generally sedentary lifestyle. An activity level of 1.375 indicates you engage in light exercise up to three times per week. Your activity factor is 1.55 if you have up to five moderately heavy exercise sessions per week. An activity factor of 1.725 is for someone who has up to six sessions of heavy exercise per week.
Formula
The Harris-Benedict equations consist of two gender-specific equations to calculate your BMR. The Weill Cornell Medical College provides the following equation for males: BMR = [6.25 x weight] + [12.71 x height] - [6.775 x age] + 66.5. Females use this equation: BMR = [4.347 x weight] + [4.7 x height] - [4.676 x age] + 655.1. You must provide your weight in pounds in your height in inches.
Example
Assume you weigh 143 lb., are 67 in. tall and are 28 years of age. Your BMR is therefore [4.347 x 143] + [4.7 x 67] - [4.676 x 28] + 655.1 = 1,461 calories per day. You engage in heavy exercise six times per week, so your activity factor is 1.725. Your daily calorie expenditure is therefore 1,461 x 1.725 = 2,520 calories per day.
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