Ask the Diet Doctor: How Many Calories Should I Eat to Lose Weight?

Q: How many calories should I eat per day to lose weight?

A: Calculating the number of calories you need to lose weight is a relatively simple, three-step process. The Harris-Benedict equation is the most widely used method of calculating your calorie needs (and thus your calorie needs for weight loss). Here is the Harris-Benedict equation for women:
BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
You just need to plug in your age, height, and weight. The number you get is the total number of calories you need each day to exist (also known as your basal metabolic rate, BMR). For example, a 50-year-woman who is 5' 7" and weighs 160 lbs has a basal metabolic rate of 1441 calories.

Since you don’t lie in bed all day, you’ll burn more calories than this. To estimate how many calories you burn during your daily activities, we’ll use the activity factors listed below.

• Sedentary: Minimal movement, lots of TV watching, reading, etc. Activity factor = 1.4
• Light activity: Office work, ~1 hour of moderate exercise/activity during the day. Activity factor = 1.5
• Moderate activity: Light physical/manual labor during the day, plus more active lifestyle. Activity factor = 1.6
• Very Active: Active military, full time athlete, hard physical/manual labor job. Activity factor = 1.9

Next, multiply your activity factor by your BMR. For the example we're using, we’ll choose an activity factor of 1.5 (common for most people) and multiply that by 1441 calories, giving us 2161 calories. This number is your total caloric needs, or roughly the amount of calories that you need to eat each day to maintain your weight. To lose weight, you need to eat less than this. How much less? That's the next step.

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Determining how many calories to cut each day is where weight loss becomes more of an art than a science, as there are many variables that can impact the calories in < calories out equation, including:

1. The type of exercise you are doing. Resistance and interval training will burn more calories after you stop exercising compared to traditional aerobic training.

2. The type of diet you are eating. High-protein diets burn more calories, as protein takes more effort for your body to digest and metabolize.

3. How much weight you have to lose. For simplicity sake I had you use total body weight instead of your lean body mass (which is your total body weight minus your body fat). Because of this assumption, if you need to lose 25 pounds or more to reach your goal weight, then your total caloric needs, which we calculated above, is probably too high. This is because we treated the calorie needs of body fat the same as lean tissue (muscles, bones, and organs), but in reality your body fat has a much lower caloric need (close to zero). I explain how to adjust for this below.

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4. Your individual metabolism. The Harris-Benedict equation or any equation that estimates your calorie needs is just that, an estimate. These equations are based on averages, and you are probably not average. Don’t take the numbers you generate after reading this article as gospel, but use them as a starting point, put them to the test, and adjust from there.

Traditionally, recommendations are made for individuals to subtract 500-1000 calories from their total calorie needs in order to lose 1-2 pounds per week. This is because a 500-calorie daily deficit yields a 3500-calorie weekly deficit—which is the number of calories you need to burn to lose one pound of fat. However, I find that these reductions are too aggressive initially and that cutting too many calories too soon sets you up for long-term weight-loss failure.

If you are weight training, doing interval cardio sessions during the week, and eating a carb-controlled/moderate-protein diet, then I recommend that you only subtract 250 calories from your total calorie needs (or 500 calories if you need to lose more than 25 pounds to reach your goal weight). You’ll see results that make you happy while being able to eat more. Using our example, the woman would need to eat 1900 calories per day to lose weight.

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The final step: Put it to the test! At the end of two weeks, see how much weight you have lost. If you aren’t losing at a rate that makes you happy, opt to do more activity before you cut out more calories. If you need to cut out more calories, remove another 250 and put that new calorie level to the test for two weeks.