Most people understand that to lose weight, you need to eat fewer calories than you use. Figuring out just how many calories you should eat, however, isn't always easy, as this depends on your weight, age, gender and the amount of activity you get. Take in too many calories, and you won't lose weight, but the same can be true if you get very few calories. Calculating your basal metabolic rate can help you estimate the number of calories you need.
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Calculating Calorie Needs for a 180-Pound Person
One of the more accurate methods for figuring out your calorie needs is to calculate your resting metabolic rate, sometimes called your basal metabolic rate, or BMR. This can then be multiplied by different numbers based on your activity level to estimate the calories needed to maintain your weight.
Men should use the formula: BMR = 66.47 + (13.7 x weight in kilograms) + (5 x height in centimeters) - (6.8 x age in years).
Women should use the formula: BMR = 655.1 + (9.6 x weight in kilograms) + (1.8 x height in centimeters) - (4.7 x age in years).
Divide your weight in pounds by 2.2 to get your weight in kilograms, and multiply your height in inches by 2.54 to get your height in centimeters. For example, if you weigh 180 pounds: 180/2.2 = 81.8 kilograms. Using these equations, a 35-year-old man who is 5 feet 10 inches tall and weighs 180 pounds has a BMR of 1,838 calories per day. A 35-year-old woman who is 5 feet 5 inches tall and weighs 180 pounds has a BMR of about 1,580 calories per day.
Next, the BMR is multiplied by a number based on activity level to get a better estimate of calorie needs. Multiply your BMR by 1.2 if you don't exercise much, by 1.375 if you exercise once or twice per week, by 1.55 if you exercise three to five days per week and by 1.725 if you exercise most days of the week. If you exercise a lot and have a physical job, multiply your BMR by 1.9.
Using these numbers, in order to maintain his current weight, the man previously mentioned would need 2,206 calories per day if he was inactive -- because 1,838 x 1.2 = 2,206 -- or he'd need 2,849 calories per day if he was moderately active. To maintain her current weight, the woman would need around 1,900 calories per day if she was inactive or 2,450 calories per day if she was moderately active.
Estimating Calories Necessary for Weight Loss
For each pound of weight loss, you need to create a 3,500-calorie deficit. This can be done by eating fewer calories, increasing activity levels to burn more calories or a combination of the two. It's important not to eat too few calories, however, as this could slow down your metabolism and weight loss. Women shouldn't go below 1,200 calories per day, and men shouldn't go below 1,800 calories per day.
To lose about 1 pound per week, you need to cut 500 calories per day. So, for example, the inactive woman previously mentioned -- who needs 1,900 calories per day to maintain -- would need to eat roughly 1,400 calories per day to lose weight. To lose a pound a week, the inactive man who needed about 2,200 calories for maintenance, can subtract 400 calories to eat the minimal 1,800 calories per day, then burn another 100 calories through exercise.
Effect of Exercise on Weight Loss and Calorie Needs
The more cardio you get, the more calories you burn. You can use these extra calories to increase your weight loss or to allow you to eat a little more while still losing weight. For weight-loss purposes, you should aim for 300 minutes of moderate-intensity cardio exercise and at least two strength-training sessions per week, recommends the Department of Health and Human Services.
While it may not seem too important since it doesn't burn a lot of calories, strength training should not be skipped. Such exercises build muscle, which increases your metabolism, and also limit the amount of muscle you lose as you lose weight. A study published in Obesity Research in 2001 found that it takes about 2 calories to maintain each pound of fat, but it takes 6 calories to maintain each pound of muscle. This means adding an extra 10 pounds of muscle to your body should theoretically increase your basal metabolic rate by 60 calories.
Metabolism and Weight Loss
It may take a little trial and error to figure out just how many calories you need to cut and how much activity you need to engage in, as calories burned can vary based on a number of factors. For example, every 10 years after the age of 20, your metabolism will probably slow down by 2 or 3 percent due to decreases in muscle. Strength training can help minimize this, however. Hormones, including thyroid hormones, can also affect your metabolism, which is why many women tend to experience a slower metabolism and increased weight gains after they turn 40, when thyroid activity tends to decrease. Dietary factors, including the amount of protein and fiber you eat, can play a role in metabolism as well, as these nutrients tend to have a beneficial effect and slightly increase weight-loss results. Keep in mind that as you lose weight, you need fewer calories to maintain your new weight, so you'll need to continue decreasing your caloric intake as your weight loss progresses.