In a culture where women's magazines and websites are frequently telling you how to cut calories from your diet, it can be easy to forget that you need to consume enough calories to function optimally. The U.S. Department of Agriculture estimates that women need at least 1,600 calories per day, but it may be safe to consume fewer calories for weight loss. Calculating your basal metabolic rate and applying the Harris-Benedict formula will give you an estimate of your personal calorie needs.
Typical Calorie Needs
Your individual calorie needs will depend on your height, weight, age and activity level. As a woman you may need fewer calories than a man of the same age and activity level. Your calorie needs will increase with your activity level and decrease with age. On the high end, an active woman in her 20s or 30s may need 2,400 calories a day, according to the USDA, and on the low end a sedentary woman over 50 may need as few as 1,600 calories a day. Even if you fall on the low end of the spectrum, you likely need more than 1,500 calories per day to maintain your current weight and energy levels. The USDA's figures do not take weight or height into account, so they aren't completely personalized.
Basal Metabolic Rate
Your basal metabolic rate is the number of calories your body needs for basic functions. According to the McKinley Health Center, "your BMR is the amount of calories you would need if you laid in bed all day and didn't move." The BMR formula for women is: 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years ). For example, consider a 30-year-old woman who is 5 feet 3.8 inches tall and weighs 166.2 pounds -- the average figures for an American woman as of 2012, the Centers for Disease Control and Prevention notes. To get her BMR, you would add 655 to the product of 4.35 and 166.2 to get 1,378; then you would add the product of 4.7 and 63.8 to 1,378 to get 1,678; finally you would subtract the product of 4.7 and 30 from 1,678 to get a basal metabolic rate of 1,537.
It's essential to remember that your BMR is just the number of calories you need for basic biological functions. To get your calorie needs for daily activities, you need to use a formula like the Harris-Benedict equation, which multiplies your BMR by an activity factor. The factor for a sedentary person is 1.2, and for a lightly active person it's 1.375. A moderately active person uses a factor of 1.55, while a very active person uses 1.725 and an extra active person uses 1.9. For example, a woman with a BMR of 1,537 needs 1,844 daily calories if she is sedentary but 2,290 if she is extra active.
Minimum Calorie Needs
If you're attempting to lose weight, you'll need to increase your physical activity level or decrease your calorie intake. You should aim to create a safe daily deficit of 500 to 1,000 calories. But you need to keep your intake above a safe minimum. For women, 1,500 calories is enough to meet this minimum, but women should never consume fewer than 1,200 calories per day unless it's under a doctor's recommendation and supervision.