If you've ever tried to count calories for weight loss, then you've probably felt confused by the numbers. The FDA recommends adults eat 2,000 calories a day, but many diets drop that number way down to 1,300 or 1,500 a day for women. As helpful as the FDA is trying to be, there's no easy answer as to how many calories everyone should eat in a day; it all boils down to personal metabolism, body composition, and activity level.
In general, your daily calorie needs are based on your resting metabolic rate (RMR), which is the amount of calories your body burns at rest to perform basic functions to keep you alive: breathing, heart beating, brain power, etc. You can get your RMR tested with a clinical RMR test, like our assistant fitness editor Tamara Pridgett did at Fitnescity, which is the most accurate way to determine how many calories your body burns at rest.
But if you don't have $250 to shell out on a metabolic test (we hear you), then you can calculate your resting metabolic rate using the Mifflin-St. Jeor equation, which is based on your total bodyweight, height, age, and sex. It's a little more complicated, but it's more accurate than quick-and-dirty formulas found online. For a woman, the calculation is:
RMR = (6.25 x height in centimeters) + (9.9 x weight in kilograms) - (4.92 x age in years)
For a woman who is 30 years old, 5'6" (167.6 centimeters), and 150 pounds (68 kilograms), her RMR would be: 1,047.5 + 673.2 - 147.6 = approximately 1,573 calories a day. Once you have that number, then we can determine how many calories you need to eat in a day, whether you want to lose weight, gain muscle, or maintain.
How Many Calories to Eat to Lose Weight
Your RMR is just that: the calories you burn at rest. It doesn't take into account all the calories you burn through exercise or other daily activities such as walking around the office and taking the stairs. Registered dietitian and personal trainer Jim White, ACSM, owner of Jim White Fitness and Nutrition Studios, told POPSUGAR that men can eat at their RMR or up to 10 percent above it to lose weight, which accounts for the calorie deficit to come from exercise and other daily activities.
"For women, some experts have found in their practice that it is best to consume at the RMR and no more than 20 percent below to promote weight loss without damageing metabolism," Jim told POPSUGAR. So for that 1,573 RMR, you can eat around 1,570 calories a day and still expect to lose weight. Just don't go below 1,258 calories a day, which is 20 percent — in general, Jim said women shouldn't eat below 1,200 calories a day to avoid negatively affecting their metabolism and energy levels.
It also depends on how much weight, specifically fat, you need to lose. "If an individual has 100 pounds of fat to lose, they can go into a larger deficit in the beginning without metabolic risk than someone with 20 pounds of fat to lose," he said.
How Many Calories to Eat to Maintain Weight
If you want to keep your weight the same, you will have to factor in your RMR and the calories you burn with exercise and daily activity. "Knowing your RMR is invaluable in estimating accurate caloric needs," Jim said. To figure out how many calories you burn each day, you can use a fitness tracker such as a Fitbit or Apple Watch. Although these aren't 100 percent accurate, it will give you a good estimate of how many calories you burn each day.
"In order to maintain weight, finding the average amount of calories burned per day (RMR + activities/exercise) and consuming around there will provide weight stabilization," Jim said. "Every person is different, therefore this may take some trial and error to find what that is for you."
How Many Calories to Eat to Gain Weight
If you want to gain weight, specifically muscle, you need to eat in a calorie surplus. Jim recommends eating an extra 300 to 500 calories a day above what your weight maintenance would be and making sure your extra snacks are rich in carbohydrates and protein to aid in muscle growth. "This, of course, needs to be in conjunction with strength-training exercises," he said. "A good recommendation is to strength train three days a week to build muscle, with 48 hours between resistance-training sessions."