The general rule is that if you eat more calories than you use, you’ll gain weight. And if you take in fewer calories than you use, you’ll lose weight. And if those numbers are more or less even, your weight will stay about the same. It seems simple, but the number of calories you need to lose weight, maintain weight, or gain weight from lean muscle depends on your activity levels, body size, hormones, sleep, and more, explains registered dietitian Wesley Delbridge, R.D., a spokesman for the Academy of Nutrition & Dietetics. So figuring out how many calories you need per day can be crazy-complicated.
And, it's also important to remember that, when it comes to cutting calories for weight loss, lower is not always better. According to the American College of Sports Medicine, your calories should never dip below 1,200. That’s because most women, unless they are very small, will burn more calories than that doing literally nothing, says registered dietitian nutritionist Jonathan Valdez, R.D.N, C.D.N, owner of Genki Nutrition and a spokesperson for the New York State Academy of Nutrition and Dietetics. “Less than that and you could shock your body into starvation mode, which will slow your metabolism, decrease your muscle mass, and likely keep you from getting the nutrients you need to sustain daily activity,” Delbridge explains.
So, if you're asking yourself, "How many calories do I need a day?" read on as experts explain what you need to know to get your calorie intake just right.
How To Determine Your Base Calorie Needs
In order to figure out how many calories you need to lose, or even gain weight, you first need to determine how many you need to maintain. As a first step, Delbridge recommends checking out the latest Dietary Guidelines for Americans, as they can give you a good estimate for what you need to stay the same weight. The guidelines say young women should aim for 1,800 to 2,400 daily calories, depending on age and activity level, but that range isn’t necessarily tailored to your specific needs—so it’s not as precise as it could be.
For a more exact number, start by finding your basal metabolic rate (BMR), which is the minimum number of calories your body burns at rest, suggests physical therapist Grayson Wickham, D.P.T., C.S.C.S., founder of Movement Vault. Your BMR accounts for 60 to 75 percent of your total daily calorie burn, according to a review in Medicine and Science in Sports and Exercise.
“To most accurately calculate your BMR, you’d need to go to a lab to have your carbon dioxide and oxygen analyzed after having fasted for 12 hours and slept for eight. But, that can be a little pricey and a rough estimation of your BMR can be found using a few different equations,"says Wickham.
One study published by the Journal of the Academy of Nutrition and Dietetics found the Mifflin-St. Jeor equation to be highly accurate, so it is now considered the gold standard when it comes to calculating BMR. For comparison's sake, however, some experts prefer the Harris Benedict equation for determining BMR.
For women, the Mifflin-St. Jeor equation equation is: BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161.
So, for a 25-year-old woman who is 5'4" and weighs 150 pounds, the BMR equation would look like this: BMR= (10 x 68) + (6.25 x163) - (5 x 25) -161 = 1,413 calories
For women, the Harris Benedict equation is: BMR = 655.1 + (9.563 x weight in kg ) + (1.850 x height in cm) - ( 4.676 x age in years).
So, for the same woman, the BMR equation would look like this:
BMR= 655.1 + (9.563 x 68) + (1.850 x 163) - ( 4.676 x 25) = 1,490
As you can see, the results for both are slightly different, but they're pretty darn close, says Wickham. When you find your BMR on your own, consider it a really good estimate, not a hard-and-fast rule, he adds.
For both equations, finding your BMR requires your weight, height, age, and gender (yes, guys have their own equation). Wondering why? “The more you weigh and the more mass you have, the more fuel you need to sustain your organs,” explains Valdez. That’s why people who weigh more have heavier BMRs.
Age is a factor in the equations because, as you get older, muscle mass declines by 5 or so percent each decade after the age of 30, Wickham explains. This might change as more women start strength training, but as a general rule, that’s fair, he says. And if you’re wondering why the formula is different for men and women, it’s because research published in The Journal of Clinical Investigation shows that a woman’s BMR is typically around 5 to 10 percent lower than a man’s.
Now what? Once you know your BMR, you know the bare minimum number of calories you would need to keep your body alive if you were going to lay in bed all day, says Wickham. But you need to take into account everything else you do that burns calories (walking the dog, folding laundry, climbing five floors of stairs to your apartment, bi-weekly CrossFit class, Thursday evening yoga…).
To do that, multiply your BMR by the factor that best represents your activity level.
- If you are sedentary = BMR x 1.2
- If you do light exercise 1-3 days a week = BMR x 1.375
- If you exercise at a moderate intensity 3-5 days a week= BMR x 1.55
- If you are exercise at a high intensity 6-7 days a week = BMR x 1.725
- If you are into two-a-days or have a physically demanding job = BMR x 1.9
How Many Calories You Need To Lose Weight
Okay, so how many calories do I need a day to lose weight? Once you know how many calories you need to maintain your weight, you simply subtract some calories to put yourself into a caloric deficit. How many calories? Well, to lose roughly half a kilo a week (a healthy goal) you need a 500-calorie-per-day deficit, he explains. In other words, just delete 500 calories from the number you found above.
But a calorie deficit doesn't have to (and, in reality, shouldn't) come solely from eating less, says Valdez. Exercise can help, too. If you are game for taking your workouts to the next level, Valdez recommends decreasing your calories from food by 250 per day, and increasing the intensity or duration of your workouts so that you are burning an additional 250 calories two to three times a week through exercise.
That means if you already take a cycling class three days a week, add in a 30-minute walk two days per week to keep the calorie burn going. Or, if you currently live a lightly active life, consider incorporating a yoga class, strength-training class, or hike into your routine.
However, that does mean that on the days you don't do any physical activity at all, you should decrease your calorie intake by closer to 500 cals, he says. Your goal is to burn about 500 fewer calories than you take in per day, through diet, exercise, or both. You do the math.
How Many Calories You Need To Gain Weight From Muscle
Not everyone who counts calories wants to lose weight. Some want to gain it from lean, powerful muscle. Gaining weight from muscle is a great way to improve your health and even decrease your body-fat percentage. Bonus: Since muscle is metabolically active, it can also help you shed fat without cutting calories, says Wickham. When you start to gain muscle, your BMR will increase, which means that your body needs more calories just to go about its daily function, he explains.
“If you want to gain weight, the simple trick is to tack on 250 to 500 extra calories in healthy, whole foods per day. Every one to two weeks, you’ll have added a pound safely,” says New York City nutritionist Brittany Kohn, R.D. To gain muscle without also gaining fat, you need to increase your protein to 1.8 to 2 grams per kilogram of body weight every day, so the majority of these additional calories should come from protein, says Valdez. And the rest should come from carbs like whole grains, fresh fruits, and vegetables, which will help power your workouts.
This article originally appeared on Women's Health US.