How Many Calories in a Day Does it Take to Gain Weight?

When you gain weight, it usually creeps on slowly over several weeks or months -- not all at once in one day. Weight gain occurs when you consume more calories than you burn. A 3,500-calorie surplus -- 3,500 calories in addition to those you use every day -- will make you gain about 1 pound. Exactly how long it will take you to gain a pound depends on your basal metabolic rate, which is based on your genetics, age and gender, as well as your activity level.

The exact number of calories required to gain weight in a day depends on your individual metabolism. Use a basic formula, known as the Harris-Benedict equation, to determine roughly how many you need if all you were to do is lie in bed all day. This basal metabolic rate, or BMR, is the energy required to fuel basic bodily functions -- including pumping blood, operating internal organs and brain activity.

For a man, add 88.4 plus 13.4 times your weight in kilograms. Then add 4.8 times your height in centimeters. Finally, subtract 5.68 times your age in years. For a woman, the equation is slightly different. Add 447.6 and 9.25 times your weight in kilograms. Add 3.1 times your height in centimeters. From this total, subtract 4.33 times your age in years. To figure kilograms, divide your weight in pounds by 2.2; to figure centimeters, multiply your height in inches by 2.54. Alternatively, you can use an online BMR calculator to do the math for you.

For a 35-year-old, 5-foot-10-inch man weighing 160 pounds, this comes out to 1,721 calories just to survive. For a 35-year-old, 5-foot-6-inch woman weighing 125 pounds, the result is 1,341 calories per day.

Take the number you determine as your BMR and multiply it by a factor that represents your activity level to come up with an estimate of how many calories you need daily to maintain your current weight. Once you know this number, you can then determine how many calories lead to weight gain. A sedentary person who does little more than sit at a desk all day multiplies the BMR by 1.2. For light activity, which involves a workout one to three times per week, use 1.375. For moderate activity, which means you are moderately active three to five times per week -- probably meeting the 150 minutes per week of moderate activity recommended by the Centers for Disease Control and Prevention -- multiply by 1.55. For the very active person who has daily sports practice, use 1.725, and for the extremely active person, who performs hard exercise and has a physically active job, use 1.9.

In the previous examples, if the man heads to the gym six times per week for intense weight training and 30 to 45 minutes of cardio, he'd qualify as very active and need 2,968 calories to maintain his weight. If the woman example previously mentioned had a desk job and never exercised, she'd need just 1,609 calories to maintain her frame.

Gaining weight quickly and without exercise usually leads to an increase in body fat, rather than healthy lean muscle. To add muscle, aim for just a 250- to 500- calorie surplus per day to add 1/2 to 1 pound to your frame per week. Choose healthy calorie additions, such as an extra serving of chicken or steak at meals or cottage cheese before bed. The protein in these added calories supports a concerted weight-training program that includes two to three total-body sessions per week using heavy resistance. Each workout should consist of one to three sets of at least one exercise for every major muscle group that contains four to eight repetitions.

Larger servings of processed foods, such as snack mixes, fast food or soda, will cause weight gain, but probably not the muscle you're seeking. You don't have to make huge changes to increase your daily calorie intake healthfully by 250 to 500 calories. For example, add 2 tablespoons of peanut butter to your morning toast to add 190 calories; melt cheese on scrambled eggs or add an ounce to a sandwich for another 114 calories; add a half an avocado to a salad for 113 calories; or blend up a post-workout smoothie that includes a banana, scoop of whey protein and a cup of milk for 400 calories.

Other high-calorie, healthy foods to feature at meals and snacks include dried fruit, whole-grain breads, seeds and starchy vegetables, such as sweet potatoes and corn.