Maintaining your current body weight relies on an equal intake and output of calories. If you take in more calories than you expend on a regular basis, you'll gain weight, but if you don't consume enough food, you'll lose weight. Your specific caloric requirements depends on your basal metabolic rate, as well as your activity level.
Basal Metabolic Rate
The basal metabolic rate, known as BMR, is the minimum amount of calories you need to support basic bodily functions, such as cardiovascular processes and digestion. Determine your basal metabolic rate by multiplying 24 by your weight in pounds, divided by 2.2. At 135 pounds, your basal metabolic rate is 1,472 calories per day. To maintain a weight of 135 pounds, you'll need to have at least this amount of calories in your diet, but you also need to account for exercise.
Accounting for Activity Level
Once you know your basal metabolic rate, multiply the amount of calories by a co-factor that represents your activity level. Being relatively sedentary has a co-factor of 1.45, light activity level is 1.60, having a moderate activity level results in a co-factor of 1.70, and if you are very active or are a trained athlete, your co-factor is 1.88. Multiply your basal metabolic rate by the appropriate co-factor to determine your exact calories. If you are older and don't get out of the house often, multiply 1,472 calories by the sedentary co-factor of 1.45, which results in 2,134 calories to help you maintain your weight. Being a fitness instructor or training for a marathon increases your caloric needs. In these cases, multiply your basal metabolic rate by 1.88, resulting in 2,767 calories per day to maintain 135 pounds.
Calories from Macronutrients
Intake recommendations for macronutrients, or carbohydrates, protein and fat, are based on a percentage of your calories. Knowing the proper amount of macronutrients and keeping track of your intake helps you maintain your weight. According to the Dietary Guidelines for Americans 2010, 45 to 65 percent of your calories should come from carbohydrates, 10 to 35 percent should come from protein and 20 to 35 percent from fat. Both carbohydrates and protein have 4 calories per gram, but fat is a concentrated energy source and has 9 calories per gram. As an example, at 135 pounds, a basal metabolic rate of 1,472 calories and a moderate activity level, you'll need approximately 2,502 calories per day. Based on 2,502 calories, you should get 281 to 406 grams of carbohydrates, 62 to 218 grams of protein and 55 to 97 grams of fat.
While the basal metabolic rate and activity level calculation is a good starting point for determining the right amount of calories for weight maintenance, it may under or overcompensate for the actual amount of calories you need. Stick to the lower end of your recommendation of macronutrients and track your intake for one or two weeks. If you are losing weight or always feel hungry, slowly add small amounts of carbohydrates, protein and fat to your diet to maintain 135 pounds. An increase in weight means that you might be consuming too much. Cut back by opting for low-fat dairy or skip that second latte in the afternoon.
Melodie Anne Coffman specializes in overall wellness, with particular interests in women's health and personal defense. She holds a master's degree in food science and human nutrition and is a certified instructor through the NRA. Coffman is pursuing her personal trainer certification in 2015.