There's also proper training, recuperation and sleep to factor in too. But when it comes to gaining lean weight, calories are the bottom line just the same. No matter what you eat and no matter how hard you train, if you're not eating enough it is physiologically impossible to gain muscle.
There are many factors involved in gaining lean bodyweight, but the starting point is to calculate your total daily energy expenditure (TDEE), which is the number of calories you require to maintain your bodyweight. According to exercise physiologists William McArdle and Frank Katch in their excellent textbook, Exercise Physiology, the average TDEE for women in the United States is 2000-2100 calories per day and the average TDEE for men is 2700-2900 per day. To calculate TDEE you must first determine your basal metabolic rate (BMR). Your BMR is defined as the minimum level of energy required to sustain the body's vital functions in the waking state.
Here's a simple formula developed by Dr. Fred Hatfield of the International Sports Sciences Association that you can use to estimate your BMR based on your bodyweight in kilograms. (One kilogram is 2.2 lbs.)
Men's BMR=1 X body weight (kg) X 24
Women's BMR=.9 X body weight (kg) X 24
You are male
You weigh 172 lbs. (78 kilos)
Your BMR=1 X 78 X 24=1872 calories
The formula above is based on total body weight, not lean body mass, therefore it will be fairly accurate provided your body fat levels are not above the average ranges (14-19% for men, 20-25% for women). If your body fat is substantially higher than average, then basing caloric needs on total bodyweight alone will overestimate calorie expenditure.
If you know your lean body mass, then you can get an even more accurate estimation of your BMR. This formula from Katch & McArdle takes into account lean mass and therefore is more accurate. The difference in calorie expenditure between men and women is due to the fact that men generally have a higher lean body mass and a larger total body surface area. Since this formula accounts for lean body mass, it applies equally to men and women.
BMR (men and women)=370 + (21.6 X lean mass in kg)
Example: You are male
You weigh 172 lbs (78 kilos)
Your body fat percentage is 14% (24.1 lbs fat, 147.9 lbs lean)
Your lean mass is 147.9 lbs (67.2 kilos)
Your BMR=370 + (21.6 X 67.2)=1821 calories
Now that you know your BMR, you can calculate TDEE by multiplying your BMR by the following activity factor.
Sedentary=BMR X 1.2
Lightly active=BMR X 1.375
Moderately active=BMR X 1.55
Very active=BMR X 1. 725
Extremely active=BMR X 1.9
Continuing with the previous example:
You are a 172 lb. male with 14% body fat and a BMR of 1821
Your activity level is moderately active (work out 3-4 times per week)
Your activity factor is 1.55
Your TDEE=1.55 X 1821=2822 calories
Once you've determined your TDEE, the second step is to increase your calories high enough above your TDEE that you can gain weight. It is a basic law of energy balance that you must be on a positive calorie balance diet to gain muscular bodyweight. If you consume the exact amount of your TDEE you will simply maintain your weight. Generally speaking, you'll need to add another 300-500 calories per day onto your TDEE in order to gain weight. To be more specific, add a minimum of two calories per pound of bodyweight on top of your TDEE to determine your optimal caloric intake to gain weight.
Continuing with our example:
Your weight is 172 lbs
Your TDEE is 2822 calories
Your additional calorie requirement for weight gain is 2 X 172=344
Your optimal caloric intake for weight gain is 2822 + 344=3166
Using the formulas above, we have determined that our "typical" 172 lb. moderately active male will need 3166 calories to gain weight. Keep in mind that this is merely an estimate: All calorie expenditure formulas are estimations. Due to genetic factors, there may be a 20% variance of BMR either way. Age is another factor that you may want to take into consideration. According to Dr. William Evans, PhD., one of the world's leading authorities on exercise and aging, we may need as much as 100 calories less per day per decade to maintain our body weight. Also consider that certain athletes train so frequently and so intensely that their TDEE can be off the normal activity scale limit of 1.9. Daily energy expenditure can be much higher for competitive athletes or extremely active individuals. Some triathletes and marathon runners have been reported to require as many as 5000-6000 calories per day or more just to maintain their weight!
Don't just focus on gaining weight. It doesn't do you any good to gain weight if most of it is fat. The goal of a weight gain program is to gain lean muscle mass with little or no increase in body fat. If you have access to body fat testing, get it done every 1 -2 weeks. If you find yourself gaining fat, first add in 20-30 minutes of cardio 3-4 days per week. If, after adding cardio you still gain fat and the quality and quantity of calories is correct, then you will need to begin cycling your calories up and down in a "zig-zag" fashion. Three high calorie days at your optimum calorie intake for weight gain, followed by three lower calorie days at or slightly below your maintenance level (TDEE) will allow you to add solid weight while keeping your body fat in check.
Using these calorie guidelines, you can expect to gain muscular bodyweight at a rate of 1/2 to 1 lb. per week, or slightly slower if you are female. If two weeks go by and you haven't gained any weight, you're doing something wrong; most likely, you're not eating enough and you should increase your calories. After 3 - 4 months, the rate of muscle gain tends to slow down closer to 1/2 pound per week. Eventually, as you get closer and closer to your genetic limit for carrying muscle mass, the rate of muscle gain will slow down to 1/4 lb per week. Even at this rate, that's still 13 pounds of solid muscle per year.
In Part Two of "How to Gain Lean Bodyweight" will discuss meal frequency, meal ratios, caloric density and proper food choices for packing on the muscle.