To the OP:
Lex pretty much told you everything you need to know. Ignore most of the other advice on here and do what Lex said.
I've been lifting for over a year now. I started out at about 173 lbs, 20% body fat. (I'm 5'11''.) I'm now at just about the same weight, but more like 13% body fat. I would have made a lot more progress, but for about the first six months I didn't have my diet locked on all the way. I simply wasn't eating enough. Then, I did a little research and reworked my diet, and I've gotten a lot bigger. Currently, I'm on a bulk. I try to take in over 3200 calories a day. I am gaining weight, and all of my lifts are improving. For example, for the longest time I was plateauing on bench press. I couldn't get over 185. The solution? EAT MORE! Since I started bulking about a month ago, I'm up to 195. (Five sets of five.)
To make it very simple:
Step 1: Weigh yourself.
Step 2: Determine your body fat percentage. To get an accurate number, you will need to use calipers, which you can purchase for pretty cheap, so it's no big deal.
To determine your body fat percentage, you measure different areas of one side of your body (typically the right side) and average them together. Usually, the calipers will come with directions on how to use them and the appropriate formulas.
Step 3: Determine your lean body mass. Lean body mass is simply your weight minus all of your body fat. Just multiply your overall weight by your body fat percentage expressed as a decimal. Then subtract that number from your overall weight.
For example:
Tom weighs 200 lbs.
Tom has 20% body fat.
200 x .20 = 40
200 - 40 = 160
Tom has a lean body mass of 160 lbs.
Step 4: Determine your base metabolic rate (BMR). Your BMR is simply the amount of calories your body is burning while resting. In other words, if you were to sit around all day and do nothing, you would burn the amount of calories your body needs to sustain it's lean body mass. You can calculate your BMR
here.
A normal BMR is in the 1600 - 1900 range.
When you use the BMR calculator, make sure you input your lean body mass, not your overall weight!
Step 5: Determine your daily calorie needs. You can do this using the
Harris Benedict Equation that Lex mentioned above. The resulting number is the amount of calories you need to eat in order to maintain your weight, not gaining or losing any lbs.
If you are basically sedentary, multiply your BMR by 1.1 or 1.2. If your are very active, multiply your BMR by 1.5. If your are lifting every day or three or four days a week, you would definitely want to multiply by 1.5. An active person can easily have a maintenance calories level of 2600 or more.
Once you know your maintenance calorie level, add 500 calories if you are trying to gain weight and subtract 500 if you are trying to lose weight. For an average guy, that means that he could easily require well over 3000 calories a day to gain weight.
Step 6: Count your calories. Go and look at the nutritional labels for everything that you eat, record the calories and then add everything up. While you're doing this it's a good idea to also record the amount of protein, carbs, and fats in all of your foods as well as any other factors you may want to track (like sodioum or cholesterol, etc.).
Does it meet your calorie needs? If not, then add in foods until you are in caloric surplus. If it's too many calories, figure out where you can cut out a few until you are at your desired level.
Here is a great website for counting calories and keeping track of everything your eat. You have to create an account, but it's totally free. You can look up a lot of foods that you are unsure about in terms of calorie levels. (How many calories are in a chicken breast? etc.)
Step 7: Adjust your diet. For a bulking phase, I would recommend eating six or seven times a day. I eat at least once every three hours. From the time I get up until I go to sleep. My stomach is constantly full. And the bit about 1.5 g of protein per pound of body weight is absolutely correct. (Some will tell you 1 g of protein per pound of lean body mass, but it's pretty similar in the end.) If you are trying to gain muscle, protein is your friend. Finding time to fit in all of your meals can be really tough with work and school and everything, but it can be done. What I do is cook a lot of things before hand and take them with me in a cooler or an insulated food bag. Most workplaces have a microwave in the break room where you can heat things up.
Example meal plan off the top of my head:
7:00 AM - Meal 1: 1/2 cup of oats -- 1 scoop whey protein - 1 apple
10:00 AM - Meal 2: 4 eggs -- 1 table spoon natural peanut butter
12:00 PM - Meal 3: 1 grilled chicken breast -- 1 potato
Work Out
3:00 PM - Meal 4: Post-Work Out Shake (ex: 2 scoops whey protein) -- 1/2 cup brown rice
5:00 PM - Meal 5: 1/2 lb lean beef -- 2 cups brocoli
7:00 PM - Meal 6: 4 eggs -- 1 table spoon natural peanut butter
9:00 PM - Meal 7: 1/2 cup cottage cheese -- 1 scoop casein protein
Step 8: Get at least 8 hours of sleep a night, every night. Sleep is actually where your body regenerates and builds back the muscle you broke down in the gym, using all that food you've been eating as fuel. If you don't sleep enough, you will not get bigger. It's as simple as that.
Step 9: Adopt a mass building work out routine. Once you've got your diet and sleep schedule completely locked on, you need to make sure that your getting the most out of every work out that you can. And most people will tell you that to gain the most muscle, you need to do 4 - 6 reps for 3 - 5 sets of a heavy weight, utilizing progressive overload. If you can lift a certain weight 7 times, it's time to go up to the next weight.
Progressive overload simply means that you are increasing in reps per set or weight each exercise.
You can start with
Starting Strength, Mad Cow's 5 x 5 program, Max OT training, etc., etc. Suffice it to say that there are plenty of mass building programs out there that you can pick from. The important thing is to stick with it. Heavy weights with low reps.
Step 10: Don't overtrain. Part of getting the most out of every work out is making sure that your muscles have had enough time to recover from the last work out in order to be put under that much stress again. I wouldn't recommend working a particular body part more than twice a week. Most mass building programs only work each body part directly once per week. If your diet and sleep are locked on and you are not increasing in reps or increasing in weight each work out, then you are probably overtraining.
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Following all of that is pretty much the best anyone can do if they want to stay natural. It may take a long time, but if you stick with it and do everything right eventually you will get to where you want to be. And, only gauge your progress off of yourself. Don't pay attention to how strong or how big anyone else is. Everyone has different genetics. As long as your are making progress and doing the best you can, that's all that matters. Good luck man.