Figuring out your daily calorie needs is the foundation of any weight goal. Gaining, losing or maintaining your weight requires you to eat different amounts and if you don't know how to figure that out, you won't be able to reach your goal.
Every single calorie that you put into your body either gets burned or stored (if everything is working properly). Think of your body as a car. When you put 20 gallons of fuel into your car, if you don't burn those 20 gallons right away, they are 'stored' in the gas tank for later use. The same thing happens in your body. If you eat 3,000 calories in a day but don't burn all that energy up, your body will store what's left over (as fat in various areas around your body and as glycogen in the liver and muscle).
A gallon of gas weighs about six pounds. If you fill your car up with 20 gallons and burn off only 10 of those gallons, your car will have gained 60 pounds. The same goes for food. A pound of body weight equals about 3,500 calories. If you eat 5,000 calories and burn 1,500 in a day, you will gain one pound.
Depending on how much you eat, you will either gain weight, lose weight or maintain your current weight. Maintaining weight is the easiest to understand. Remember, if you eat too much, your body will store the extra calories for later use which will lead to weight gain. If you don't eat enough, your body will go to that stored fat and start burning it causing weight loss.
How Many Calories You Burn
To figure out how many calories you use each day use the calorie calculator. It uses the Harris-Benedict Equation which estimates your daily calorie expenditure using your height, weight, age, gender and activity level. It's just an estimate so you will need to monitor your weight and adjust your calorie intake accordingly.
Once you figure out how many calories you burn each day, all you need to do is eat that much. If you constantly burn and eat 2,500 calories per day, your body won't start using its fat stores for energy. It also won't store any fat for later use because all the energy is coming from the food you eat.
Losing weight requires you to eat less than you burn. If you don't provide your body with 100% of its energy needs, it will find that energy mainly in your body fat. Over time, this will lead to weight loss. With some diets, you can probably lose a lot of weight in a short period of time. These weight loss solutions are usually temporary because the amount of calories you need to cut will leave you hungry all day long. You won't be able to sustain that for very long and once you start eating regularly, the weight will come back.
Healthy weight loss is about 1-2 pounds per week. Those that lose weight at this rate are much more likely to keep it off. Remember, to lose one pound, you need to burn 3,500 calories more than you eat. You can't do this in one day (healthy weight loss isn't 1 pound per day, it's 1-2 pounds per week) so you need to spread this calorie deficit (difference between calories eaten and burned) over an entire week. If you divide 3,500 calories by seven days per week, you get 500 calories per day. That means that each day, you need to burn 500 calories more than you eat to lose 1 pound.
If you want to lose 2 pounds per week, double your calorie deficit to 1,000 calories per day. This might seem like a huge deficit but you can create it with a combination of eating less an exercising more. You don't have to cut 500 calories out of your diet. You can burn an extra 250 calories each day and cut 250 calories out of your intake to reach the 500 calorie per day deficit.
Gaining weight is the exact opposite of losing it. To gain weight, you need to eat more than you burn so that your body can use that excess energy to build muscle. There is no way to gain weight if you aren't creating a calorie surplus (eating more than you burn). The same numbers that apply to losing weight can be applied to gaining it. To gain 1 pound per week, you need to create a surplus of 500 calories per day. Double it to 1,000 calories per day to gain 2 pounds per week.
If you're trying to gain muscle, you also need to engage in strength training. If you don't exercise on a regular basis, your body will store those extra calories as fat instead of building up your muscles. Gaining weight is easy, gaining muscle is a bit harder. Healthy weight gain is 1-2 pounds per week. If you try and gain weight faster than that, you'll be putting on more fat than muscle.
Even though at the end of the day, weight is controlled by how many calories you eat, not all calories are equal in health. If you're trying to lose weight, you can't just remove healthy foods from your diet to decrease your calorie intake. You have to get rid of the bad foods so that your health isn't negatively affected. The same goes for gaining weight; you can't just eat everything in sight because then you'll start to get unhealthy.
The Bottom Line
To be successful, you need to monitor your progress. You need to weigh yourself on a regular basis to ensure that you're moving in the right direction. Not losing weight a week or two after you start a diet isn't reason enough to quit. If you aren't moving towards your goal and think you're taking all the right steps, you're not. Your exercise levels and/or calorie requirements might need to be tweaked. Tweaking leads to success while quitting leads to digging yourself further into your problem.