Have you ever wondered how many calories a pound of muscle burns in a day? You often hear that muscles are responsible for a large percentage of the calories you burn in a day. When I was in undergraduate school, I was taught that a pound of muscle burned upwards of 40+ calories per pound per day.
I was taught wrong! Recent research has debunked this estimation and discovered that a pound of muscle actually burns…wait for it…I hope you're sitting down…just 6 calories per day! That means if you pack on 10lbs of muscle, you'll only burn an extra 60 calories in a day. So while it's true that you will increase your metabolism by adding on some muscle mass, it will not increase by much. In fact, muscle mass is only responsible for 25% of the calories you burn every day.
But wait, there's more… these numbers are for when you are at rest! This is precisely why exercise is so important for fat loss. Your muscles are not very active when at rest, so they take a vacation while your organs (brain, liver, intestines) are more active, accounting for upwards of 60% of your metabolism. However, while you exercise, your muscles become much more metabolically active and burn calories at a significantly higher rate. This is also the reasoning behind exercising frequently and consistently. In order to lose fat, you have got to have a negative calorie balance and the only way to increase your caloric output is through exercise. The rest of the caloric deficit is the responsibility of your diet. The saying is true that "you cannot 'out exercise' a bad diet."
One last thing, the more intensely you exercise, the more rapidly your muscles will burn through calories. Think about that while you're working out today.
You can learn more about this topic from Lyle McDonald's research based article.