Fat has a volume 3 times greater than muscle: this is why you can be in the best shape of your life and end up being heavier than when you were a couch potato! The difference in the resting metabolic rate of the 2 tissues is 3-6 calories per day: 1 pound of muscle will burn 3-6 calories more per day than 1 pound of fat. Disappointing as this stat is (there are many places which will tell you it’s 50 calories per day) you must bear in mind that this is when resting/inactive. As soon as your muscles are active they make calorific demands on your body which mean you will use more energy (there’s a reason Michael Phelps can eat that breakfast!) Fat is more buoyant than water: this is how hydrostatic body fat testing works – by comparing your ‘land-weight’ with your ‘water-weight’ you can calculate the percentage which is fat.
Think bathtub, Archimedes and a calculator and you’re pretty much there! It’s a much more accurate measure than scales such as BMI which are based on height vs. weight without the density of fat and muscle being taken into account. 1 pound of muscle working at its full capacity will burn 4.7 calories per hour: this may not sound as much as you would hope but if you imagine a 150lb person, roughly 40% of their weight being taken up by muscle, in one hour they have the potential to burn 282 calories and that’s solely in their muscular output. There are still ‘essential’ expenditures which are constantly running in the background (a bit like your Tribesports on your desktop!) which are burning calories at the same time as your muscles are working.