Your Basal Metabolic Rate is a measure of how much energy your body uses when you are resting just for normal bodily function. It does not take into account any exercise you may do or how active your lifestyle is (or isn't). Your BMR contributes 50-80% of your energy used in a day.
Increased lean muscle mass results in a higher BMR; so often as you get older your BMR decreases as you lose muscle mass. Consequently, if you consistently participate in some resistance training you will build muscle mass and therefore have a higher BMR!
You can figure out your average BMR if you know your age, gender, height and weight. An average male may have a BMR of around 7100kJ per day while an average female 5900kJ per day. To find out exactly a DEXA scan may be done to determine the composition of the body in relation to muscle and fat.
My husband just askled me why he would want to know this.
The reason this is important is firstly so that you know how many calories your body uses on a daily basis. If you want to lose weight you need your calorie balance to be in negative. You need to know how many calories (approx) your body uses in a day, added to the exercise you do -- to know how many calories you are expending. Eat less than that - you will lose weight.
The second reason this is important is embeded in the fact that muscle uses more energy than fat!! More lean muscle = more calories burnt while you sit and do nothing. Now -- That sounds good to me!! So add some weights to your workouts people!!