How to measure metabolism rate?

Metabolic rate is the rate at which your body uses energy to carry out certain activity, physiological functions etc. BMR (basal metabolic rate) is standardized measure of energy expenditure when at rest. BMR is a rate of energy expenditure for an individual at rest in a supine position, measured immediately after at least 8hrs of sleep & at least 12hrs of fasting. This value reflects the minimum amount of energy required to carry on your body’s essential physiological functions. Your BMR is directly related to your fat free mass, body surface area. Other factors that affect your BMR are age, body temperature, stress, hormones.
As your body shifts from resting state to some activity, your metabolic rate increases. Rise in metabolic rate is directly proportional to rate of doing work. You can estimate the energy expenditure based on whole body oxygen consumption and its caloric equivalence.

Eva is right. The BMR(basal metabolic rate) uses the variables of height , weight , age & gender to calculate the Basal metabolic rate. This helps to calculating calorie needs based on the body weight alone.
There is a formula for BMR (Basal metabolic rate)
For women BMR=655+(9.6*weight in kilos)+(1.8*height in cms)-(4.7*age in years).
For men BMR=66+(13.7*weight in kilos)+(5*height in cms)-(6.8*age in years). This is metric form formula .