The standard measurement for metabolism is called the Basal Metabolic Rate (BMR). This describes the minimum amount of calories needed to simply keep your body going, sustaining vital functions such as digestion, breathing and heartbeat.

BMR varies a lot between different individuals. These variations can make the difference between someone being overweight or of normal weight. For example, many obese people don’t eat any more food, and might eat less than non-obese people. For many obese people, this is the result of an extremely low level of physical activity. For others, it may simply mean that the obese person has a more efficient metabolism that requires fewer calories. 

BMR generally accounts for around 1200-1400 calories per day in females, and 1400-1800 calories in males. This is the number of calories it takes for you to stay alive. Many factors affect BMR including:

• Gender. Men tend to have more lean muscle in their bodies than women. Lean muscle requires more calories, so men’s BMR is greater than women’s.
  
• Age.  Because of the increased activity of cells undergoing division, the younger the person, the faster their metabolism. There is therefore a decline in BMR of approximately 2% for every 10 years after age 30.
  
• Weight. The heavier a person is, the higher their energy requirements, so they need more calories to keep their body functioning and moving. Their BMR is higher, but that doesn’t mean their metabolism is faster; a point of confusion sometimes!
  
• Height. The taller a person is, the higher their BMR as they have more body surface area (skin) exposed to the outer elements, equating to greater heat loss.
  
• Environmental temperature. People living in tropical or very cold climates tend to have BMR's 5 - 20% higher than those living in moderate climates.
• Physical activity. People who are physcially active generally have a higher BMR than those who are not.