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Many factors can significantly impact your RMR:

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Genetics

People are born with faster metabolisms, while others naturally have slower metabolisms. 

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Gender

Men, on average, have a greater muscle mass and a lower body-fat percentage than women, they generally have a higher RMR.

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Age

RMR is greater in childhood than in adulthood. After age 20, RMR is estimated to drop about 2 to 3 percent each decade. 

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Weight

The more an individual weighs, the higher his or her RMR will be. For example, the metabolic rate of very overweight individuals is generally higher than that of their normal-weight counterparts.  

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Height

Taller people typically have greater body surface area and more lean body mass (LBM).

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Body Fat %

Fat Percentage- all other things being equal, people with a higher body-fat percentage have a lower RMR than those with a lower-body fat percentage. 

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Diet

Starvation or serious abrupt calorie-reduction can dramatically reduce RMR by up to 30 percent. Likewise, restrictive, low-calorie weight-loss diets may cause RMR to drop by as much as 20 percent. 

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Body Temperature/Health

For every increase of 0.5 degrees C in internal temperature of the body, RMR increases by about 7 percent. The chemical reactions in the body actually occur more quickly at higher temperatures. Therefore, a person with a fever of 42 degrees C (about 4 degrees C above normal) would have an increase of about 50 percent in RMR. 

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External temperature

Temperature outside the body also affects RMR. Exposure to cold temperature causes an increase in the RMR, as the body tries to create the extra heat needed to maintain its internal temperature. A short exposure to hot temperature has little effect on the body’s metabolism because of compensatory increases in heat loss. However, prolonged exposure to heat can raise RMR. 

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Glands

Thyroxin (produced by the thyroid gland) is a key RMR regulator that speeds up the body’s metabolic activity. The more thyroxin produced, the higher the RMR. If too much thyroxin is produced (a condition known as thyrotoxicosis), RMR can actually double. If too little thyroxin is produced (myxoedema), RMR may shrink to 30 percent to 40 percent of normal. Like thyroxin, adrenaline also increases the RMR, but to a lesser extent. 

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Exercise

Physical exercise not only influences body weight by burning calories, it also helps raise RMR by building extra muscle. The greater the exercise intensity, the longer it takes the body to recover, which results in a longer and higher excess post-exercise oxygen consumption (EPOC).