BMR and RMR
No two people are the same and no two diets should be the same, take the time to work out your calorific needs and activity level, a little time spent now will pay off in the future.
To start with BMR and RMR are estimates of how many calories you would burn if you were to do nothing but rest for 24 hours. They represent the minimum amount of energy required to keep your body functioning, including your heart beating, lungs breathing, and body temperature normal.
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BMR stands for Basal Metabolic Rate, and is synonymous with Basal Energy Expenditure or BEE. BMR measurements are typically taken in a darkened room upon waking after 8 hours of sleep; 12 hours of fasting to ensure that the digestive system is inactive; and with the subject resting in a reclining position.
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RMR stands for Resting Metabolic Rate, and is synonymous with Resting Energy Expenditure or REE. RMR measurements are typically taken under less restricted conditions than BMR, and do not require that the subject spend the night sleeping in the test facility prior to testing.
As you can see from the two explanations of the different Metabolic rates the RMR is probably going to be a more accurate, realistic number to work with.
Here is an online calculator for your BMR, or you can do it the old fashioned way with your brain and a pencil 
The Harris-Benedict equation for BMR:
The Mufflin equation for RMR:
Where:
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w = weight in kg
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h = height in cm
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a = age
To determine your total daily calorie needs, multiply your BMR or RMR by the appropriate activity factor, as follows:
If you are sedentary (little or no exercise) : Calorie-Calculation = BMR/RMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR/RMR x 1.375
If you are moderately active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR/RMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR/RMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR/RMR x 1.9
It is a bit of a math lesson i know, but once calculated it can be a very useful guide.
One word of caution, DO NOT OVER ESTIMATE YOUR ACTIVITY LEVEL, this is where most people go wrong and will cause plateau's and possibly weight gains!
Body Composition.
The equations do not take into account body composition, a measure of the percentages of muscle and fat composing your body. It is therefore less accurate if you have a non-typical amount of muscle. This is because muscle burns calories, while fat store do not. There is a calculation for the average person which is the BMI, or Body Mass Index, this is a calculation which uses height and weight to come up with a number which can be used to determine the catagory your body size falls into. Under 18.5 is underweight, 18.6-24.9 is normal, 25-29.9 is overweight and 30+ is obese. This calculation does not take in to account high muscle content, so body builders for example would be classed as obese! To calculate your BMI click here
A person with an above average amount of muscle will also have a higher BMR or RMR than calculated against a person with a below average amount of muscle.
The vast majority of users will never read this far into these notes, and they will do just fine using the calculators so long as they remember that the calculations are only estimates. But for those who want to fully understand how the calculators work, we would like to clarify one last point.
The calculator does not make use of your BMR or RMR calculations in calculating calories burned. Instead, it uses a constant based on your weight alone. Thus the parameters for sex, age and height are ignored in calculating calories burned.
It is therefore somewhat of an "apples and oranges" disparity to compare your BMR or RMR calculation with your calories burned calculations. You can see the discrepancy by calculating Sitting - quietly for 24 hours. Ideally, this calculation would equal your RMR.
Perhaps it is beginning to sound as if the calculators are flawed. While it is true that they only make estimates, we believe that they are the most accurate you will find. Something we do that is unique is to explain how they work, inaccuracies and all.
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