You use energy no matter what you're doing, even when sleeping. The BMR Calculator will calculate your Basal Metabolic Rate (BMR); the number of calories you'd burn if you stayed in bed all day.
If you've noticed that every year, it becomes harder to eat whatever you want and stay slim, you've also learnt that your BMR decreases as you age. Likewise, depriving yourself of food in hopes of losing weight also decreases your BMR, a foil to your intentions. However, a regular routine of cardiovascular exercise can increase your BMR, improving your health and fitness when your body's ability to burn energy gradually slows down.
Once you know your BMR, you can calculate your Daily Calorie Needs based on your activity level using the Harris Benedict Equation.
BMR Formulas Used:
Note that all formulas are in metric notation.
HarrisBenedict Equation (Original):
The original harrisbenedict formula is one of the most used formulas on the internet to calculate your daily energy needs, however it is also one of the least accurate.
Females: 655.0955 + (9.5634 x Weight [kg]) + (1.8496 x Height [cm])  (4.6756 x Age)
Harris Benedict Equation (Revised):
In 1984, the harrisbenedict formula was revised by Roza and Shizgal. A larger research group was used.
Females: 447.593 + (9.247 x Weight [kg]) + (3.098 x Height [cm])  (4.33 x Age)
MifflinSt Jeor:
In 1990 the MifflinSt Jeor formula was introduced. In 2005 The American Dietetic Association (ADA) compared the BMR formulas of HarrisBenedict, MifflinSt Jeor, Owen and WHO/FAO/UNU and found that MifflinSt Jeor was the most accurate, predicting RMR within 10% of measured values.
Females: (9.99 x Weight [kg]) + (6.25 x Height [cm])  (4.92 x Age)  161
Schofield:
The Schofield equation was published in 1985 and used by FAO/WHO/UNU (World Health Organization and others). However, a disproportionate number of subjects in the data set were Italian men with on average higher BMR values. This skewed the results for other communities.
Males:

Females:

Oxford (Most Reliable):
As the Schofield equation above was proven not to be very reliable for many, a new series of equations was developed in 2005 which consisted of a database of 10,552 BMR values that had a more diverse set of subjects.
Males:

Females:

KatchMcArdle:
Both the KatchMcArdle and the Cunningham formulas use lean body mass to estimate your resting metabolic rate. If you know your body fat percentage, lean body mass can be calculated by the following formula: (1  Body Fat Percentage / 100) x Weight
. Note that in the BMR/RMR calculator above the lean body mass is automatically calculated using the Boer formula if body fat percentage is not provided.
Cunningham:
The Cunningham equation is more accurate for very athletic people.
Note: 1 kilogram (kg) = 2.2 pounds (lb), 1 meter = 3.28084 feet