If you want to know how many calories you should eat in a day, learning roughly how many calories your body expends on a daily basis is an important part of learning how to properly fuel your body, and how to lose, gain, or maintain weight.
If anyone gives you a set number of calories to eat per day without intensely monitoring your every activity, around the clock - there's a 99% chance that they don't know what they're talking about. Not only is each person different, every single day for every single person is different. Imagine the difference between a lazy day sitting on the couch and one where your day is full of running errands and a good workout session. The amount of calories that you burn per day can easily vary by 1,000 calories or more.
An important part of learning about the number of calories to consume has to do with figuring out your basal metabolic rate (BMR). Your BMR is the number of calories your body uses to do absolutely nothing, aside from run itself - it's the number of calories you would burn each day if you didn't move a muscle. It's the energy required for your body to complete all of it's complex and very important basic functions that keep you alive and breathing.
There are many different BMR calculators online that will do this work for you, but here's the general equation in both standard & metric forms:
Standard/Imperial BMR Formula
Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years )
Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year )
Metric BMR Formula
Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) - ( 4.7 x age in years )
Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) - ( 6.8 x age in years )
Since this is the bare minimum for bodily functions to run properly, you are obviously going to need many more calories than this when it comes to all of the normal daily activities that you do - things as simple as feeding yourself, cleaning the house, or walking around at work. It's easy to see that the number of calories you would need to fuel yourself can quickly climb, especially if you are doing any kind of intense physical activity, or if you have a physically demanding job.
How many calories should I eat to lose weight?
Just like there is no one right answer for how many you might need to eat to maintain, the number of calories you will need to consume to drop weight is also going to vary. We recommend eating 500 calories less than what you burn in a day, if you are trying to lose weight - but you should never eat less than 1200 calories. This rate of deficit can also be attained by burning 250 extra calories through exercise, and consuming 250 less in calories, and it will result in a weight loss of 1 lb a week (a safe, sustainable rate that will help you keep the weight off for good).
There are many different ways to figure out your daily caloric expenditure (BMR calculators, daily calorie burn estimation calculators, relatively accurate calorie counting mechanisms that you wear, etc) and while it is helpful in the beginning to have an idea of what those figures are, we also recommend taking a more natural approach. Learn to listen to your body. Eat when you're hungry, stop when you're full. Stay active, do cardio and strength training on a regular basis. Watch for changes in your body and make exercise and nutritional changes when necessary, but try not to obsess about every single calorie consumed, and every single calorie burned. In the end, it makes for weight loss and bodily changes that are much easier to sustain, and it helps you keep a healthy mindset about food and working out.