To best calculate your daily caloric needs, you can use the Compendium of Physical Activities Tracking Guide [link]. It’s a huge list of very specific daily activities, including physical activities at different intensity levels as well as general tasks, such as eating in church. For each activity, calorie output is represented by a number called “METS” (metabolic equivalents).
To get calories from METS, multiply your weight in kilograms (pounds divided by 2.2), and multiply by the number of hours you performed the task for.
For example, if you weigh 70 kilograms and went to dig worms with a shovel for 30 minutes, you look that up on the chart. It’s METS = 4.0. Calories burnt = 70 x 4.0 x 1/2 = 140 calories.
To find out your daily calorie needs, take an average day, and track all of your activities during that 24 hour period, including sleeping, walking, studying, exercising, eating, etc. The more detail you put in, the more accurate your results will be. Here’s an example for one of my days:

Daily Calories Example
As you can see from my example, I’m burning about 3600 calories in that day. But if you saw my previous post, my average output is 3200 calories a day. This is because for this sample day, I’ve included my morning bike ride - but I do not do this everyday. To compensate, you may need to average a few days worth of calories.
Of course, these figures will be different based on your age, gender, metabolism, etc. But overall, they are pretty accurate, especially if you are exercising a lot. This is because metabolic differences will become small relative to calories consumed during exercise.
Now all you need to do is consume a number of calories that’s approximately equal to your caloric needs if you want to maintain your weight. And if you’re trying to lose or gain weight, then eat less or more! A good estimate for calories to burn is that ~500 calories per day = 1 pound of loss a week, or 3500 calories = 1 pound of fat. Healthy weight loss is about 0.5-2 pounds a week, depending on your original body weight. I’m not a qualified nutritionist, so you should contact a professional before starting any weight-loss/gain plan.