It is surprisingly easy to estimate how many calories you lose while running.
The truth is, it only depends on two factors.

1. Your weight
2. The distance you run

You would also expect it to depend on how fast you run, the truth is, it doesn't.
Running faster means you are out running for less in terms of time, i.
e.
you are working harder, but for less time, so the two cancel each other out.
A simple formula for calculating calories lost while running is given by, Calories=Distance*weight Where distance is measured in kilometres, and weight is measured in kg.
If you want to work in miles, multiply your answer by 1.
6.
If you want to work in pounds, multiply your answer by 2.
2.
As an example, I weigh 73 Kg, so in running a mile I am going to use up 1*73*1.
6=117 calories.
This formula works well across a range of different distances, and gives you a quick easy way of calculating calories lost while running.
There are plenty of calculators on the internet that will work out the answer for you (including one on my website), they all work on variations of this basic formula though.
If you have a watch with a built-in heart rate monitor, that can also tell you what your calorie expenditure is.
These work on the basis that if your heart rate is higher, you are working harder, so your calorie expenditure will also be higher.
Bear in mind though that these are estimates as well, but they will be more accurate than this general formula.
If you have one of these watches already, it would be interesting to see how the two figures vary.