Running costs you roughly 100 calories per mile. That figure is good enough for planning, and it hides a lot. What you actually burn depends mostly on how much of you there is to move, and only slightly on how fast you move it.
The rule of thumb, and where it comes from
Moving your body over a mile takes a fairly fixed amount of work, and that work scales with body weight. A heavier runner burns more per mile than a lighter one over the same ground, because there is more mass to carry.
So 100 calories per mile is a middle estimate for a middle-sized adult. Someone at 60 kg is meaningfully under it. Someone at 100 kg is meaningfully over. If you want a number closer to your own, scale it: a runner twice the weight of another burns roughly twice as much covering the same distance.
Pace matters less than people assume
Here is the part that surprises everyone. Running a mile faster does not burn dramatically more than running the same mile slower.
You burn more per minute at a quicker pace, obviously. But you also finish sooner, and those two effects mostly cancel out over a fixed distance. Distance is what drives the total. Speed mainly changes how long you have to be uncomfortable for.
That does not make hard running pointless. Intervals build the engine, and they leave you breathing hard for a while afterwards. As a way of spending calories, though, an extra two miles beats an extra 30 seconds per mile.
Terrain does move the number
Hills and trails are a different story. Running uphill adds vertical work on top of horizontal work, and the cost climbs steeply with the gradient. A hilly route can cost half again as much as the same distance on the flat.
Trails add their own tax. Uneven ground, loose surfaces and the constant small corrections to stay upright all take energy that a smooth road does not ask for. Trail running has been estimated to cost up to 30% more than road running over the same distance, which is a large enough gap to notice in how hungry you are afterwards.
What barely matters
Running form gets talked about as a calorie lever. It is really an efficiency and injury question, and a more efficient runner burns slightly less for the same pace, not more.
Hydration is the same kind of thing. Being properly hydrated lets you hold intensity and feel better doing it. It does not add calories to the ledger by itself. Drink because dehydrated running is miserable and slower, not because you think it burns more.
If you are running to lose weight
Two things will trip you up.
The first is that your watch is generous. Wrist-based estimates run high, sometimes considerably, and eating back a burn that never happened is one of the most common reasons people stall. There is more on why in our piece on fitness trackers and calorie burn.
The second is appetite. Running makes people hungry, and a five mile run costs about the same as one large muffin. That is not an argument against running. It is an argument for deciding what you eat afterwards before you get home.
Set your intake from your maintenance calories and treat running as something that shifts the total a little, rather than something that buys back dinner. The running is doing plenty for your heart, your legs and your head regardless of what the scale says.
Getting more out of the same hour
Add distance before you add speed, if calories are the goal. Run somewhere hilly when you want the same distance to cost more. Keep the easy runs genuinely easy so you can do more of them, because total volume across a week beats any single hard session.
And stay consistent. Four steady weeks of running beats one heroic week followed by a fortnight nursing a sore knee.