Running burns roughly 1 kilocalorie per kilogram of body weight per kilometre, at almost any steady pace. A 55 kg runner burns about 550 kcal over 10 km, a 70 kg runner about 700, and a 90 kg runner about 900. A wrist watch’s calorie figure for the same run can be off by 20% or more.
The rule of thumb goes back to Margaria’s 1963 measurements of the energy cost of running. It is crude, but it is the right order of magnitude. Two corrections matter on top of it: one about pace, and one about what your watch is telling you. Put your own weight, distance and time into the calories burned calculator for the exact figure. The calculator runs a slightly fuller equation, so its numbers come out a little above the rule:
- 55 kg, 10 km at an easy 5:30/km — about 600 kcal
- 70 kg, 10 km at an easy 5:30/km — about 770 kcal
- 90 kg, 10 km at an easy 5:30/km — about 990 kcal
Does running slower burn fewer calories?
No: per kilometre, running slower burns about the same, or slightly more. The calculator runs on the ACSM running equation, which splits the cost of a run into two parts. One part scales with speed. The other is roughly fixed, the price of staying upright and breathing, charged every minute however fast you move. Run faster and you shorten the time the fixed cost applies, but you raise the speed-dependent part. The two nearly cancel out.
For a 70 kg runner on flat ground, a kilometre costs about the same energy at any steady pace between 4:00 and 7:00 per km. The ACSM running equation, which the calories burned calculator uses, puts it at about 75 kcal at a brisk 4:00/km. At an easy 7:00/km, the same kilometre comes out at about 79 kcal: slightly more, not less. The reason is the near-fixed cost of staying upright and breathing, which builds up for longer when you run slower. Over 10 km, that is about 749 kcal in 40 minutes at the fast pace and about 786 kcal in 70 minutes at the slow one. Nearly double the time on your feet, and the energy bill barely moves. So a runner who trims a long run’s fuelling because “today is slow and easy” is cutting the wrong thing. Distance, not pace, sets how much fuel the run needs.
Why is my watch’s calorie number different?
Your watch’s calorie number is a proprietary estimate, and wrist devices are known to miss by a wide margin. A 2017 Stanford study tested seven wrist-worn devices, including an Apple Watch, a Fitbit Surge and a Samsung Gear S2. It compared them with indirect calorimetry, the lab method for measuring energy use, across sitting, walking, running and cycling. Heart rate came out accurate: six of the seven devices stayed within 5% of the reference reading. Calorie estimates did not. Error ranged from about 27% at the best device to about 93% at the worst, and none of the seven got below 20% (Shcherbina et al., 2017).
The calories burned calculator runs the published ACSM formula instead of an on-wrist algorithm. Studies like that one use the same kind of reference equation to judge the watches. It is still an estimate, not a metabolic-ward measurement. But it is a more defensible number to plan fuelling against than whatever your wrist reports after the run.
What the number doesn’t see
The equation assumes flat ground and a steady pace, and that assumption does real work. Running economy, wind, temperature and terrain all move the true cost of a run. None of them show up in distance and time alone. A hilly loop or a run into a headwind costs more than the flat, calm version of the same route, and the calculator can’t see either. A heavier pack, trail underfoot and day-to-day variation in how efficiently you run push the real number around the estimate too. Use the figure as a planning baseline for a typical flat effort, not a ledger to balance to the last kilocalorie.
How many calories does a long run burn per hour?
A 70 kg runner on an easy long run burns roughly 770 kcal an hour. For fuelling, that hourly rate matters more than the headline total. At 6:00/km, two hours covers about 20 km and burns in the region of 1,550 kcal. Standard fuelling guidance of 30–60 g of carbohydrate an hour supplies something like 120–240 kcal of that in real time. That is nowhere near a full replacement, and it isn’t meant to be. A gel on a long run keeps the tank from running empty before you finish. The rest of the ledger gets paid out of what you started with.
Work out your own hourly carbohydrate target on the gel calculator. How spend and intake fit together across a whole race is covered in three ways marathon fuelling goes wrong.