How it works
VO₂ = 0.2·S + 0.9·S·grade + 3.5; kcal = VO₂ · weight ÷ 1000 · 5 · minutes
Step 1 — describe the run you are about to do. Use the planned distance and a realistic finish time. The tool applies the ACSM running equation, which prices every minute from your speed and body mass, so the hourly figure is what your body will spend while you are out there. Step 2 — read the burn rate, then drop the idea of replacing it. A 160 lb runner planning 16 miles in 2:40:00 (10:00 per mile) gets 2,072 kcal in total, 129 kcal per mile and a burn rate of 777 kcal per hour. Nobody absorbs that much on the move: common endurance guidance is about 60 g of carbohydrate per hour for runs beyond two hours, up to roughly 90 g with a trained gut and mixed-sugar products (Jeukendrup 2014; ACSM/AND/DC 2016 position stand). At 60 g per hour, 2 hours 40 minutes is 160 g — about 640 kcal, or six to seven 25 g gels — which covers only about 31% of the burn. Stored glycogen and fat pay for the rest, and teaching your body to do that is part of what long runs are for. Step 3 — scale it to race day. A 140 lb runner aiming for a 3:45:00 marathon spends about 2,930 kcal (112 kcal per mile, 69 per kilometer, 781 per hour). Sixty grams per hour over 3.75 hours is 225 g, or nine gels, still only about 31% of the cost, which is why the carbohydrate you store in the days before matters as much as the gels you carry. Step 4 — total the week. The per-mile cost barely moves with pace, so weekly mileage times the per-mile figure is a fair estimate: a 150 lb runner logging 35 miles at around 9:30 per mile spends 121 kcal per mile, roughly 4,230 kcal across the week, or about 600 kcal a day averaged out.
Sources
- ACSM metabolic equations American College of Sports Medicine, “ACSM’s Guidelines for Exercise Testing and Prescription.” The running equation: VO₂ = 0.2·S + 0.9·S·grade + 3.5 (S in m/min).
- Oxygen-to-energy conversion Approximately 5 kcal are released per litre of oxygen consumed — the caloric equivalent used to convert VO₂ to energy expenditure.
- MET concept One MET = 3.5 ml O₂·kg⁻¹·min⁻¹ (resting metabolic rate); intensity is often expressed as a multiple of this baseline.
FAQ
How many gels should I carry on a long run?
Size the plan by hours on your feet rather than by calories burned. Under about an hour most runners need nothing; between one and two hours, around 30 g of carbohydrate per hour is the usual target; beyond two hours, around 60 g per hour, with up to 90 g for a trained gut (Jeukendrup 2014; ACSM/AND/DC 2016 position stand). With standard 25 g gels, a 2:40 long run at 60 g per hour works out to six or seven gels. Practice that exact amount on training runs so race day holds no surprises for your stomach.
Why can I not eat back what I burn during a marathon?
Because the gut, not the burn, sets the ceiling. A runner spending close to 800 kcal per hour can absorb only about 240–360 kcal of carbohydrate per hour (60–90 g), so running a deficit during the race is normal and expected. Glycogen stored in muscle and liver fills the gap, which is why carbohydrate-rich meals in the day or two before a marathon count as much as anything you eat on the course.
How should I refuel after a long run?
Treat the run as a withdrawal you repay over the rest of the day, not in one sitting. A snack or meal with carbohydrate and some protein within a couple of hours helps you recover for the next session, and ordinary meals cover most of the remainder. The total on the calculator is a sense of scale — a 2,000 kcal long run is on the order of a whole day of eating for many adults — rather than a number to match exactly.
What does a high-mileage week do to my energy needs?
Weekly running adds up faster than most runners expect. Thirty-five miles at about 121 kcal per mile is roughly 4,230 kcal, and a 50-mile peak week at the same per-mile cost passes 6,000. Chronic under-eating through that kind of load is a common reason training stalls and fatigue lingers, so during a heavy block most runners do better matching intake to the work than chasing a deficit at the same time.
Is a treadmill run comparable to running outdoors?
Close enough for planning, with one caveat about the inputs. A belt has no wind or air resistance (many runners add a 1% incline for that), but the bigger difference is the distance itself: treadmill calibration varies from machine to machine, and the estimate can only be as right as the distance and time you enter. If the display says 6 miles but the belt runs slow, the burn is overstated by the same share, so log treadmill runs by the pace you can confirm. To see what a belt setting means in road pace, use the treadmill speed-to-pace converter.
Does the energy come from fat or carbohydrate?
Both, in a mix that shifts with effort. At easy long-run pace a substantial share comes from fat; as you speed up toward half-marathon or marathon race effort, carbohydrate does more and more of the work. That is why the same 2,900 kcal marathon can go smoothly with steady gels and fall apart without them: the total stays similar, but the carbohydrate portion draws on a limited store you have to protect.
These burn figures come from a population equation applied to the run you describe, and your own running economy can shift them noticeably. The fueling numbers reflect general sports-nutrition ranges, not a personal plan, and nothing here is medical or coaching advice — if you have a medical condition, a history of disordered eating or specific weight goals, get individual advice from a registered dietitian or physician.