How energy use is calculated
The calculator uses the metabolic equations from the American College of Sports Medicine (ACSM). For running above about 8 km/h: VO2 (ml/kg/min) = 0.2 × speed (m/min) + 0.9 × speed × incline + 3.5. For walking below 8 km/h, a different equation applies: VO2 = 0.1 × speed + 1.8 × speed × incline + 3.5. From the VO2 value, body weight and duration in minutes, energy use follows the conversion that 1 litre of oxygen equals about 5 kilocalories: kcal = VO2 × weight × minutes / 1,000 × 5.
At 70 kg and 10 km/h over 10 kilometres (60 minutes), that gives a VO2 of 0.2 × 166.7 + 3.5 ≈ 36.8 ml/kg/min, so 36.8 × 70 × 60 / 1,000 × 5 ≈ 774 kcal, roughly 77 kcal per kilometre.
Comparison with the rule of thumb
A common rule of thumb says roughly 1 kcal per kilogram of body weight per kilometre. For 70 kg and 10 km, that gives 700 kcal, close to the roughly 774 kcal from the ACSM equation, but not identical. The rule of thumb is easier to remember but ignores pace and incline. The faster and steeper you run, the larger the gap between the rule of thumb and the formula.
Carbohydrates during a race
For efforts beyond about one hour, the body needs extra carbohydrate to avoid depleting glycogen stores. According to Jeukendrup (2014), the guideline is 30 to 60 grams of carbohydrate per hour for efforts between 1 and 2.5 hours, up to 90 grams per hour for efforts beyond 2.5 hours. The higher amounts generally require glucose-fructose mixes, because the gut can only absorb a limited amount of pure glucose.
For a 3:30-hour marathon and a target of 60 to 90 g/h (average 75 g/h), that works out to about 262 grams of carbohydrate for the whole race, or roughly 10 to 11 gels at 25 grams each. Below one hour of effort, this source does not identify a separate extra requirement during the activity.
Limits of the calculation
The value shown is the gross energy cost of the activity, including the resting metabolism your body would have used anyway, not a pure extra on top of your daily needs. The ACSM equation assumes a steady pace on a flat or evenly inclined course; wind, surface, heat and individual running economy are not accounted for. How much carbohydrate you can actually tolerate during a race has to be tested in training beforehand, never for the first time in competition.
Frequently asked questions
How many calories does 10 km in 60 minutes burn at 70 kg?
By the ACSM equation that comes to about 774 kcal, roughly 77 kcal per kilometre. The 1 kcal per kg per km rule of thumb would give 700 kcal.
Why do the ACSM equation and the rule of thumb differ?
The rule of thumb ignores pace and incline and assumes a flat 1 kcal per kg per kilometre. The ACSM equation uses actual speed, so it diverges more from the rule of thumb at very slow or very fast paces and on inclines.
How much carbohydrate do I need for a 3:30 marathon?
By Jeukendrup (2014), the target for efforts over 2.5 hours is 60 to 90 g/h. For 3.5 hours, that works out to about 262 grams of carbohydrate in total, roughly 10 to 11 gels at 25 grams each.
Is the calorie value my extra energy need for the day?
No, the value is the gross cost of the activity including the resting metabolism you would have used anyway. The pure extra burn from running is somewhat lower.
At what pace does the calculator switch from walking to running equation?
The calculator uses the ACSM running equation from 8 km/h upward and the ACSM walking equation below that. The threshold is a practical convention, because walking and running are different movement patterns with different oxygen demands.