Quick answer: riding for an hour at 15 mph on flat roads takes about 88 W for a 160 lb rider on a 20 lb bike, which works out to roughly 379 kcal. If your power meter shows 150 W average for an hour, you burned about 645 kcal.
Why power is the better starting point
Calories on a bike follow from mechanical work. A power meter measures that work directly: 150 W for an hour is 540 kJ. Human muscles turn only part of the food energy they use into work on the pedals; the rest leaves as heat. That share is called gross efficiency, and a 2019 review in Sports Medicine Open reports it has mostly been around 18 to 20% at 150 W. At 20%, 540 kJ of work needs 540 / 4.184 / 0.20 = 645 kcal. Choose power mode whenever you have a meter or a smart trainer reading.
From speed to watts
Without a meter, the calculator estimates power with the standard road-cycling physics model (the kind validated by Martin and colleagues in 1998): rolling resistance, climbing and air drag, plus a small drivetrain loss. The power needed against air drag rises with the cube of speed, which is why 20 mph costs far more than 15. In this model the 160 lb rider needs about 53 W at 12 mph, 88 W at 15 mph, 138 W at 18 mph and 180 W at 20 mph. At 15 mph roughly two thirds of the effort goes to pushing air (about 59 W) and the rest to tyre rolling (about 27 W).
The drag area (CdA 0.32 m2), rolling coefficient (Crr 0.005) and air density (1.225 kg/m3) in the form are typical assumptions, not measured facts about you or your bike. Published values for CdA and Crr vary widely with position, clothing, tyres and road surface, so change them if you know better. Sitting more upright with CdA 0.40 pushes the 15 mph figure from 88 to about 103 W.
Limits of the estimate
- Speed mode assumes still air and a steady pace. Wind, drafting, stops and surges all change the real power.
- A hilly loop has an average grade near zero, but the climbs cost more than coasting downhill gives back, so the flat estimate will be low there.
- Efficiency differs between riders and with intensity. Changing it from 20% to 18% raises the calorie figure by about 11%.
- Bike computers and watches use their own models. All of these are estimates and say nothing certain about weight change.
Training indoors? The stationary bike calorie calculator works from the watts on the display. Compare with the running calorie calculator. Racing a triathlon? Put your bike speed into the triathlon calculator. Choosing gearing for a ride? Try the Bike gear calculator.
Sources: Matomaki, Linnamo and Kyrolainen (2019), cycling mechanical efficiency, Sports Med Open; Martin et al. (1998), Validation of a mathematical model for road cycling power, J Appl Biomech. Rules checked . Found a problem? Tell us.
Questions
How many calories does cycling burn per hour?
It depends mostly on power. With the default assumptions, a 160 lb rider on a 20 lb bike burns roughly 228 kcal an hour at 12 mph, 379 at 15 mph, 592 at 18 mph and 775 at 20 mph on flat ground.
How many calories does a 30 minute bike ride burn?
With the default assumptions on flat roads, a 160 lb rider on a 20 lb bike burns about 78 kcal in 30 minutes at 10 mph, 114 kcal at 12 mph and 189 kcal at 15 mph. Hills, wind and a heavier bike raise it.
How do I convert cycling kilojoules to calories?
Divide kJ by 4.184 to get kcal of work, then divide by gross efficiency. At 20% efficiency that is about 1.2 kcal burned for each kJ shown on a power meter.
Why do my bike computer calories differ from this?
Bike computers without a power meter guess power from speed and heart rate in their own way, and with a meter many assume a different efficiency. Use one method to compare rides.
Does climbing burn more calories?
Yes. At 15 mph, a steady 3% grade raises the estimated power for a 160 lb rider on a 20 lb bike from about 88 W to about 253 W, almost three times the energy.