Calories burned cycling

Speed or watts: which to trust

Cycling is the one activity where speed tells you least about effort. A tailwind, a long descent or a wheel to sit behind can halve the work at exactly the same number on the computer. That makes speed-based estimates the least reliable on this site — call it ±20–30% — and it is why every calculator that only asks for speed is guessing more than it admits.

If your bike or trainer records power, use it. Power is measured, not inferred, and the arithmetic from there is short.

Why kilojoules are roughly kilocalories on a bike

Riders repeat that 1 kJ of work equals about 1 kcal burned, and it is very nearly true — but by coincidence, not by physics. Converting kilojoules to kilocalories means dividing by 4.184. Converting mechanical work to metabolic energy means dividing by your gross efficiency, roughly 0.2 to 0.25, because most of what you eat becomes heat rather than motion. At an efficiency of 23.9% the two factors cancel almost exactly.

Which means the shortcut is only right at one efficiency. At 22%, closer to typical and what several head units assume, a kilojoule is about 1.09 kcal. At 20% — a less trained rider — it is 1.19. Less experienced riders genuinely burn more per kilojoule of work, which is the opposite of what most people assume.

Net and gross, and why apps disagree

The headline here is net: the cost of the ride above the energy you would have spent anyway, which is what Apple Health records as active energy. Head units and training apps variously report gross metabolic energy, mechanical kilojoules, or a figure with their own efficiency assumption baked in — which is why the same ride comes out different in three apps. None is lying; they are answering different questions.

What the speed bands mean

The MET values come from the 2011 Compendium of Physical Activities, interpolated between published bands. They assume a flat road, no wind and no drafting, which is exactly the assumption a real ride breaks. The 2024 revision moves some of these — racing above 32 km/h goes from 15.8 to 16.8 METs — so a calculator citing a different edition will disagree with this one by several per cent before any of the other uncertainty is counted.

Indoor riding and e-bikes

On a stationary bike or in a spin class, power is the honest input — the Compendium indexes indoor cycling on wattage rather than speed for exactly this reason. On an e-bike, the motor is doing a share of the work that never passes through you, so a speed-based figure will overstate your effort substantially. Use power, or accept that the number is decorative.

For what any of this means against a goal, thedeficit calculator handles the intake side, which is where most of a deficit has to come from anyway.