Work in kilojoules is average power times duration. Because human gross efficiency is around 24 percent and a kilocalorie is 4.184 kilojoules, the two figures come out nearly identical - a genuine coincidence, not an approximation.
Get it from the ride, not a form WattLog.PRO computes kilojoules from every recorded second of your session, so a ride with surges and coasting is counted properly rather than averaged into a guess.
Open WattLog.PROWhy power-based calories are accurate
A power meter measures mechanical work directly. Multiply average watts by seconds and you have kilojoules of work done, with no modelling assumptions at all. Converting that to dietary calories requires knowing your gross efficiency - the fraction of metabolic energy that becomes pedal force - which sits close to 24 percent for nearly all trained cyclists.
Here a happy coincidence takes over. Dividing by an efficiency of about 0.24 and converting kilojoules to kilocalories by dividing by 4.184 very nearly cancel out. So kilojoules of work and kilocalories burned come out within a few percent of each other, and you can read one as the other without doing any arithmetic.
What the number is good for
Calorie estimates that come from speed, distance or heart rate are considerably worse, because they infer effort rather than measuring it. Speed-based figures cannot tell a headwind from a tailwind. Heart rate figures inherit every drift caused by heat, caffeine and fatigue. If you have power, use power.
For fuelling during a ride, the useful figure is rate rather than total: most riders can absorb 60 to 90 grams of carbohydrate an hour, well short of what a hard session burns, so a deficit during a long ride is normal and expected. Use the total for recovery planning afterwards, and remember it excludes the energy you would have spent existing anyway.
Frequently asked questions
Are kilojoules and calories really the same?
Not physically, but numerically they land within a few percent for cycling. One kilojoule of mechanical work costs roughly 4.2 kilojoules of metabolic energy at 24 percent efficiency, and 4.184 kilojoules is one kilocalorie. The two conversions almost exactly cancel.
Why does my heart rate monitor give a different number?
Because it is inferring effort from heart rate, which drifts with heat, hydration, caffeine and fatigue. Those estimates commonly differ by 20 to 30 percent from power-based figures. If you have a power meter or smart trainer, trust the power number.
Does rider weight affect calories burned?
Not for this calculation. The work you do is measured at the pedals, so 200 watts for an hour is the same work regardless of who produces it. Weight affects how much power you need to move at a given speed, but not the calories per watt.