Last updated: 2026-06-21

Reading Your Ride as Energy

Every modern power meter and smart trainer reports a single number at the end of a ride: kilojoules of work. It is a genuine physics measurement — the mechanical energy delivered to the pedals — and it turns out to be one of the most useful numbers in endurance training, because it doubles as a remarkably good estimate of the calories you burned.

This calculator takes the two things you already know — your average power and your ride time — and returns both the mechanical work and the dietary calories your body spent to produce it.

How It Works

The work done on the pedals is simply power multiplied by time:

A watt is one joule per second, so 200 watts held for one hour (3600 seconds) is $200 \times 3600 = 720{,}000$ joules, or 720 kJ.

Your body, however, spends far more energy than that. Muscles are only about 24% efficient — roughly three-quarters of the metabolic energy is lost as heat. The food energy burned is therefore the work divided by efficiency, converted from joules to kilocalories (1 kcal = 4184 J):

The 1 kJ ≈ 1 kcal Shortcut

Here is the quirk that makes cycling power data so convenient. Dividing by an efficiency of 0.24 multiplies the work by about 4.17; dividing by 4.184 to convert joules-to-kilocalories divides by almost the same amount. The two nearly cancel:

So the kilojoules on your head unit are within a couple of percent of the kilocalories you burned. A 720 kJ ride costs roughly 717 kcal. You can read one as the other and rarely be wrong by more than a chocolate bar.

Worked Example

A steady endurance ride at 200 W for one hour:

Why the Calorie Number Is Bigger

It is a common surprise that a 720 kJ ride "costs" 717 kcal when 720 kJ only equals 172 kcal of pure energy. The difference is efficiency. The pedals see 720 kJ; your metabolism spent about four times that in heat and work combined. The work figure measures output, the calorie figure measures cost — both are real, they just answer different questions.

Using the Result

• Fuelling. Aim to replace 30–60 g of carbohydrate per hour on long rides; the calorie figure tells you the hole you are digging.
• Training load. Total kilojoules per week is a simple, honest measure of how much work you are doing — more robust than time alone, because it accounts for intensity.
• Comparisons. Because the kJ figure comes straight from a power meter, it is far more reliable than heart-rate or speed-based calorie estimates, which guess at your effort.