January 17, 2026 · 4 min read

Burning Kilojoules on the Trainer — How to Calculate Carb Needs Precisely

Every smart trainer and power meter counts kilojoules (kJ) — the total mechanical work done during a ride. That number isn't just trivia: you can use it to calculate how many carbohydrates you're really burning, and how much you need to replace to avoid hitting the wall mid-workout.

What the trainer is actually counting

The kilojoules on screen are the integral of power over time — the mechanical work done at the pedals. That's not the same as calories burned: muscles work at a gross efficiency of roughly 20–25%, so to produce 1 kJ of mechanical work, the body uses 4–5× more metabolic energy.

Why, in practice, kJ ≈ kcal

Here a convenient coincidence shows up. 1 kcal is 4.184 kJ of metabolic energy. At a typical efficiency of ~24%, that multiplier almost exactly cancels out the kcal→kJ conversion. The effect: the kilojoule figure shown by the trainer is roughly equal to the number of kilocalories burned.

This is an approximation, not a lab result — real efficiency depends on cadence, position and training level, so an error of 10–15% is normal. For planning nutrition during a workout, though, it's entirely good enough.

From kilocalories to grams of carbohydrate

Not all the energy comes from carbohydrate — fat covers part of the effort too, and the split depends on intensity. The higher up the power zones you go, the bigger carbohydrate's share of the burn.

1 gram of carbohydrate is 4 kcal. To convert kilojoules burned into grams of carbohydrate, multiply the kcal by the carbohydrate share for that intensity and divide by 4.

An example

An hour-long ride at Sweet Spot, the trainer shows 650 kJ at the end. At a carbohydrate share of ~65%:

That's already more than you can typically absorb during a ride (the gut realistically takes in around 60–90 g of carbohydrate an hour) — which is why longer, intense workouts always create a deficit you need to make up afterwards.

How much to eat during, how much to replace after

• Workouts up to 60 minutes, low-to-moderate intensity — usually no need to eat during; glycogen stores are enough.
• Workouts 60–90 minutes, Sweet Spot / threshold — 30–60 g of carbohydrate per hour during the ride (a gel, an isotonic drink).
• Workouts 90+ minutes, high intensity — 60–90 g/h during, plus making up the difference afterwards (a carb-rich meal within 1–2 h).

It's worth comparing your calculated kJ figure with the workout's TSS — a high TSS with relatively low kJ burn often means a lot of time in zones with a higher fat share rather than carbohydrate, despite significant fatigue.

Frequently asked questions

Is kJ from the trainer the same as calories burned shown in a fitness app?

Close, but not identical. Fitness apps often add resting metabolic rate (BMR) for the duration of the workout; the trainer counts only mechanical work. For estimating carbohydrate needs, it's better to base your calculation directly on the trainer's kJ figure — it's the closest to the actual work done by your muscles.

Does this method work outside too, not just on the trainer?

Yes, the mechanics are identical — what matters is the kJ recorded by any power meter, whether on a trainer or measured on the road.

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