Calories and kilojoules on the bike

If your computer says 650 kilojoules, you burned about 650 calories. Here is why those two numbers are nearly the same — and where the other three quarters of the energy went.

## The coincidence that makes this easy

If your head unit says you did 650 kilojoules of work, you burned roughly 650 calories. Not approximately in a hand-waving sense — genuinely close, and for a reason worth understanding.

Two conversions sit between the two numbers and they very nearly cancel:

1. A dietary calorie is 4.184 kilojoules. So 650 kJ of work would be only 155 calories if your body were perfect.
2. But your body is about a quarter efficient. To put 650 kJ through the pedals it has to spend roughly four times that.

Multiply a quarter by 4.184 and you get almost exactly one. At 23.9 per cent gross efficiency the two cancel perfectly, and real human efficiency on a bicycle sits close enough to that for the shortcut to hold.

So the rule is: kilojoules of work ≈ calories burned. Read the kJ number off your computer and stop there.

## Why this is more trustworthy than most calorie numbers

Almost every other calorie estimate you will see — from a watch, a phone, a gym machine — is working backwards from heart rate, age, weight and a statistical model. Those models carry very large error bars, because heart rate responds to heat, caffeine, stress, sleep and illness as much as to effort.

A power meter is not estimating. It is measuring force and rotational speed and multiplying them. The only uncertainty left is your personal efficiency, and that varies much less between people than the things heart rate models have to guess at. It is one of the few honest calorie figures available to anybody.

## Where the rest of the energy goes

This is the part that connects to everything else about training indoors.

If you are riding at 180 watts and you are 24 per cent efficient, your body is spending about 750 watts. Only 180 of those reach the chain. The other 570 leave as heat.

That is not a metaphor. You are a 570-watt heater, and indoors there is nowhere for it to go. On the road you are moving through air fast enough to carry it away as quickly as you make it; on a turbo you are sitting still in a closed room, warming the room and then being warmed by it.

This is why:

• indoor sessions feel harder at the same power
• you sweat in a way that looks alarming to anyone who has only ridden outside
• a decent fan is worth more than almost any other piece of indoor kit
• your FTP tested indoors is usually five to ten per cent below your outdoor figure

The calculator shows the heat figure alongside the work figure, because for indoor riding it is arguably the more useful of the two.

## The efficiency setting, and why it is there

Gross efficiency is the share of the energy your body spends that arrives at the pedals. Across trained cyclists it runs roughly from 18 to 26 per cent. It is mostly a property of your muscle fibre composition, it changes very slowly with training, and you cannot easily measure it without a laboratory.

The practical consequence is small: at 20 per cent, 650 kJ becomes 777 calories; at 26 per cent it becomes 598. A spread of about 25 per cent either side of the tidy one-to-one answer. For most purposes the coincidence is good enough, and the dropdown is there so you can see what the uncertainty actually looks like rather than pretending it does not exist.

## One thing worth saying plainly

Treat all of this as arithmetic about a bike ride, not as a budget. Work out your kilojoules because it is a clean, honest measure of what a session cost and a good way to compare one week with another. If you want to use it for anything to do with eating, that is a conversation for a doctor or a registered dietitian who knows you, not for a calculator on a cycling website.