Let’s take it back to 12th grade physics class. We’re talking energy.

More specifically, we’re talking energy and how it’s consumed, burned and expended. In the context of cycling, these functions of physics are thrown around, but do we really know what it all means? Well, you’re about to.

Energy in cycling refers to a couple things: the energy that is stored in the form of chemical energy from nutrition, and the heat and mechanical energy that’s doled out when we’re on the bike. Now we must consider the different terms we have for that energy.

Calories or calories?

First thing’s first — what is a calorie? In order to perform on the bike, we need energy. Simply put, a calorie is a unit of that energy and is most commonly thought of in two ways: the small calorie (often referred to as the gram/physics calorie) and the large Calorie (known as the dietary calorie, often indicated by a capital “C”).

  • The small calorie is most often used in chemistry, as it represents the approximate amount of energy required to increase the temperature of one gram of water by one degree celsius.
  • The dietary Calorie is what you’re used to seeing. We consume food as a source of energy and the Calorie is simply a measure of stored chemical energy from the food we ingest. That chemical energy transfers into heat and mechanical energy for work in the body.

Why did we need to know that?

Because 1 dietary Calorie = 1000 physics calories. This explains why Calories are often referred to as kilocalories (kcals), because kilo means “thousands”. So when you say “I burned 500 calories today”, you’re really saying that you burnt 500 kilocalories (dietary Calories) or 500,000 physics calories.

Wait, isn’t the SI measurement for energy in joules?

Yes, it is! The International System of Units (SI) deems the ‘joule’ to be the standard base unit to quantify energy, work and heat. So although Calories/calories are on the metric system, they have been superseded by the SI standard of the joule. So we think of Calories as a measure of the potential energy stored in food to be burned in our body, and joules as the measure of work done on the bike. This means it’s useful to know their conversion:

1 calorie (the small calorie) = 4.184 joules

Now, knowing the difference between Calories and calories, one can assume that we’ll need to express joules in a form that converts to the Calorie.

1000 calories = 1 Calorie (dietary)

1,000 joules = 1 kilojoule (kJ)

1 calorie = 4.184 joules → 1 Calorie = 4.184 kJs

“If I’m getting this right, you’re saying for every 4.184 kJ I output I burn 1 Calorie to do so?”

That’s where things wash out a bit. The body’s efficiency of turning fuel combustion into useable work at the muscles being engaged is called an individual’s Gross Metabolic Efficiency (GME). The typical GME ranges from roughly 20% to 25%. What that means is, of the 1 Calorie your body plans to burn to produce 4.184 kJ of work, the average human body is only going to be effectively using 20-25% of that Calorie to do so. The remaining 75-80% of the stored chemical energy (Calories) is lost to the external environment in the form of heat.

This boils down to a 1:1 ratio as you can see below:

1 Calorie × 25% = .25 → .25 × 4.184 kJ = 1.045 kJ

For practical reasons, most cyclists approximate this to: 1 kJ to 1 Calorie.

In order to give riders an idea of their kJ/Calorie reading, the TrainerRoad workout page gives users the ability to see just how many kJ/Cal they can expect to output/burn on any given workout.

The accuracy

It takes a gas exchange lab test to pin down exactly what percentage your GME is, so a majority of people use the approximation of 25% to keep things simple with the kJ-to-Calorie conversion. To help avoid the hassle of getting your precise GME, there are devices to give you precise Calorie calculations — some better than others:


Power devices give us the ability to see how many kJ’s were outputted in a particular workout session. To brush on some old physics definitions, a joule is defined as watts x seconds. A kilojoule being 1000 x that number. Therefore, there really isn’t a more objective and precise tool to determine a kJ output, and subsequently an estimate of Calories burned. In fact, most agree that it’s within 5% accuracy when calculating Calories burned.

Using what we’ve just learned above, we can firmly make the following statement: The approximate 1:1 ratio of kJ to dietary Calories can be relied on far more when using a device that directly measures power, since we have an objective measure of kJ outputted.

Heart Rate

Calorie calculations based off Heart Rate have improved over the last decade or so, but the accuracy is still somewhere in the neighborhood of 10-20%. Even when HR is coupled with an individual rider’s specifics (age, weight, gender, height), the algorithms can still fail to account for things like: different stroke volume, different max HR’s, and different levels of output at different percentages of HR from rider to rider. Some methods that include VO2 max have been able to close this gap slightly, but again, determining VO2 max is another lab test out of the reach of many.

Before understanding why Calorie calculations are variably less reliable with Heart Rate than they are with Power, we need to drive-home the following point: stress vs. strain. When the body applies forces to the pedals you produce stress; strain however is the response to that applied stress. Power objectively measures what you did on the bike (stress) and Heart Rate measures how your body reacted to that stress (strain). Heart Rate should be used to inform you how your body is reacting to the stress you’re applying to it.

Having said that, Heart Rate can still be useful in determining Calorie calculations, but it’s clear why a Caloric measurement is best estimated by a direct measure of kJ rather than using an algorithm in combination with HR readings. As technology advances, so has the ability to get closer to the precision of Caloric calculations from Power, but we’re still not there yet.

Gym equipment — Time/Distance

Time/Distance is the go-to method in the absence of Power and HR. When you hop on a stationary bike, treadmill or stairmaster at the gym, odds are you’re used to entering in your weight before starting your exercise. The reason is to try and dial-in a more precise measure of your energy expenditure. Once you complete a certain time and distance, you’re categorized based on the weight you’ve inputted. The outcome? A best-guess estimate of the effort you’ve just completed or plan to complete.

There are applications and tools that use a similar calculation to attempt to measure energy expenditure with cycling, but you can make sense of why they’re often seen as unreliable. Notably about 65-80% accuracy because the systems can’t take into account the effort that’s expended to ride a particular distance. You run the risk of not taking into account things like headwind/tailwind or perhaps a climb/descent. Some systems that use GPS elevations can bring the accuracy closer to reality, but calculating Caloric burn based off the typical Time/Distance approach is the least reliable.

Going from kJs to Calories — The Math

Say you get done with a workout and your power meter reads you had a 1500kJ ride. How can we approximate Calories burned going from kJ to Calories? We’ll simply divide the total kJ by 4.184, then factor in the efficiency. See below:

1500 ÷ 4.184 = 358.51

A rider’s GME is now factored in:

358.51 ÷ 25% → 358.51 ÷ .25 = 1434.03

For a rider with a GME of 25%, it took 1434.03 Calories (kcals) to output 1500 kJ.

Let’s see what it would be if we were on the lower end of the 20-25% GME:

1500 ÷ 4.184 = 358.51

358.51 ÷ 20% → 358.51 ÷ .20 = 1792.55

For a rider with a GME of 20%, it took 1792.55 Calories to output 1500 kJ

To Conclude

Given the info above, we all should be fairly confident in using our new physics lesson in the real world. The next time you get a Calorie reading thrown at you, you’ll know how to discern reliable readings from the less reliable. You’ll also know which devices give you the more dependable data to determine more accurate approximations. And if you really want to show off, you can break down the math to show all your buddies exactly how to determine a proper Calorie reading.

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Nick Kanwetz

Nick Kanwetz is a writer of all things cycling. Whether he's creating content, interviewing coaches or digging through studies, he's doing it all to make sure you become a faster cyclist and all around better athlete.