How to Convert Kilojoules to Calories for Cycling

Kilojoules and calories are both units of energy; they quantify the amount of energy stored in an object or the amount of work (force times distance) done. While joules are normally used for scientific pursuits, calories have been adopted by the dietetics community to describe the energy stored in the foods we eat. While there is a simple conversion between joules and calories, each calorie is equivalent to 4.184 joules, the conversion between these units becomes muddier when trying to apply them to nutrition and fitness.

\text{Energy} = \text{Work} = \text{Force} * \text{Distance}\\ \text{Calorie} = 4.184 \text{ Joules}

Calorie Versus Kilocalorie

In the US, a calorie (cal) is actually a kilo-calorie (kcal). Specifically, the calorie value seen on a nutritional label in food is equivalent in energy to 1000 calories (4184 joules) as described in science. In an attempt to simplify the subject, nutritionists started using the term calorie instead of kilocalorie with the general public. In other parts of the world, nutrition labels will specifically use the acronym ‘kcal’ to represent what Americans know as a calorie. Confusing enough? To continue along this line of conversion, if a calorie is 4.184 joules, a kilocalorie is 4.184 kilojoules (kJ). To further put this into context, that banana you had for breakfast is about 100 kcal or 418.4 kJ. 

1000 \text{ Calories} = \text{Kilocalorie (kcal)} = 4184 \text{ Joules} = 4.184 \text{ Kilojoules (kJ)}

Calculate Kilojoules Burned Cycling

When working out, we expend energy (duh) and do work. We produce force to move a distance. This is all simple enough. In cycling, the work done is from the pedal moving in a circle and the cyclist moving down the road. The number you see on your power meter is in the units of watts and watts are a unit of power; this is where the term ‘power meter’ comes from. Power is defined as the amount of force per unit time. Specifically, watts are defined as joules per second. So when we ride at 300w for 20 minutes, that can be converted to 360,000 joules or 360kJ. 

\text{Power} = \text{Force } / \text{ Time} \\ \text{Watts} = \text{Joules } / \text{ Seconds} \\ \text{Watts } * \text{ Seconds} = \text{Joules} \\ 300 \text{ Watts } * 1200 \text{ Seconds} = 360,000 \text{ Joules} = 360 \text{ kJ}

Kilojoules to Calories

Knowing that we produced 360kJ during that effort, how many calories did we burn? This is where it becomes muddy. If you reference the article on the human body energy systems, each of our body’s energy systems uses stored energy at a different efficiency. Aerobic work is some 18x more efficient than Anaerobic work and Anaerobic work is twice as efficient as our Sprint energy system.

So the true number of calories we burned in our 360kJ effort is unknown, but we have a pretty good idea: the human body’s ability to produce energy is about 23-25% efficient. So it takes about 4 kilojoules to put 1 kilojoule of energy into the bike. Since the conversion from kilocalories to kilojoules is also about 4, the result is that each kilojoule produced on the bike requires 1 – 1.1kcal. At higher intensities, for example, VO2max intervals, it would be a good estimate to assume each kilojoule produced used 1.1 kcal. Whereas for an endurance ride, 1kcal per kilojoule is a better estimate.

\text{Human Efficiency} = 23\%-25\%\\ \text{Calorie} = 4.184 \text{ Joules } * \text{ Human Efficiency} \approx 1 – 1.1 \text{ kcal} \\ \text{Calories burned} = \text{kJ produced on bike } * (1 – 1.1 \text{ kcal})

At the end of your ride, check your power meter for the number of kilojoules to get a good estimate of the number of calories you burned. Then make sure you eat enough to refuel and recover for the next ride.

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