Many cyclists ride in order to maintain or lose weight, so it is important to have an accurate count of calories burned for each ride. If you’ve been riding long and using tools such as Zwift or Strava to track your rides you know each service delivers different calorie numbers. Why is this, and which numbers should you trust?
To answer this question you must understand the basics of calories, kilojoules, and efficiency.
A Calorie (or more accurately, a kilocalorie) is a unit of energy. How much energy? Scientifically speaking, a kilocalorie is the amount of heat required to raise the temperature of 1 kilogram of water one degree Celsius.
A kilojoule (kJ) is also a unit of energy. How much energy? 1 Calorie=4.186 kJs. The number of kJs you put into your pedals over the course of a ride is equal to (average watts × ride length in seconds) / 1000.
- If you ride for 60 minutes (3600 seconds) at 200 watts you will have put 3600 x 200 / 1000 = 720 kJ of energy into the pedals.
- If you go harder and ride for 60 minutes at 300 watts you will have put 3600 x 300 / 1000 = 1080 kJ of energy into the pedals.
If you have an accurate power meter then you are accurately measuring your kilojoules on each ride.
(Note: in the absence of a power meter, calories calculators use some combination of heart rate, body weight, age, distance, time and incline to approximate your work. These approximations produce widely differing calorie numbers. Since this post is for Zwifters I’m assuming you have an accurate measurement of kilojoules–that is, an accurate power meter.)
Efficiency: the Big Question
When you ride, your body converts stored calories into kinetic (movement) energy. Those movements show up as wattage in Zwift, which we summarize in kilojoules at the end of the ride.
But here’s the rub: not all of your calorie’s energy is converted into kinetic energy. In fact, most of your calories are converted into heat energy, not kinetic energy–and that’s why you sweat when you ride.
How much of each calorie becomes heat, and how much becomes movement? That’s the efficiency question. And there’s no way to answer it without lab testing, so in everyday life we rely on approximations.
Scientists agree that when cycling, 20-25% of each calorie burned is actually applied to your pedals. The more efficient your body is, the higher that percentage will be. And remember, we also know 1 calorie can produce 4.186 kJs of work. Using those numbers, let’s look at some example calorie burn calculations.
Examples (using our rides above):
- 720 kJ ride at 25% efficiency burns (720/4.186)/.25 = 688 Calories
- 720 kJ ride at 20% efficiency burns (720/4.186)/.2 = 860 Calories
- 1080 kJ ride at 25% efficiency burns (1080/4.186)/.25 = 1032 Calories
- 1080 kJ ride at 20% efficiency burns (1080/4.186)/.2 = 1290 Calories
Different tools factor in different efficiency levels, and that’s why you will see different calorie numbers for any given ride if you compare Strava to Zwift to TrainingPeaks to Garmin and so on.
So Which Calorie Number is Correct?
The answer is: none of them are. Unless you are in a lab which can accurately measure your complete energy output, you must rely on tools which crunch numbers and deliver approximations. Here is what I’ve observed between the different services:
- TrainingPeaks and Garmin Connect return the same calorie numbers when using Zwift files as input.
- Zwift’s calorie counts are very close, but slightly lower than TrainingPeaks and Garmin Connect.
- Strava’s calorie counts are consistently the highest.
Tracking your calories to lose weight? Use the service that gives you the lowest calorie counts.