How to Convert Watts to Calories Burned While Cycling

Traditionally speaking, trying to get a good idea for calorie expenditure from exercise is about as precise as an Imperial Stormtrooper with a laser blaster. There are plenty of apps and calculators out there that will help you estimate calories burned by inputting data points like heart rate, running/cycling speed, body weight and other factors, but they’re invariably far from accurate. Cardio machines along with Fitbits and other fitness trackers will also attempt to give you an idea on calories burned but those tend to make inaccurate assumptions (like that everyone is a 180 lb. man) and also often include Resting Metabolic Rate (the calories you would burn simply sitting in bed all day) which can lead to double dipping.

In the cycling world, we have data available to us that other sports can only dream about, one such metric is wattage. Measuring power output in watts has become one of the most important metrics in training but there is another side to measuring power output which many don’t consider. You actually know how much energy you put into moving your bicycle over the course of a ride very accurately when using a power meter, which means you can quite accurately determine how many calories you burned while cycling.  

From Watts to Calories

If you know duration  (time spent cycling) and you have average power during that time, you  can quite easily calculate energy.

Power(Watts) = energy(Joules) / time(seconds)

We can figure out our energy using the equation above, but unfortunately, it’s in joules. We need to get to calories so we use the following conversion:

1 Joule =  0.238902957619 calories

Note, that what we commonly call a calorie, is actually a kcal or Calorie. So we need to divide by a factor of 1000 to find our common usage kcal (when you look at a nutrition label those calories are in fact kcals).

There are two types of calories: A small calorie(sympbol: cal) – 1cal is the amount of energy required to raise one gram of water by one degree Celsius. A large Calorie (symbol: Cal, kcal) – 1Cal is the amount of energy required to raise one kilogram of water by one degree Celsius.

We also need to account for human inefficiency. Like any machine the human body is far from perfectly efficient – it has to burn more than 1 joule of real energy to output 1 joule of measured energy through a power meter. The efficiency of cycling humans is between 20-25% (so your body’s many systems and inefficiencies burn 4-5J of energy for every 1J you deliver to the pedals).

This means we should divide any measured joules figure by 0.2 to 0.25 to find out our actual expenditure.

By coincidence, a joule is  0.000239006 calories, or if we multiply by 1000 to get kilocalories approximately 0.24. How convenient!

So, assuming a human with 24% efficiency, you can cancel those last two steps exactly, and assume that measured joules = real calories burned.

Take all the above into account and we can build a very simple equation for calculating calories burned while cycling from average wattage:

energy (kcal) = avg power (Watts) X duration (hours) X 3.6

You might be wondering where that 3.6 came from. It’s simply an adjust for time and the factor of 1000 reduction to get from cal to kcal (60 seconds/minute * 60 minutes/hour * 1/1000 cal/kcal = 3.6).

A few examples:

During a Zwift group ride the other night I put out an average of 205 watts for one hour and 30 minutes, my kcal expenditure would be as follows:

205 Watts * 1.5 hours * 3.6 = 1,107 kcal

Chris Froome can put out an astonishing 414 watts for 30 minutes, his kcal expenditure during that time period would be:

414 Watts * 0.5 hours * 3.6 = 745 kcal

In many ways, the difference between an amateur athlete and a pro is their ability to burn more calories more quickly.

Obviously, there are some limitations of our equation. Namely, the human inefficiency part of our calculation. But even with that fudge factor we are easily within the realm of plus/minus 4%. Not bad, especially compared to cardio machines and calculators which are often found to be off by a factor of 30% or more.

Watts to Calories Calculator

Moving from Watts to Ridding Yourself of that Spare Tire

The energy density of fat is ~3500 kcal per pound. Which fits quite nicely into our equation:

energy (kcal) = avg power (Watts) X duration (hours) X 3.6

Using the formula above, we can see that a watt-hour (1 watt for 1 hour) is worth 3.6 kcal.

So, let us say you commute at an average of 100W, every ten hours of commuting time will burn a pound of fat. If your commute takes half an hour, two weeks are all you need to lose a pound of fat. Doesn’t sound amazing, but multiply that out for a year and you’ve shed 20+ pounds (depending on how much vacation you get).

If you commute at an average of 100W, every ten hours you will burn a pound of fat. Let’s suppose your commute lasts half an hour, so it takes 20 journeys (two weeks) to lose that pound of fat.

Similarly, if I get on Zwift three times a week for an hour and a half and put out an average of 200W I can expect to burn 3,240 kcal a week. Just about a pound of fat a week.

Certainly allows for some interesting thought on how you’re fueling your body as well. A single Butterfinger candy bar is 275 kcal, which equates to 23 minutes of riding at 200W.

3 thoughts on “How to Convert Watts to Calories Burned While Cycling”

  1. The energy a more advanced cyclist uses is much less. 250 watt hours is about 216 kcalories, not more than the watt hours. The body loses a lot of this to inefficiencies on a bicycle and in general output. Perhaps say it is 40%, that would change it to 300 kcalories for a 250 watt hour average.

    My biggest note is the change for an advanced cyclist. Say during a 1 hour commute you get from point A to point B with 250 average watts, this might burn 305 calories or so for you, but an advanced (track?) cyclist rides at 400 watts average for an hour and gets to point C instead. Say point A to B was 23 miles, and A to C was 35 miles, the faster cyclist used less energy to go from A to B, and in fact, if they rode at a normal pace of 250 watts instead to B, they could have only used 290 or 280 kcalories as their riding (Notaccounting at all for aerodynamic posture nor for gear changing) Has given the more advanced rider better recruiting of muscles and better blood work, allowing them a higher efficiency. The more advanced riders ratio of use of calories at 400 watt averages also improves. This means they might burn instead of 40% additional to a 500 watt workout (which would be 430 kcalories+40 % or 600 calories) but more like 30 % or about 545 kcalories.

    In short, the pure muscular fitness of a more advanced cyclist makes their numbers actually decrease for the same distance, or the same watts. This gets more impressive with aerodynamic mechanical and line focus improvements, and then even better when gear improves.

    It is no wonder some people don’t commute by bike, your cycling efficiency may increase to allow you to burn as little as 30% of the calories and thus be doing 30% of the work another person is doing when commuting. That’s why I’m always saying you need road bikes with drop handlebars to commute, you need more aerodynamics clothing, you need an athletic shirt T, etc. It makes a difference sometimes the difference between willing to do the work or not.

    Racing is all the more. You must give yourself primarily the time it takes to build base fitness when prepping to ride for real.

    1. I think you might be conflating a few things here. Yes, in general a professional cyclist can go the same distance using less energy as a fit body is certainly more efficient metabolically than an unfit one. That said, going fast in cycling means exponentially more energy is needed as air resistance is an exponential function of speed. Some quick calculations: a 165lb cyclist on a flat road can ride at 15 mph at about 88 watts. They can speed up to 16 mph by putting out 105 watts, a 17 watt bump. Now let’s take a pro cyclist in a breakway at 25 mph, they need to hold about 330 watts. If they want to speed up to 26 mph that will require about 365 watts. That’s a 35 watt increase. So you can see, going the same distance at a higher speed actually requires more energy expenditure, even when accounting for metabolic efficiency. Hence, why fitness can be boiled down to the ability to burn calories quickly, a pro cyclist can burn 10-15 calories a minute during hard effort.

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