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Cyclists have a big advantage over runners, swimmers, weight lifters, yoga practitioners, and pretty much every other athlete out there in terms of calorie counting. They don’t have to rely on inaccurate calorie calculators or apps. Thanks to watt meters, we cyclists are able to measure the exact amount of the work we’ve done on the bike. So, how do you calculate how many calories are burned during an hour at 200W?

## There’s a simple formula

All you need to know is the time spent cycling and the average power during that time, and you can get the energy burned using this general formula.

Energy (Joules) = Power (Watts) * Time (Seconds)

The problem is that you want to know your energy expenditure in kcal, not joules. Note that what is commonly referred to as calories is in fact kilocalories, that’s why the abbreviation says kcal, not Cal. So, we would use a conversion rate of 1 Joule = 0.238902957619 kcal. We spend a lot of energy on heat production, balance and other things while riding a bike.

## Human efficiency is only 24%

Another problem is that humans aren’t perfectly efficient engines. We spend a lot of energy on heat production, balance and other things while riding a bike. In fact, efficiency of cycling humans is around 25%. We burn 5 joules of energy for each joule delivered to the pedals. That means we should divide the measured joules by 0.25 to calculate the actual expenditure. By coincidence, the joule to kcal conversion mentioned above is 0.2389. So, the last two problems cancel each other out, and we can just swap joules for kcal.

## How to calculate it

There’s just one last adjustment to make. The fact that we multiplied calories by 1,000 to get our desired kilocalories (kcal) means we also need to adjust time. We will multiply it by 3.6 to get hours. This is the final formula you can use. Just multiply your average power output on the bike by the number of hours you spent riding times 3.6 and there you have it – the number of calories burned.

Energy (kcal) = Power (watts) * Time (hours) * 3.6