I use the stationary bike in my apartment's gym to train in winter. The bike does not give me power data, but it does give me "calories burned per hour." (as a rate, not a count) Is there any way to use this to calculate my power output? I assume the machine is measuring my power, because I don't see how it could calculate calories burned otherwise.


Calculating calorie burn from power output is pretty straightforward, but the reverse simply cannot be done. The same number of calories can be burned by doing an hour of 30 second efforts and 2 minute recoveries compared to doing a one hour steady endurance effort. Beyond that, you also have the inaccuracy of the gym bike measurement which will be the biggest factor to consider. Any bike which does not display power in watts to you also doesn't measure it in a way that is accurate enough to be useful in the slightest when comparing to data from any other unit.

In short, you can use the calorie number to compare between workouts on the same gym bike to know that you're improving if you burn more calories in the same time at the same level of exertion, but nothing more than that.

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    I don't understand. If there's an easy conversion one way, how can there not be one in the opposite direction by reversing the calculation? Nov 18 '18 at 19:12
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    @DavidRicherby most of these gym bikes do a very crude calorie calculation with the worst being simply X kcal/hr, and the 'best' adding a multiplier for resistance setting and/or speed. You will find many bikes for example that give 400kcal/hr at 15speed units, but that 400 will not change regardless of if resistance is set to 1 or 20. And that's before considering that they are uncalibrated and vary wildly in states of maintenance. If you go to a local gym and try 3 bikes next to each other, the resistance they produce for each level is likely to be out by 20-30% compared to its neighbour
    – Andy P
    Nov 19 '18 at 13:09

Calories/hour is actually a power measurement (power is the rate of transfer of energy). So naively one could just convert from calories/hour to joules/second (watts). 1 kilocalorie/hour = 1.162 watts. (Remember ‘calorie’ in a diet or exercise sense is actually a kilocalorie.)

However, I strongly suspect that exercise bikes calorie rate readings are quite inaccurate. No inexpensive exercise bike is going to have a proper accurate power meter in it. I suspect that calorie rate is estimated from speed and current resistance level.

[Update] As pointed out in comments, the calorie burn rate is an estimate of calories used by your body, not an estimate of power output to the cranks.

You also do not know what time interval the calorie rate is calculated over, making the readings not very useful.

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    Argenti: I think you've forgotten to adjust for gross metabolic efficiency. Human beings on a bike have a GME around ~20%, so only ~20% of calories burned produce energy. Each kilocalorie burned per hour can be used to produce somewhere around a quarter of a watt, so if Apperson123 were pedaling at a rate of 750 kilocalories per hour (and if the exercise bike were absolutely accurate) his average power output would be about 750/4 = 187 watts.
    – R. Chung
    Nov 18 '18 at 17:43
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    @DavidRicherby do you use a heart rate monitor that strava understands? HR can be used to estimate when you go anaerobic, which is less efficient (and could be why you get <20% efficiency). Without calibrating yourself on a bike in a lab with some fairly expensive kit this is only a guesstimate but better than just using ~20%.
    – Chris H
    Nov 18 '18 at 19:26
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    @ChrisH I was using a HRM today, yes. Was really struggling up any kind of hill so probably doing about 0.20% efficiency! :) Nov 18 '18 at 20:48
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    @DavidRicherby For example, on the ride I did today, Strava estimates that I burnt 2200kcal (9200kJ) but only transmitted 1300kJ to the pedals. A kcal is 4184 J - a bit over 4 kJ. But your body only outputs about 20-25% of that energy as usable power. So that 1300 kJ to the pedals needs to be multiplied by 4 or 5 to tell you how many kcal you burned. Using 5 (20% metabolic efficiency) means to put 1300 kJ to the pedals you burned 6500 kcal. Strava is likely miscalculating your kJ along with overestimating your kcal burned. FWIW, I ride with a power meter and use 1 kJ = 1 kcal. It works. Nov 19 '18 at 13:57
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    @ChrisH I figured it out while slogging into a headwind on my way into work. I was slogging into a headwind that day, too, and Strava's estimated power is based on speed, gradient and weight, so it thought my power was low because I was going slowly. And it's probably estimating calorie burn from stuff that includes heart rate, which was about normal. Nov 20 '18 at 12:20

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