[cycling] formula for watts to calories?

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jjpptt2
jjpptt2 Posts: 5,650 Member
edited January 2019 in Fitness and Exercise
Is the a generally accepted formula for converting watts to calories after a ride/spin? I did some googling, and the only article I found seemed more complicated than I expected, and the result of the formula differed from what my spin bike gave me by about 20%.

Today's spin was 50 minutes and averaged 125w. Bike told me 481 calories... googled formula gave 375 calories.

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  • inglysh731
    inglysh731 Posts: 42 Member
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    According to my power meter/Garmin, my 50 min spin this AM burned 443 cal. My normalized power was 159 watts, my average power was 154 watts and my average hr was 130 bpm. It was a fairly steady effort (no intervals).

    That said, stationary bike power accuracy is apparently quite poor from what I've read. I would take that number with a bit of a grain of salt.

    I think you'd be right to take 20% right off the top of the figure the stationary bike provides.
  • kcjchang
    kcjchang Posts: 709 Member
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    You need to convert the work done, kj to kcal. Since the recording frequency is one second, just sum the recorded wattage produced during the entire workout (1 joule = 1 watt/sec). Divide that by 1000 to convert to kj. Divide that by 4.184 to convert it to kcal. Finally, divide the result by your efficiency factor (anywhere from 22% to 28%, measured in lab but generally 25% is used). I would use average power as opposed to normalized power (derived expression of steady state power need to complete the work as opposed to actual measurement). The estimate is as accurate as the measurement, wattage and efficiency factor.
  • sijomial
    sijomial Posts: 19,811 Member
    edited January 2019
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    A simple formula you can do by mental maths (I do it while I'm cooling down...) is
    Average watts x 3.6 to give your hourly net calorie burn.

    125w x 3.6 would be 450 for an hour. (Which would 375 cals for 50 minutes.)

    Here's a good article that explains how you can simplify the very complex formula
    ((100W x 3600s) / 4.18 ) / 0.24 = 358,851cal = 358.9kcal down to the simple one above by using 24% as the assumed efficiency ratio.
    http://mccraw.co.uk/powertap-meter-convert-watts-calories-burned/

    It's even more simple if I link my Garmin Edge to the indoor trainer and then it does it for me and passes on the power figure and calorie data to Strava as well.
  • sijomial
    sijomial Posts: 19,811 Member
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    Regarding Spinning sessions though - if you are standing cycling for a significant period of time the efficiency ratio plummets dramatically as you are weight bearing instead of non-weight bearing and you would be under estimating.
  • jjpptt2
    jjpptt2 Posts: 5,650 Member
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    sijomial wrote: »
    A simple formula you can do by mental maths (I do it while I'm cooling down...) is
    Average watts x 3.6 to give your hourly net calorie burn.

    125w x 3.6 would be 450 for an hour. (Which would 375 cals for 50 minutes.)

    Here's a good article that explains how you can simply the very complex formula
    ((100W x 3600s) / 4.18 ) / 0.24 = 358,851cal = 358.9kcal down to the simple one above by using 24% as the assumed efficiency ratio.
    http://mccraw.co.uk/powertap-meter-convert-watts-calories-burned/

    It's even more simple if I link my Garmin Edge to the indoor trainer and then it does it for me and passes on the power figure and calorie data to Strava as well.

    Thanks. That was the article I found and the formula I used as a comparison to what the bike told me.
  • sijomial
    sijomial Posts: 19,811 Member
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    Could be your bike is trying to estimate gross calories rather than net perhaps?
    If it's asking for your weight (an irrelevance for net cals) I would suspect that is what's going on, could just be using a wacky algorithm too of course. :smile:
  • aokoye
    aokoye Posts: 3,495 Member
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    In addition to what @sijomial said, the last I checked Garmin uses the same formula for their calorie numbers when the Garmin head unit or watch is connected to a power meter in terms of cycling (who knows abt running power and I'm assuming it doesn't when connected to a rowing machine - I've never (successfully) tried).
  • NorthCascades
    NorthCascades Posts: 10,970 Member
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    For our purposes, you can just change the label from kJ to kCal. The 4.1 calories per Joule reverses your efficiency factor. Purely by coincidence.
  • jjpptt2
    jjpptt2 Posts: 5,650 Member
    edited January 2019
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    sijomial wrote: »
    Regarding Spinning sessions though - if you are standing cycling for a significant period of time the efficiency ratio plummets dramatically as you are weight bearing instead of non-weight bearing and you would be under estimating.

    It's not a spin class... just steady state base work on my own (on my lunch break, using a spin bike at work). Sounds like the formula you posted, and the one I found via google, is a good sanity check for what the bike tells me. It also lines up pretty well with what my garmin tells me based off HR and no power.

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