kCals vs kJ - why do they differ so much?

Hi - love the podcast and the app. 5 stars all around.
I’m trying to discern the correlation between kCals and kJ. I’ve created a spreadsheet with some metrics grabbed from a few sources, here: https://docs.google.com/spreadsheets/d/1ZbKfx-w-lgIlVrOOhJvBWIZ3yJZfGHGfaJuCXjvaWJI/edit#gid=0

Essentially, what I’ve noticed is that my kJ and kCal numbers can differ wildly, and it seems to depend on my average heart rate. When my average heart rate is above 130bpm, kCal and kJ numbers are similar. When it’s lower, there’s a huge discrepancy.

I’ve read that 1kJ is basically 1kCal. I’ve also read that most apps use heart rate/age/weight/etc to calculate calorie burn. FWIW, I used a wahoo bolt, wahoo tickr, and quarq dzero power meter for all of the rides in my spreadsheet. I’d love to get a handle on this so I can have a better idea of how many calories I’m actually burning per day. From a bodyspec scan, I’ve got a pretty good idea of my resting metabolic rate.

Thanks!!

Ivor

because you are using a Wahoo bike computer. Switch to another bike computer and discrepancies will vanish.

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Just to be clear here, a kilojoule and a (kilo)calorie are both units of energy measurement. Like miles and kilometres are units of distance. You can convert between them.

Yes, the conversion is roughly 1:1 as a crude approximation. So 1000 kJ measured by a power meter is roughly 1000 calories.

this is the problem:

593 calories and 1458 kJ is a completely wacky and wrong conversion. As I understand it, Wahoo bike computers sometimes report wrong conversion, and then Wahoo syncs to Strava/TP/TR and unfortunately Strava and TrainingPeaks trust and use the wrong value.

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So yes, there is a standard conversion: 1 kJ = 0.24 kCal, or 1 kCal = 4.18 kJ.

But by convention in cycling, kJ is used to measure the work done at the pedals – it is calculated by integrating all the instantaneous power measurements over the time you spent riding. By convention, kCal is used to measure the energy that your body burns during this activity, typically measured from heart rate.

On average – for the average person, at average temperature, air pressure, health etc – people are about 25% efficient, meaning that about 1/4 of the energy that your body puts out [kCal] is reflected in the work done [kJ]. This means that under those conditions, 1 kCal of energy put out gives about 1 kJ of work done at the pedals.

But, if you are less efficient in general [get less work for energy expended], then kCal > kJ. Conversely if you are more efficient. And similarly for different conditions. If one day, your heart rate is high for a particular power output [you are ill, tired or whatever], then energy will be high compared to work done.

So kCal and kJ measures should be approximately equal, but there will be differences in the ratio between people and under different conditions. But, as barrbera says, 593 calories and 1458 kJ is a completely wacky.

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I’ve not yet come across an algorithm that uses HR to estimate calories burned that works with any accuracy.

Generally, it appears when HR is used, most devices overestimate the calories burned.

Until I learn of a proven methodology, I’ll stick with the 1:1 approximation.

This is the correct answer to his specific question

Your profile is private so I cannot pull exact numbers from your rides to give you examples - but what you’re seeing is one of the main problems with the Wahoo bike computers - they use your age and heart rate data to calculate calories burned, even in the presence of a power meter

What this means for most well trained cyclists is that on an easy recovery ride Wahoo will give you a significantly lower number than something based on your power. Whereas it will be closer, but still inaccurate, on a harder ride. This is because their formula is based on population averages and not well trained cyclists who can burn significantly higher calories at lower heart rates.

An example of this from my data…

Recovery ride

• Average HR 95 BPM
• ‘Calories’ per my Bolt 603
• KJ per my power meter 2382

Race

• Average HR 139 BPM
• ‘Calories’ per my Bolt 2468
• KJ per my power meter 2659

What to do about this? Complain to Wahoo - it’s ridiculous to use HR when Power data is available

Also - use the KJ, don’t use the HR based calorie information if you have the power data. If you don’t have power data - you’re kind of guessing no matter what so you can use their number, but if you are well conditioned you need to recognize the significant discrepancies on your easier rides

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I figured out the issue… In my wahoo app, the HR zones we’re all jacked up. Basically they were somehow set so that my computer thought I was burning only calories ‘at rest’ under 125bpm. I fixed the zones and my calorie count was sane on my commute home.

That said…I’m just going to operate on kJ from now on and see how that goes, in terms of my weight.

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I think I am missing something here.
It takes 4Kcal of energy to create 1Kj of work on the bike (assuming we’re 25% efficient), correct?
But the Kcals we burn on a bike ride are not 4 times the number Kj’s of work we did right?
I think I am missing something here.
Any clarification would be greatly appreciated.

The 1kJ calculated from the power measured at the power meter. But since we are ~25% efficient, the body has to expend 4 times that amount total to deliver that power to the pedals. So energy expenditure is 4kJ. Then you convert this to kCal which equals 0.956 kCal (1kJ = 0.239 kCal). The human body efficiency and the kJ to kCal conversion factor happen to work out that 1kJ of work done on the bike equates to ~1kCal of work done by the body.

Example: If you do 200W on the bike for an hour, that is 200J/s for 3600 seconds so you delivered 720,000 J (or 720kJ) to the pedals. It takes your body about 4x this amount of energy 2880 kJ. Convert this to kCal using 0.239 and you get 688 kCals that your body burned.