# Calorie/Watt Calculator



## msohio

http://www.braydenwm.com/calburn.htm

Saw this on Velo's home page. Very slick if it is fairly close. Is it? Those of you with a power tap should know if it is in the ballpark or not.


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## Kerry Irons

*Pointless*



msohio said:


> http://www.braydenwm.com/calburn.htm
> 
> Saw this on Velo's home page. Very slick if it is fairly close. Is it? Those of you with a power tap should know if it is in the ballpark or not.


Any calculator claiming to be able to estimate calories burned based on heart rate is going to be wrong most of the time. Like most HR meters and fitness center equipment, this one reads high (15% off for me). To suggest that there's a 6% difference in calories burned for two identical people, based on a 10 bpm difference in HR, shows how lame this kind of calculator is. If you want some decent numbers, go to analyticcycling.com, and multiply the wattage numbers by 4 to get calories per hour (metabolic efficiency for a fit person is pretty close to 25%).


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## StillRiding

Interesting, but inaccurate. I just ran the figures from my last workout and the error in the online calculator was about 8%. For my wife's last workout, the error was a whopping 37%. We both ride well tested power meters. Calculations were done using data taken over a 20 minute period riding at a nearly steady state.


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## leon2982

*Compared to Power Tap*

I compared 2 rides downloaded into Training Peaks from my Power Tap 2.4 and the calculator was not close at all. Each example was over 100 watts off.


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## yz_387

I just don't understand why people seem to think that the calories determined from using a power meter are so accurate. As stated above, to calculate the calories burned from power output, one must assume a metabolic efficiency. Can anyone tell me why this is more accurate than the assumption made in a HRM?


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## STARNUT

Because a power meter measures the actual amuount of work being down. It does not saythat you burned X amount of calories, only that you performed X kj of work. The kj of work is roughly equal to calories.

An HRM back calulates from your HR, obviously. I know for me personally depending on how rested I am, how many cups of coffee/red bull I've had and how much I've been training I can have a varing HR for a given power/work load.

its is simply reaction v/s action measurment.

Starnut


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## asgelle

yz_387 said:


> Can anyone tell me why this is more accurate than the assumption made in a HRM?


Simply put, because the data says so. There's plenty of data to show that while efficiency varies from individual to individual, that variation is over a fairly small range. On the other hand, there is very little statistical correlation between heart rate and calories burned, so there is no foundation for calories to correlate with heart rate.


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## Squint

Try managing your diet using the calculated numbers and see how much unwanted weight you gain.


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## yz_387

asgelle said:


> Simply put, because the data says so.


Can you point me to any of this data? I would love to see it. As someone who has spent his career in research laboratories, I have grown very cautious about claims and have much more faith in hard numbers; particularly if I can see how the numbers were calculated.

Maybe if I can come up with enough good reasons, I can eventually justify purchasing a PM. :thumbsup:


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## Dwayne Barry

yz_387 said:


> Can you point me to any of this data?


Pubmed.

Look for studies about cycling efficiency. What you'll find is if you put someone on a bike almost everyone is in the 20-25% efficient range in converting chemical energy to mechanical work.

OTOH, you'll find almost nothing to suggest that HR corresponds to work rate (and therefore calories burned) across individuals. Why? Because we all have a nearly similar HR range yet we vary greatly in our work capacity.

IOW, if my grandmother, me and a professional cyclist were all working at 200 watts, we'd all be burning roughly the same calories yet our HRs would vary widely (lets say 180, 150 and 120 respectively). Or if we were all exercising at HR of 150 bpms the pro would be working a lot harder than me, who in turn would be working a lot hard than my granny.

Basically, little variability in efficiency means work rate correlates well with energy consumption, large variability in the HR/work rate relationship means not a good predictor of energy consumption.


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## estone2

yz_387 said:


> I just don't understand why people seem to think that the calories determined from using a power meter are so accurate. As stated above, to calculate the calories burned from power output, one must assume a metabolic efficiency. Can anyone tell me why this is more accurate than the assumption made in a HRM?



Lets use an example of me. Anecdotal, but it'll show you at least why I personally prefer power over heart rate.
<table>
<td>Power</td>
<td>Heart Rate</td>
</tr>
<tr>
<td>150w</td>
<td>140bpm</td>
</tr>
<tr>
<td>160w</td>
<td>142bpm</td>
</tr>
<tr>
<td>170w</td>
<td>144bpm</td>
</tr>
<tr>
<td>180w</td>
<td>150bpm</td>
</tr>
<tr>
<td>190w</td>
<td>156bpm</td>
</tr>
<tr>
<td>200w</td>
<td>165bpm<td>
</tr>
<tr>
<td>210w</td>
<td>165bpm</td>
</tr>
<tr>
<td>220w</td>
<td>170bpm</td>
</tr>
<tr>
<td>230w</td>
<td>170bpm</td>
</tr>
<tr>
<td>240w</td>
<td>170bpm</td>
</tr>
<tr>
<td>250w</td>
<td>171bpm</td>
</tr>
<tr>
<td>260w</td>
<td>171bpm</td>
</tr>
<tr>
<td>270w</td>
<td>171bpm</td>
</tr>
<tr>
<td>280w</td>
<td>172bpm</td>
</tr>
<tr>
<td>290w</td>
<td>172bpm</td>
</tr>
<tr>
<td>300w</td>
<td>172bpm</td>
</tr>
</table>


This is for me outdoors. My LT is 300W, my LTHR is 172bpm. Indoors my LTHR is 187bpm, though my LT is still 300w. A heart rate monitor will tell me that indoors, I'm burning more calories, even though I'm still putting out the same power... and seeing as (Kj of mechanical work)/(metabolic efficiency)=kcal burnt, HR must be wrong!

A powermeter will tell you I'm burning more calories at 300w than 230, despite the fact that my heart rate is almost exactly the same... a HRM would miss that.

One thing about me is that my heart's very abnormal... I don't think most people's heart rate graphs resemble the graph of a square root so much as the graph of an exponential, in which case an HRM would work a bit better, and most peoples HRs dont skyrocket indoors. Indoors, I've been on a recovery between intervals, talking and breathing normally, with my heart rate at 189bpm... but that's just me.

Additionally, a human body is 19-21% efficient according to Friel, though these boards say 21-25%. 4.1kJ=1kCal, so 1kJ is 24% of a calorie. So (kJ of mechanical work * .24)=Calories of Mechanical work. Calories of Mechanical Work/metabolic efficiency=Calories burned.
That goes to:
(kJ*.24)/(~.24)=kC. It's not an exact conversion, but it's close enough for measuring roughly how many calories you burned.


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## GTScott

Not meaning to dig up an old post, but I am interested in the conversion from wattage to calories burned. I ran across some discussion that suggested the following:

[((Average power in watts / 1000)x(duration in seconds))/4.18] / 0.25 

So, for my 40 minute ride at 199 watts average output, it is showing a burn of 465. This seems inline with the general rule of thumb of 100 calories per 10 minutes that I have typically used. 

Any comments on this formula? It is predicting a burn slightly above what I thought but I assume it is in the ballpark. 

I am not trying to use this for any huge diet or other purpose...just curious.

-GT


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## den bakker

GTScott said:


> [((Average power in watts / 1000)x(duration in seconds))/4.18] / 0.25
> 
> 
> -GT


(((Take power and convert into Kwatt)*(time to reach number of Kjoule burned)/(conversion from Kjoule to cal)/(efficiency i.e. for each watt placed in the pedal, some number of watts are going into heat). 
The only place there is a little wiggle room is is the 0.25 factor, all others are definitions. 
As far as I know, the variations around the value of 0.25 from person to person are very small.


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## GTScott

I hear you on the 0.25. Obviously lowering that number makes the calorie count go up. I hear that 0.26 is about as high as you would ever get and the low range I see is 0.22.


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## TurboTurtle

GTScott said:


> I hear you on the 0.25. Obviously lowering that number makes the calorie count go up. I hear that 0.26 is about as high as you would ever get and the low range I see is 0.22.


I've seen numbers as low as 18-19%. - TF


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## Kerry Irons

*The range*



TurboTurtle said:


> I've seen numbers as low as 18-19%. - TF


The statistics I have seen for human metabolic efficiency are "18 to 30%". I believe that numbers on the low end are for people who are very out of shape, and numbers on the high end are for genetic freaks.

Converting watts to calories is very simple: the conversion factor is 3.6 (assumes "typical athlete metabolic efficiency of 24%"). After that, you just ratio the time. If you did 2 hours, then multiply by 2. If you did 40 minutes, multiply by .666. Etc.


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## yz_387

Kerry Irons said:


> The statistics I have seen for human metabolic efficiency are "18 to 30%". I believe that numbers on the low end are for people who are very out of shape, and numbers on the high end are for genetic freaks.


This is exactly what makes me so skeptical about the claims that the power meter numbers are so accurate. Run the numbers to calculate calories burned based on average power and time and check the difference between 18% and 30% metabolic efficiency; it's a whopping 40%! Look at the more conservative estimate that metabolic efficiency varies from 22% to 26% and you still see a 15% difference in calories burned. I am not saying that a HRM is necessairly any more accurate, but I don't see how you can draw the conclusion that a HRM is incorrect by comparing it to data generated from a power meter.


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## gatorling

yz_387 said:


> I just don't understand why people seem to think that the calories determined from using a power meter are so accurate. As stated above, to calculate the calories burned from power output, one must assume a metabolic efficiency. Can anyone tell me why this is more accurate than the assumption made in a HRM?


Because calories are a measure of energy and power is a measure of energy/unit time.
If you know your power and the duration of the workout then you know how much energy is expended. 

HRMs make the assumption that at a certain HR a certain amount of work must be done, which isn't necessarily true.


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## TurboTurtle

yz_387 said:


> This is exactly what makes me so skeptical about the claims that the power meter numbers are so accurate. Run the numbers to calculate calories burned based on average power and time and check the difference between 18% and 30% metabolic efficiency; it's a whopping 40%! Look at the more conservative estimate that metabolic efficiency varies from 22% to 26% and you still see a 15% difference in calories burned. I am not saying that a HRM is necessairly any more accurate, but I don't see how you can draw the conclusion that a HRM is incorrect by comparing it to data generated from a power meter.


While the power meter is somewhere around + 10% (for those using this forum), the HRM has no basis from which you can even calculate an error.

It would be like calculating the amount of gas you burned on your last vacation only knowing the RPM of your engine. You can't.

TF


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## gleasonll

TurboTurtle said:


> While the power meter is somewhere around + 10% (for those using this forum), the HRM has no basis from which you can even calculate an error.
> 
> It would be like calculating the amount of gas you burned on your last vacation only knowing the RPM of your engine. You can't.
> 
> TF


Are you accounting for the fact that in the Polar HR monitor, the calorie calculation takes into account maxHR amd VO2max (both input by user) and uses the recorded HR as a percentage of maxHR to calculate that amount of O2 being used, based upon the input VO2max The amount of O2 consumed is then related to the number of calories burned. What has been said in this forum is that if you are at a HR of 180 (for the sake of discussion) and start coasting, power goes to zero, and no calories are being used. I would contend that until your HR approaches some nominal value, excess calories are being burnt; Polar uses a value of 90bpm if you have input a test value for VO2max.

The following is taken from the Polar site (OwnCalS is used in the Polar S series):
In OwnCal the energy expenditure assessment is based on gender, heart rate and body weight and in OwnCalS on individual VO2max and HRmax along with heart rate and body weight. In OwnCal the calorie counting starts from the heart rate 100 bpm and in OwnCalS from 90 bpm. The limit is set because the OwnCal/ OwnCalS -feature has been created to calculate only total calories consumed during exercise and heart rate values below 100 bpm do not correspond to such a situation in most individuals. In OwnCalS the counting starts at 90 bpm because athletes have lower heart rates than people with lower fitness level at the same exercise intensity


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## winstonw

There's some good points raised in this thread, and as I've been messing with these concepts, thought I'd add my 2 cents worth to clarify a few things said above. 

Regarding converting watts to Calories expended. 

1 watt = 1 joule / second
1 calorie = 4.184 joules. 
1000 calories = 1 Calorie

from these, to calculate Calories expended when average watts for a period of time is known

average watts 200
period of time 30 minutes
Calories expended to generate 200 watts for 30 minutes. 
= watts * 3.6 * 30/60 
= 200 * 3.6 /2 
= 360 Calories

If you want to know Calories expended for exactly 1 hour, all you have to do is multiply watts by 3.6. How is this derived? 

100 watts is a work rate of 100 joules per second. 
There are 3600 seconds in an hour, therefore 
100 watts is a work rate of 360,000 joules per hour = 360 kJ per hour. 

To convert kJ to Calories, we divide by 4.184 -> 360/4.184 = 86 Calories per hour.
However, the body is roughly 24% efficient at converting chemical to mechanical energy, therefore we have to divide by 0.24 -> 86 / 0.24 = 358 Calories / hour. 

Putting all this into one formula reveals that the conversion for kJ to Calories is cancelled by the 24% efficiency conversion i.e. 

for a work rate of 100 watts

100 J/s * 3600/1 (s/hr) * 1/1000 (kJ/J) * 1/4.184 (Calories/kJ) * 1/0.24 (human energy efficiency)

units left = Cals/hr

derived value 
= 100 * 3600 1/1000 * 1 / (0.24 * 4.184)
= 360000/1000 * 1/ (0.24 * 4.184)
= 360 * 1/ (0.24 * 4.184)
= 360 * 0.998
= ~360 Cals/hr


Therefore, to convert watts to a work rate per hour, multiply watts by 3.6.
A 200 watt hourly work rate 
= 200 * 3.6 = 720 Calories per hour. 

However, keep in mind the above derivation only considers the energy expended to produce the power measured. 
It does not include the energy expended to sustain other body functions, as represented by basal metabolic rate (BMR). 
BMR is roughly 0.8 Calories / kg bodywt / hour. Therefore an 80 kg male has a BMR roughly around 80 * 0.8 = 64 Calories / hour. 

This would be added onto the Calories expended generating work, to establish total Calories expended during exercise. i.e. for an average work rate of 200 watts over one hour, 720 + 64 = 784 Calories. 

However, Calories expended during exercise is usually worked out to establish what ADDITIONAL Calories are required in a diet. So BMR is generally not included. 

When I want to include BMR, I get a rough estimate by multiplying watts by 4. This has < +/- 7% error for most people, which is good enough for ballparking additional food intake.

This above is the mathematical proof to convert measured watts to Calories. Note that many stationary bikes that try to convert watts to Calories expended overestimate by up to 100% because they either try to estimate based on heart rate or on the University of South Carolina's Compendium of Energy Expenditures. The Compendium is known to heavily overestimate energy expenditure, especially in relation to cycling.


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## Undecided

winstonw said:


> This above is the mathematical proof to convert measured watts to Calories.


No, it's a fine detailed explanation of a relationship, *assuming* a particular metabolic efficiency. Given that every computer (that I've seen) that reads from a power meter spits out the kJ number, though, you can just say kJ=Calories, more or less, if you're willing to make that same assumption.


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## Alex_Simmons/RST

winstonw said:


> Regarding converting watts to Calories expended.
> 
> 1 watt = 1 joule / second
> 1 calorie = 4.184 joules.
> 1000 calories = 1 Calorie
> 
> from these, to calculate Calories expended when average watts for a period of time is known
> 
> average watts 200
> period of time 30 minutes
> Calories expended to generate 200 watts for 30 minutes.
> = watts * 3.6 * 30/60
> = 200 * 3.6 /2
> = 360 Calories


No.

Replace Calories with kilojoules and this would be correct. It is a measure of mechanical work done at the cranks.

Further down you get the right answer.

As Undecided mentions, the conversion of mechanical work done to total energy metabolised depends on an individual's gross metabolic efficiency. In trained cyclists that can range from 18-25% and is largely an inherit trait. Perhaps trainable over very long time (decade) with very high volumes of riding. Slowtwitch dominant riders tend to be more efficient. 

It also varies in an acute sense due to a range of factors, inter alia: absolute and relative power output, variability of power output, environmental conditions, diet and state of glycogen stores which can affect fuel substrate utilisation ratio, hydration levels, muscular fatigue, cadence and so on.

24% is pretty high efficiency, at one end of the bell curve.


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## dkilburn

*Garmin for Calories burned?*

Hi, Does anyone know how accurate the Garmin 750 is for the meassure of calories? If it said you burn 2000 calories in a ride is it about correct or something less? I have just used it as a guide in the past, if I work hard the number goes up and the same if it is a easy ride it is less. 
Thank for your help.


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## Undecided

dkilburn said:


> Hi, Does anyone know how accurate the Garmin 750 is for the meassure of calories? If it said you burn 2000 calories in a ride is it about correct or something less? I have just used it as a guide in the past, if I work hard the number goes up and the same if it is a easy ride it is less.
> Thank for your help.


No. Nobody knows how accurate it is for any given person. That's the problem.


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## Kerry Irons

*Test it*



dkilburn said:


> Hi, Does anyone know how accurate the Garmin 750 is for the meassure of calories? If it said you burn 2000 calories in a ride is it about correct or something less? I have just used it as a guide in the past, if I work hard the number goes up and the same if it is a easy ride it is less.
> Thank for your help.


Within reason, riding on the flats with no wind at 20 mph is 600 calories per hour for a 150 lb rider, 630 for a 180 lb rider. Do ride like that and see what your Garmin says. Previous posters have given Garmin numbers that are 30% high, but that is an approximation as they haven't all been that much off (some more, some less).


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## Undecided

Kerry Irons said:


> Within reason, riding on the flats with no wind at 20 mph is 600 calories per hour for a 150 lb rider, 630 for a 180 lb rider. Do ride like that and see what your Garmin says. Previous posters have given Garmin numbers that are 30% high, but that is an approximation as they haven't all been that much off (some more, some less).


If you're basing those numbers on the assumptions reflected on the analytic cycling site, I would suggest that the drag coefficients seem low, to me; i.e., I need to significantly increase the coefficient to make the estimates on that site accord with (my) reality, although I'm not all that much heavier than the default weight (and probably "denser" than a "default person"!). Another calculator that includes models for several of my local climbs does accord with my observed times and power outputs (and my performance in both Merckx-style and TT-bike TTs against very solid pros suggests that my coefficient of drag isn't exceptionally bad).


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## dkilburn

Kerry Irons said:


> Within reason, riding on the flats with no wind at 20 mph is 600 calories per hour for a 150 lb rider, 630 for a 180 lb rider. Do ride like that and see what your Garmin says. Previous posters have given Garmin numbers that are 30% high, but that is an approximation as they haven't all been that much off (some more, some less).


 Thanks, 
Last evening I did a 29.2 mile ride, averaged 18.5 mph, 983' of ascent and 1913 calories burned. I'm 170 plus depending on the day, and I did have some side wind before the rain came. My heart rate was in zone 3 and 4 for most of the ride. I do have the Garmin set with numbers of the bike and my weight. So it's close and just a guide that I,m doing something to burn off the meals of the day or yesterday. 
Thanks,


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## Undecided

dkilburn said:


> Thanks,
> Last evening I did a 29.2 mile ride, averaged 18.5 mph, 983' of ascent and 1913 calories burned. I'm 170 plus depending on the day, and I did have some side wind before the rain came. My heart rate was in zone 3 and 4 for most of the ride. I do have the Garmin set with numbers of the bike and my weight. So it's close and just a guide that I,m doing something to burn off the meals of the day or yesterday.
> Thanks,


Kerry's point, if I may be so bold, is that it may not be "close," and that the algorithms used by the computer to estimate caloric expenditure may be far off even when you've entered accurate inputs. But you could try to test it as he described, to get an idea of whether it's plausibly close or not. If it's off by 30%, or more, is it much help as guidance in burning off those meals? Based on personal experience riding with a power meter, I'd take a *guess* that the ride you describe, at your weight, reflects something closer to 1,200 or 1,300 kilojoules than 1,900.

The HR zone that you were in says nothing about the calories you burned; the amount by which the Garmin's estimate is off may vary from ride to ride.


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## Kerry Irons

*Reality*



Undecided said:


> If you're basing those numbers on the assumptions reflected on the analytic cycling site, I would suggest that the drag coefficients seem low, to me; i.e., I need to significantly increase the coefficient to make the estimates on that site accord with (my) reality, although I'm not all that much heavier than the default weight (and probably "denser" than a "default person"!).


How are you measuring that "reality"? The numbers I quote have been confirmed by several different riders using PowerMeters and SRMs. The drag coefficient used as the default by your site is 30% higher than the AnalyticCycling site. What is the basis for this?


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## Undecided

[Wrong place.]


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## Undecided

I don't know how the authors of the site I mentioned decided on their default drag coefficient, but as that site is focused on (actual) climbs, only, I think they have the luxury of presuming one position (on the tops, I suspect) that is likely to be generally applicable. I don't know how the site I mentioned deals with the variability of gradients on those climbs. I would struggle to find a "flat" section of road of any meaningful duration, but analytic cycling seems (to me) to generate speeds that are more like what I might expect on the TT bike. Play around with a hypothetical male pro racer at 5.5 w/kg (assuming 8kg of bike and equipment) and I think you get speeds that wouldn't seem to far off the mark for "flat" 40k TTs on the road. Anyway, I'm not defending one or the other and have no idea what their assumptions are, but I know that you seem like a guy who's interested in these things. 

And to answer your questions, I am measuring with either of two PowerTaps (and have in the past used two other PowerTaps, and they're regularly stomp tested and serviced when needed) and a Garmin EDGE 500 (and previously a PowerTap computer, although I didn't have independent elevation data with that), with the altitude data adjusted to reflect surveyed elevations. My reference is primarily two climbs that I do regularly (and that are in the set on the site I mentioned) on a road bike, typically climbing seated and on the tops.


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## cevita11

^good


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