
RoadBikeReview Member
Reputation:
Calorie/Watt Calculator
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.

Pointless
Originally Posted by 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.
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%).

What? Me worry?
Reputation:
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.

leon2982
Reputation:
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.

RoadBikeReview Member
Reputation:
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?

extremely biased
Reputation:
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

Originally Posted by yz_387
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.

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

RoadBikeReview Member
Reputation:
Originally Posted by asgelle
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.

Originally Posted by yz_387
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 2025% 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.

Originally Posted by 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?
Lets use an example of me. Anecdotal, but it'll show you at least why I personally prefer power over heart rate.
Power 
Heart Rate 
150w 
140bpm 
160w 
142bpm 
170w 
144bpm 
180w 
150bpm 
190w 
156bpm 
200w 
165bpm 

210w 
165bpm 
220w 
170bpm 
230w 
170bpm 
240w 
170bpm 
250w 
171bpm 
260w 
171bpm 
270w 
171bpm 
280w 
172bpm 
290w 
172bpm 
300w 
172bpm 
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 1921% efficient according to Friel, though these boards say 2125%. 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.
"It's hard to tell the poison from the cure, so enjoy the disease."
Mohair_Chair

Local Bigfoot Expert
Reputation:
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

Originally Posted by GTScott
[((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.

Local Bigfoot Expert
Reputation:
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.

NeoRetroGrouch
Reputation:
Originally Posted by 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.
I've seen numbers as low as 1819%.  TF
"Don't those guys know they're old?!!"
Me, off the back, at my first 50+ road race.

The range
Originally Posted by TurboTurtle
I've seen numbers as low as 1819%.  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.

RoadBikeReview Member
Reputation:
Originally Posted by Kerry Irons
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.

RoadBikeReview Member
Reputation:
Originally Posted by 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?
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.

NeoRetroGrouch
Reputation:
Originally Posted by yz_387
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
"Don't those guys know they're old?!!"
Me, off the back, at my first 50+ road race.

RoadBikeReview Member
Reputation:
Originally Posted by TurboTurtle
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
Last edited by gleasonll; 03202009 at 06:37 PM.

RoadBikeReview Member
Reputation:
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.
Last edited by winstonw; 08042011 at 05:09 AM.

but thinking about it
Reputation:
Originally Posted by winstonw
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.

Originally Posted by winstonw
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 1825% 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.

RoadBikeReview Member
Reputation:
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.

but thinking about it
Reputation:
Originally Posted by dkilburn
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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