Go Back  Bike Forums > Bike Forums > Road Cycling
Reload this Page >

Calories expended. Garmin vs Powertap

Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Calories expended. Garmin vs Powertap

Old 09-03-08, 05:23 PM
  #1  
merlinextraligh
pan y agua
Thread Starter
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,329

Bikes: Wilier Zero 7; Merlin Extralight; Co-Motion Robusta; Schwinn Paramount; Motobecane Phantom Cross; Cervelo P2; Motebecane Ti Fly 29er; Calfee Dragonfly Tandem

Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 873 Post(s)
Likes: 0
Liked 3 Times in 2 Posts
Calories expended. Garmin vs Powertap

Rode today with the Powertap and a 705, mostly playing around, and to check the calibration of the Powertap's mileage.

The inaccuracy of the Garmin's calorie measurement was laughably bad, to the point of being worthless.

On a relatively easy one hour flat ride, average speed 19.4 mph, the Powertap indicates I burned 780 calories (applying the formula below). The Garmin calculated it at 1625.

And how do we know the PT is right and the Garmin wrong?
The PT measures actually energy expended in Kilojoules. 4.18 KJ = 1 calorie. However, given a cyclists efficiency in converting dietary calires to energy at the hub, the relationship is closer to 1 to 1.

Lab tests show the efficiency is between .18 and.24, which means that calories expended is actually between 1.05 and 1.15 x KJ.

Using kJ x 1.1 =Calories means you'll be accurate within plus or minus 5%. (which is what I used for the PT calories) So even a 5% varience in the PT calculation would still leave the Garmin data off almost 100% high

http://www.saris.com/CalorieCalculator.aspx

So if you're calculating you calorie replacement needs off your Garmin, you might not be losing too much weight.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.
merlinextraligh is offline  
Old 09-03-08, 05:30 PM
  #2  
rizz
I'm that guy that I am.
 
Join Date: Jul 2007
Location: Maryland
Posts: 1,153
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
People actually use the calorie numbers off a Garmin?
rizz is offline  
Old 09-03-08, 05:34 PM
  #3  
Iamkar33m
Senior Member
 
Join Date: Jun 2008
Posts: 725
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
I halve the number my Garmin gives me (which is usually close enough to the accurate calorie count).

EDIT: I use the Garmin Edge 305

Last edited by Iamkar33m; 09-03-08 at 05:41 PM.
Iamkar33m is offline  
Old 09-03-08, 05:37 PM
  #4  
yobtah
Senior Member
 
Join Date: Aug 2006
Posts: 71
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Yep... I also usually divide my Forerunner 305's calorie estimate for cycling by two. I normally guess around 40 calories per mile when I ride, and the 305 estimates approximately double that.

Oddly, the 305 seems much closer to accurate with running than it does with cycling. For running, it's around 120 calories per mile which I think is not far off.
yobtah is offline  
Old 09-03-08, 06:12 PM
  #5  
dgasmd
shedding fat
 
dgasmd's Avatar
 
Join Date: Aug 2006
Location: South Florida
Posts: 3,149

Bikes: LOOK 595 Ultra/Campy Record 10Sp, restored Guerciotti/Campy C-Record 6 Sp, TIME RXR/Campy SR 11Sp, and Colnago C-60 with Campagnolo SR 11sp.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked 0 Times in 0 Posts
The calorie/energy used feature of the PT is one thing that I love about the unit. Prior to it, I was always under-eating and feeling like crap on longer rides. This feature alone has made the PT a good buy for me at least. I keep up much better now and if anything, it is a constant reminder to eat and drink more. I can't wait until I can pair the 705 to the PT to have one simplified unit. Right now I am only using the 705 as a GPS unit when I go riding out of town. Poor use or justification for it really, but it will pay for itself here pretty soon when I go to Europe for 2 weeks riding my bike.
__________________
Arguing with ignorant people is an exercise in futility. They will bring you down to their level and once there they will beat you with their overwhelming experience.
dgasmd is offline  
Old 09-03-08, 06:20 PM
  #6  
umd
Banned
 
umd's Avatar
 
Join Date: Sep 2005
Location: Santa Barbara, CA
Posts: 28,387

Bikes: Specialized Tarmac SL2, Specialized Tarmac SL, Giant TCR Composite, Specialized StumpJumper Expert HT

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Although yes, the garmin's calculations are laughably bad, one thing to keep in mind with it & most HRMs is that it is supposed to include your base calories as well. For example, my rides often include long descents, 15-20 minutes where I may not be pedaling at all. I've done no work but my body is still burning calories, and likely still at a high rate due to just having worked hard, and all the core muscle involvement. I'm not sure when doing work, the RMR is included in the efficiency assumption or not.
umd is offline  
Old 09-03-08, 06:34 PM
  #7  
merlinextraligh
pan y agua
Thread Starter
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,329

Bikes: Wilier Zero 7; Merlin Extralight; Co-Motion Robusta; Schwinn Paramount; Motobecane Phantom Cross; Cervelo P2; Motebecane Ti Fly 29er; Calfee Dragonfly Tandem

Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 873 Post(s)
Likes: 0
Liked 3 Times in 2 Posts
Originally Posted by Iamkar33m View Post
I halve the number my Garmin gives me (which is usually close enough to the accurate calorie count).

EDIT: I use the Garmin Edge 305
Problem with this approach is I'm not sure the Garmin is consistently wrong. Today 1625 calories an hour on easy flat ride.
This weekend averaging around 1400 calories an hour going up 8-10% grades busting it.

Depending on wind, grade, aero position, it just seems so much throws it off, that it's not worthwhile.

Just a simple example today , riding along and droping from the hoods to the drops, my speed went up and watts stayed the same. So E on the PT clicked off at the same rate, but Calories on the Garmin started going faster.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.
merlinextraligh is offline  
Old 09-03-08, 06:43 PM
  #8  
Iamkar33m
Senior Member
 
Join Date: Jun 2008
Posts: 725
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by merlinextraligh View Post
Problem with this approach is I'm not sure the Garmin is consistently wrong. Today 1625 calories an hour on easy flat ride.
This weekend averaging around 1400 calories an hour going up 8-10% grades busting it.

Depending on wind, grade, aero position, it just seems so much throws it off, that it's not worthwhile.

Just a simple example today , riding along and droping from the hoods to the drops, my speed went up and watts stayed the same. So E on the PT clicked off at the same rate, but Calories on the Garmin started going faster.
Look at it this way... halving the number for the most part gets me closer to the right number or it under-estimates the calories burned (better than over-estimating).
Iamkar33m is offline  
Old 09-03-08, 06:51 PM
  #9  
wfrogge
Senior Member
 
Join Date: Feb 2007
Posts: 3,917
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Hate to break it to you but the Garmin calorie count isnt even close to accurate *it grossly overestimates*
wfrogge is offline  
Old 09-03-08, 06:54 PM
  #10  
aham23
grilled cheesus
 
aham23's Avatar
 
Join Date: Jun 2005
Location: 8675309
Posts: 6,951

Bikes: 2010 CAAD9 Custom, 06 Giant TCR C2 & 05 Specialized Hardrock Sport

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by Iamkar33m View Post
Look at it this way... halving the number for the most part gets me closer to the right number or it under-estimates the calories burned (better than over-estimating).
we have a winner. it always blows my mind that people trying to lose weight cant figure this out. they always want to do the opposite and over estimate burn / under estimate intake. makes them feel better about cheating on the diet i guess.

as someone mentioned my forerunner 305 does seem to be more accurate when calculating calories burnt for running, which just seems odd. wouldnt it use the same formula for both running and riding? for me it doesnt really matter. i have both the forerunner and edge 305. i never look at the calorie number for cycling on the edge. never. later.
__________________
aham23 is offline  
Old 09-03-08, 07:31 PM
  #11  
operator
cab horn
 
Join Date: Jun 2004
Location: Toronto
Posts: 28,349

Bikes: 1987 Bianchi Campione

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 31 Post(s)
Likes: 0
Liked 6 Times in 6 Posts
They are both worthless for "calories expended" anyways. So laugh all you want.
operator is offline  
Old 09-03-08, 08:12 PM
  #12  
Ritterview
Tandem Vincitur
 
Ritterview's Avatar
 
Join Date: Sep 2005
Location: Northern California
Posts: 3,317

Bikes: BMC Pro Machine SLC01, Specialized Globe, Burley Rock 'N Roll tandem, Calfee Dragonfly tandem.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Oh, if my body only looked like what my Garmin calorie expenditure says it should.
Ritterview is offline  
Old 09-03-08, 09:50 PM
  #13  
gregf83 
Senior Member
 
Join Date: Jun 2008
Location: Vancouver, BC
Posts: 8,754
Mentioned: 9 Post(s)
Tagged: 0 Thread(s)
Quoted: 903 Post(s)
Liked 23 Times in 16 Posts
Originally Posted by merlinextraligh View Post
And how do we know the PT is right and the Garmin wrong?
The PT measures actually energy expended in Kilojoules. 4.18 KJ = 1 calorie. However, given a cyclists efficiency in converting dietary calires to energy at the hub, the relationship is closer to 1 to 1.

Lab tests show the efficiency is between .18 and.24, which means that calories expended is actually between 1.05 and 1.15 x KJ.
No comment on the accuracy of the Garmin numbers but if human efficiency varies between 18 and 24% it isn't possible to estimate calories expended to within +/- 5% without doing further lab tests to actually determine calories consumed. Without the help of a sophisticated lab the PT accuracy will be more like +/- 15%.
gregf83 is offline  
Old 09-03-08, 09:55 PM
  #14  
furiousferret 
Senior Member
 
furiousferret's Avatar
 
Join Date: Jan 2006
Location: Redlands, CA
Posts: 5,823
Mentioned: 28 Post(s)
Tagged: 0 Thread(s)
Quoted: 633 Post(s)
Liked 47 Times in 27 Posts
Its pretty obvious its off but its a marker, and that's really all calories are. I use a forerunner and edge pretty much for all my training and while the big numbers are nice, I know that I can't pig out and make the difference, heh.
furiousferret is offline  
Old 09-04-08, 12:34 AM
  #15  
pista
Senior Member
 
Join Date: May 2008
Posts: 195
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Why do you guys bother with calorie calculators! The utlimate guide is, if you are overweight, whether you are losing weight. After a ride don't even think about rewarding yourself with a milkshake or something stupid like that. I went from 235 to 165 in one summer. And that's without a computer at all.
pista is offline  
Old 09-04-08, 06:35 AM
  #16  
umd
Banned
 
umd's Avatar
 
Join Date: Sep 2005
Location: Santa Barbara, CA
Posts: 28,387

Bikes: Specialized Tarmac SL2, Specialized Tarmac SL, Giant TCR Composite, Specialized StumpJumper Expert HT

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by pista View Post
Why do you guys bother with calorie calculators! The utlimate guide is, if you are overweight, whether you are losing weight. After a ride don't even think about rewarding yourself with a milkshake or something stupid like that. I went from 235 to 165 in one summer. And that's without a computer at all.
Hmm, well seeing as how I've never been close to 235 lbs because I pay attention to what I eat and how much I need, maybe you shouldn't be so critical of those that do care about such things...
umd is offline  
Old 09-04-08, 06:47 AM
  #17  
Lithuania
Oh The Huge Manatee
 
Lithuania's Avatar
 
Join Date: Feb 2006
Location: mabra
Posts: 4,528
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by pista View Post
Why do you guys bother with calorie calculators! The utlimate guide is, if you are overweight, whether you are losing weight. After a ride don't even think about rewarding yourself with a milkshake or something stupid like that. I went from 235 to 165 in one summer. And that's without a computer at all.
maybe because some people are trying to make sure they are eating enough.
Lithuania is offline  
Old 09-04-08, 06:50 AM
  #18  
Looneytuna
Every day is a good day!
 
Looneytuna's Avatar
 
Join Date: May 2005
Location: Brooklyn, NY
Posts: 428

Bikes: 2006 Specialized Tarmac, 2005 Seven Elium, 1989 Cannondale R600, 1973 Raleigh Supercourse, 2007 Electra Ratrod 3

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by Ritterview View Post
Oh, if my body only looked like what my Garmin calorie expenditure says it should.
+1 - if I burned as many calories as my 305 says, I would weight of disappeared a long time ago..
Looneytuna is offline  
Old 09-04-08, 07:03 AM
  #19  
merlinextraligh
pan y agua
Thread Starter
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,329

Bikes: Wilier Zero 7; Merlin Extralight; Co-Motion Robusta; Schwinn Paramount; Motobecane Phantom Cross; Cervelo P2; Motebecane Ti Fly 29er; Calfee Dragonfly Tandem

Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 873 Post(s)
Likes: 0
Liked 3 Times in 2 Posts
Originally Posted by gregf83 View Post
No comment on the accuracy of the Garmin numbers but if human efficiency varies between 18 and 24% it isn't possible to estimate calories expended to within +/- 5% without doing further lab tests to actually determine calories consumed. Without the help of a sophisticated lab the PT accuracy will be more like +/- 15%.

From the Saris Link in my first post:

In the Applied Exercise Science Laboratory we actually measure mechanically efficiency and calculate personal conversion factors that allow athletes to convert mechanical Kjoules to metabolic Kcals. This conversion ranges from about 1.05 Kcals per Kjoule for the most efficient athletes to about 1.15 for our least efficient athletes. It’s important to note, that a number of factors like training status, temperature, and biomechanics may change a person’s efficiency. When it comes to calculating energy expenditure, however, the potential errors introduced by changes in an athlete’s efficiency are small compared to the potential errors that may occur when heart rate is used to calculate energy expenditure. While there is no perfect way to calculate the total Kcals used during a ride, measuring power output is currently our best estimate in real world conditions.

Thus if you use 1.1, you'll be within .05 of the range of test results.

I'm guessing the .18 to .24 range might include noncyclists or untrained riders.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.
merlinextraligh is offline  
Old 09-04-08, 07:32 AM
  #20  
Kurt Erlenbach
Senior Member
 
Kurt Erlenbach's Avatar
 
Join Date: Jul 2007
Location: Space Coast, Florida
Posts: 2,465
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 20 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by umd View Post
Although yes, the garmin's calculations are laughably bad, one thing to keep in mind with it & most HRMs is that it is supposed to include your base calories as well. For example, my rides often include long descents, 15-20 minutes where I may not be pedaling at all. I've done no work but my body is still burning calories, and likely still at a high rate due to just having worked hard, and all the core muscle involvement. I'm not sure when doing work, the RMR is included in the efficiency assumption or not.
This point is extremely important and is usually overlooked in these discussions. I have always been unclear whether the basal metabolism rate is included in these calories-expended numbers. The average person burns 1500 - 2500 calories per day (based on sex, age, weight, etc.) just sitting around breathing and digesting and pumping blood. The calculation in the OP does not take that expenditure into account. I suspect that the actual calorie count (the number of calories used by your body from the time you step on the bike to the time you step off) is the basal rate plus the energy expended as calculated through the PT. That doesn't mean the Garmin is right, but I don't think it's as bad as protrayed.
Kurt Erlenbach is offline  
Old 09-04-08, 07:41 AM
  #21  
merlinextraligh
pan y agua
Thread Starter
 
merlinextraligh's Avatar
 
Join Date: Aug 2005
Location: Jacksonville
Posts: 30,329

Bikes: Wilier Zero 7; Merlin Extralight; Co-Motion Robusta; Schwinn Paramount; Motobecane Phantom Cross; Cervelo P2; Motebecane Ti Fly 29er; Calfee Dragonfly Tandem

Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 873 Post(s)
Likes: 0
Liked 3 Times in 2 Posts
Ok add in 100 calories for basal metabolic rate, and the Garmin was only off 745 calories an hour.

I didn't buy the thing to track calories, but its logic is so laughable, it's a disservice to people that aren't aware of how bad it is.

The thing has an HRM. If it used that data as something of a reality check, along with the speed, and elevation data, they certainly could get a lot closer than they currently do. With all the computer power its got , it has to be a matter of poor programming.

IMHO, Garmin has to know it's whacked, but if they overstate Calorie consumption and make a bunch of recreational cyclists fat dumb and happy, and therefore feeling good about their Garmins, they don't see a big problem.

And for clarity, I don't see this as a reason not to buy a Garmin, but just a note for the unfamiliar that they should take the calorie data with a huge grain of salt.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.
merlinextraligh is offline  
Old 09-04-08, 07:50 AM
  #22  
wanders
going roundy round
 
wanders's Avatar
 
Join Date: Dec 2005
Location: High Point, NC
Posts: 6,084
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
In 3 hours of riding, my 705 said I had burned 5000 calories. I thought "hell yeah" After reading this thread and other reviews of Garmins calorie calculations, I think of this about Garmins' calorie calc.-

"You're a crook. You're a cheat and a swindler! That's what you are! How can you do a thing like this?! Build up a little boy's hopes, and then smash all his dreams to pieces?! YOU'RE AN INHUMAN MONSTER!!"

I still love the 705.
wanders is offline  
Old 09-04-08, 08:13 AM
  #23  
Tequila Joe
Living the n+1
 
Tequila Joe's Avatar
 
Join Date: Mar 2005
Location: Off the back
Posts: 2,746

Bikes: 2019 RM Blizzard, 2013 SuperX, 2007 Litespeed Vortex, 1970 Falcon Olympic, 2008 RM Metropolis IGH, 2004 Specialized Enduro, 2006 Langster

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Trust your Garmin.... and replace those hard spent calories by treating yourself to a double burger with bacon, jalapeno jack cheddar, fries and a large chocolate milk shake.
Tequila Joe is offline  
Old 09-04-08, 08:17 AM
  #24  
umd
Banned
 
umd's Avatar
 
Join Date: Sep 2005
Location: Santa Barbara, CA
Posts: 28,387

Bikes: Specialized Tarmac SL2, Specialized Tarmac SL, Giant TCR Composite, Specialized StumpJumper Expert HT

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by Kerlenbach View Post
I suspect that the actual calorie count (the number of calories used by your body from the time you step on the bike to the time you step off) is the basal rate plus the energy expended as calculated through the PT. That doesn't mean the Garmin is right, but I don't think it's as bad as protrayed.
Oh I still think the Garmin isn't even close, but I think estimating off a Power Meter's KJ will be low unless the NP is close to the AP.

Originally Posted by merlinextraligh View Post
And for clarity, I don't see this as a reason not to buy a Garmin, but just a note for the unfamiliar that they should take the calorie data with a huge grain of salt.
I agree. I ignored the calorie data on the Garmin from day 1.
umd is offline  
Old 09-04-08, 08:20 AM
  #25  
brianappleby
Senior Member?
 
Join Date: Oct 2005
Location: Denver
Posts: 1,975

Bikes: orbea onix, Cervelo SLC, Specialzed Allez, Cervelo P3 Alu

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Nobody mentions that the efficiency numbers for metabolism are always calculated based on aerobic respiration. When you go anaerobic for any period of time you are being grossly inefficient.

I'm a physicist, but if I remember correctly from HS Biology class: the lactic acid cycle produces 4 ATP per glucose while the electron transport chain (using Oxygen) produces closer to 36. This makes us 800% more efficient during aerobic exercise than when we're anaerobic.

What I'm getting at: the KJ=Kcal conversion will underestimate calories if you ever go lactic in a ride.
brianappleby is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.