Usually it's the other way round for me..... Both methods have thier inaccuracies so I go with a calories per mile formula.

25 calories per mile for recovery rides
30 calories per mile for normal pace
35 calories per mile for all out time trial effort


This is based off of using a power meter in the past and talking with folks that use them now who are my size and weight. For an example, I rode 103 miles this past Saturday and it calculated out as 3090 calories.

I have wondered if my HRM is accurate in its calorie count. I always read that HRMs are the best way to measure calorie output because formulas, online calculations, and averages do not always take in to account the size of the person or their fitness level. So I did some brief research....and I came across this website below.

Basically the more information you can feed in to a model/calculation the better it will predict. So if you have VO2 max and HR you will get the most accurate result. But who has access (or money) to VO2 max lab tests all the time?? And unless you are a serious athlete I don't know that you really need to know your VO2 max, other than it would be cool to know. I did find some equations online to measure VO2 max using some formulas and your heart rate, but who knows how accurate they are? If anyone has done this calculation it would be interesting to hear about it. Especially if you have had your VO2 max tested in a lab and done the self field test.

So, If you have only HR it will be more reliable than using a general calculator. You also have to take in to account your max heart rate. There is an equation for this (220-age), but depending on individual differences, your actual heart rate may be different. Maybe the "best" thing to do is to find your max heart rate and use that in your monitor and then use the results from your Garmin.

Although the website points out that, on average, the HRMs underestimated calorie burn I sometimes feel like my HRM overestimates calorie burn. And remember it is just that, an estimate, be it a pretty darn good one. I do know that when I ride hard I am ravenous, and can usually tell that I burned 1,000-2,000 calories.

The calories per mile is interesting. Based on a typical 40 mile ride I would burn about 1200 calories, but I know that I need to eat a lot more than that or I feel loopy and out of it. I am seriously hungry after my rides. For me, I don't think that is the best estimate...but if it works for you that's great!

And most of this journey, and life, is about finding what works for YOU! :-)

http://journals.lww.com/nsca-jscr/Abstract/2009/08000/Validation_of_Heart_Rate_Monitor_Based_Predictions.18.aspx

I've found the MFP figure agrees pretty closely with the Runtastic app I use on my phone which takes account of speed, distance and weight. I don't bother with the HRM for cycling as I'm commuting but I do use it for running and the running figures tally also.