HR montiors vs. activity estimate

I have a Garmin HRM (which is great when I go hiking as it gives me elevation gained, etc. too!) I entered my sex, weight, and max HR. When I take a spin class and work my butt off, my HRM says I burned about 1000 calories and the activity estimator says about 500--That is a HUGE difference. There is a difference on the hiking I do too. I have asked my Physical Therapist (who has a doctorate degree in whatever they have to have.....) and he says that it should be an equation in the HRM that takes into consideration your weight AND your heart rate and that the HRM should be more accurate than using the info off of the bike/ellipitical/threadmill or the "charts"/activity estimator.

So--do you really think I burned that many calories?!

depends which garmin you have.

yep!  I do.  One of my spin instructors who's been teachin for over a decade said his hour long class was about 1000 cal...

Granted, you obviously weren't in the same exact class but I do believe it.

I have the Garmin 305. The spin studio has the Kaiser machines with rpm, watts and calories, heart rate (need a polar HRM), distance, and a timer. (I LOVE these machines!) I don't trust the calories because I don't use the polar HRM so I don't think it is accurate for me (~300cal). Though I think I am in pretty good shape and pretty strong (I am 5'2" and 48yo.) compared with a REAL cyclist I am a wimp! I follow the teacher's instructions on HR and cadence and adjust my gear accordingly. (These bikes actually have "gears" 1-24). At the end of a class, the bike displays your ave. watts. (Watts are the same for everyone in that it is power--an equation of gear level and rpm. Simplifying the info... the stronger you are the more gear you can add, the stronger your heart the more gear you can add. The more rpm's the harder your heart has too work.... therefore, if you are really strong and have a strong heart, you can add lots of gear and peddle fast and your HR isn't going to be as high as mine. Or, if Lance and I were in the same class on these bikes, with an rpm of 100 and gear of 5, my HR would be 140bpm and his would be 65bpm but we would have the same watts.) The higher the gear and rpm the higher the watts. Lance Armstrong would probably be at gear 24 rpm 110 and 400 watts!) I KNOW I have been working hard (yesterday was mostly 80 and 85% MHR with a 10 min. w/u and cool down). However, because I am still not super strong, my ave. watts was around 100, which according to the activity estimator would be considered "light" and about 400 cal. I considered it medium to heavy or closer to 800 cal. My HRM said 1002.

Any thoughts? The more I learn, the more confused I feel! Thanks.

I just started spinning with a HRM and I'm getting much different results than most of you.  I'm 32/F/117lbs/5'3" and at and keeping my HR at an average of 88% it is saying I'm burning between 450-500cals. 

I do have to say that using a HRM is the best thing I could have ever done.  You don't realize how quickly your rate will drop and with the HRM you know what you have to do to keep it up.  Also, how much harder I have to work!

Next up, using it in Bikram yoga!