Calculating excess calories burned exercising

rapat

Maybe I'm going a bit too deep into this, but I really just want to get a more acurate sense of how exercise affects my calorie surplus/deficence

I got tested with the BodyGem, which estimated my RMR at 1,880 -- which I believe means the number of calories I'd burn if I just slept or laid in bed for 24 hours without doing anything and without eating.

Then I multiple by that by 1.2 to get an estimate for the calories I burn based on my lifestyle (no/light activity and a desk job):
So I get:
2,256 calories

I've bought a HR monitor, which is providing me with calorie estimates for cardio exercises.
It says I burned 400 calories in 30 minutes of running/walking (couch to 5k)

Now I want to know how much extra calories I burned.

Do I take 2,256, divide by 48 (# of half hours in a day) = 47.
And subract that from the HR monitor estimate -- 400 - 47 = 353 additional calories?

Am I missing any steps? Using the wrong numbers? Anything I can add to make the number more accurate -- 2,256 / 48 might not be quite as accurate since the running is specifically replacing the calories burned sitting on the sofa.

now_or_never12

What heart rate monitor do you have? Is it just a watch one? DOes it have the chest strap? Do you have to input the height, gender, weight, sex?

rapat

Polar FT7 -- Watch with chest strap. Yes I had to input all that --
But from what I've read, the heart rate monitors in general don't factor in calories you would've burned sitting at a desk?
Or is that wrong -- do they show the extra calories?

EnchantedEvening

I always subtract my BMR from my HRM reading.

For the sake of easy math: if my BMR for one hour is 50 calories, then I subtract 50 calories from the HRM readout if I worked out for an hour. If I worked out for a half-hour, I subtract 25. I do that because I would have burned it even if I'd been sitting on the couch.

Also, multiple lab studies have shown that HRMs are usually off by 9-13% when compared to the actual calorie burn (using all the lab equipment to measure oxygen, heart rate, etc), so I subtract 20% from my HRM just to be safe.