How accurate are HRM

It will be most accurate for steady state cardio, such as running. For 30 day shred, it's probably fairly accurate. I think around 200 calories would be right for me, but I'm much heavier than you. I'd probably reduce those calories if you're only 116 lbs. Of course, I'm also old, so if you're young, that might negate the lower weight.

Yea 200 is a pretty good ballpark number for a 25 minute workout. To be safe only eat back like 50-60% of the cals burned until you get a good gauge of your weight loss. If the weight starts coming off to fast then you can start eating back more. And vice versa.

This MFP blog post from five years ago is still pretty much spot on. Unless you have a high-end HRM that uses Firstbeat's technology to analyze the gaps between individual heartbeats, the equations that they use are accurate only for steady-state cardio, and even there, there's a substantial amount of variance that heart rate, weight, gender, and age don't account for.

You can use your HRM to get a ballpark estimate, especially if your activity level is fairly constant, but keep in mind that it's only an estimate, and that you should compare results against your estimates to see how useful it really is.

Why are the algorithms so different when I use my Digifit app and the Polar app? An hour of cardio on Digifit is over 1100 cal and a similar hour using Polar is only around 500. I put the same metrics in (weight, height) and I’m using the same chest strap so the difference has to be in their algorithms. When I’m on the stationary bike or the AMT the machine’s HR monitor when you hold the handles says almost the same as either app so it has to be how they figure out calories burned.

How do I find out which is most accurate representation of my calories burned?

HRMs are for tracking steady state cardio only. It is important to note that HRMs are not accurate for weight lifting and will not give you an accurate burn. They are also not meant for HIIT. Temperature extremes and daily burn tracking (e.g. I wear it all day to find out what I should be taking in) are also inaccurate. HRMs are for steady state aerobic exercise only!

This is a helpful blog post for understanding HRMs:

http://www.myfitnesspal.com/blog/Azdak/view/the-real-facts-about-hrms-and-calories-what-you-need-to-know-before-purchasing-an-hrm-or-using-one-21472

HRMs are not 100% accurate even when they are used for their intended purpose (steady state cardio). They still have a margin of error. That being said, when HRMs are used for purposes they are not intended for they can become grossly inaccurate. It really isn't much better than pulling a number out of nowhere if the device is being used for things it was never designed to handle.

I personally have both a Bodymedia Fit (for daily activity tracking) and a HRM (for my steady state cardio). I love them both! However, it is important to know what these devices are actually designed to track. They are very helpful when they are used correctly!