A heart rate monitor can be great for managing your workout and making sure that you stay in the zone of activity best suited for weight loss (or maintenance, endurance, strength, etc; whatever your goal is).
Some of them will tell calories but this can be a very subjective figure, much the same as the treadmill display at your local health club. Any device that tells you calories burned will (hopefully) be more accurate if it allows you to input certain body factors such as age and weight, and this goes for treadmills, life-steppers, or an HRM you attach to yourself.
The best way to tell "actual calories" is to first get your resting daily caloric usage tested. I had mine done some years ago at a health club where I used to have a membership. They use a device that measures how much CO2 your body produces while at rest (and having been at rest for the preceding 24 hours or so). What will likely surprise you the most is how LOW that number will actually be, even if you are in pretty good shape. Mine was only 2200 without working out. That's two slices of pizza, and I'd have to scrape the cheese off of one of them!
Working out would raise that number in two ways (actually there are more, but this is already becoming a dissertation).
1) Direct calorie expenditure of an exercise (for example, a 45 minute treadmill run that shows 600 calories on the display, assuming that the machine is anywhere near accurate).
2) Continued raised metabolism from exercising. That 600 calorie run might result in an additional 100 or 200 calories to be burned over the next several days as my body recovers.
There are lots of HRMs available, and I think that there's no "perfect" one yet. A few years ago I bought a Polar HRM and I use it when I go walking, running, and even weight lifting. It's both fabulous and frustrating at the same time.
I think my HRM is fairly accurate, but who's to say? Yes, it knows my weight and my birth date and it can tell how fast my heart is beating, but it's not measuring my CO2 production during an exercise and it's not measuring my blood sugar levels. I trust my HRM, knowing that it's only as accurate as it can be given the inputs that it is receiving. The same thing will apply to any fitness machine with a built-in calorie display.
But even so, here's how I can still use that information:
If I run on the same brand treadmill at my health club 3-4 days per week and I perform at roughly the same levels each day, then they all "should" produce consistent results. Whether or not this is an accurate number is still doubtful, but there is (ostensibly) some consistency. So if I run for 45 minutes one day and per the display I burn only 500 calories, but I know that 600 is my norm, then maybe there's an explanation for that. Did I run it slower? Did I run it flatter (no incline)? Could one of the machines be faulty?
I decided some time ago NOT to get myself into a nit-picking mental mode where I'm worried about the accuracy of my recording device for a given workout. Kitchen-table math tells me it's not worth the time or trouble!
Let's take my 2200 resting daily intake needs as a baseline. I trust that figure, within some small margin of error unknown to me.
Let's add to that my 600 calorie intake needed to fuel an "average" run on a treadmill or outdoors.
Let's add to that 100 calories (a guess) intake needed to fuel my body's fitness recovery requirements and because I climb the stairs instead of riding the elevator.
Ignoring other factors for a moment (sleep, illness, metabolism, increased muscle mass from weight workouts, etc), I've got to take in approximately 2900 calories per day. Less and I will likely lose weight, more and I will likely gain weight).
Now let's assume that the treadmill I used on Tuesday told me a lie, and I only burned 550 calories instead of the 600 it seduced me into thinking I burned. 50 / 600 = .08 right? That's an 8% rate of error for that machine (which of course is unknown to me). Okay, maybe significant. But here's why I decided not to worry about it:
Actually burning 2850 calories instead of the 2900 I "thought" I burned, would be a 1.7% rate of error. Not enough for me to get fat from!
Accuracy is important, yes. Our consumer devices are "mostly" accurate, I'm sure. But not razor sharp accurate. And in the big scheme of things, I very much doubt that it matters. I'm still going to have a glass of wine when I take my lady friend out on Saturday night. And if she wants desert, you can bet we're going to get desert!
Best of luck on your weight loss goal!