Strength Training.

That's a real tough one to answer. There are so many factors to consider with strength training, one of the biggest is time between sets.

I've heard that the BodyBugg is one of the better monitors as it takes into consideration body temperature. It has a monthly cost to use, which I think sucks and is why I don't have one. But all my strength training is done in circuits, so the HRM works well there - rest breaks are between 10 - 60 seconds.

Here are reasons why it's so hard to calculate calories burned while lifting weights:

1. Weight and reps can differ.
2. Time between sets can differ.
3. Muscle groups being used can differ.

If person A spends 20 minutes doing squats and is doing maximum weight and minimum reps (lets say average 400 lbs at 3 reps), and takes 3 minutes rest breaks between sets. Person B spends 30 minutes doing bicep curls using moderate weight and moderate reps (lets say average 30 lbs at 10 reps) and takes 60 seconds rests.

Have they burned the same number of calories? No way. Has their heart rate gone up about the same? In terms of heart rate, it would be close.

So both people have spent 20 minutes lifting weights, but there is no way they have burned the same number of calories (I'd bet person A burned more because of the much heavier weight and the muscle group being hit).

The bottom line, you will be hard pressed to get an accurate figure. The best you can do is guess. Like sradis, said, you can add strength training as a cardio exercise and use the numbers they give. That's better than nothing.