Using exercise machines

Really depends on the machine. I find that MFP does a fantastic job at getting the cals pretty close. I use a HRM and even when i log with that the cals that MFP gives is pretty close. So if you dont have a HRM, use MFP's calculations and you wont go wrong.

My main experience with devices that "do" your calories is the Precor elliptical trainer (the EFX 576i). As you say, it asks you (or actually, gives you the option of telling it) your age and weight. I hit the thing pretty hard for 30 minutes on an "interval" program, after which it credits me with calories in the low 500s, and a simulated distance of about 3.4 miles. If I tell MFP that I've done the elliptical trainer for 30 minutes, it assigns me 541 calories ... pretty close. Also, I go to spin classes three mornings per week, and we just got some new bikes ("FreeMotion") which have an electronics box that keeps some statistics, including power in watts and total calories used. This bike has a USB port, and if you plug in a thumb drive, it writes a CSV file, one record per second, in which it shows crank speed (RPM), power (watts), and some other things. Just for fun, I opened one such ride as a spreadsheet and integrated (power)·(time) for the ride, to compare the total work done on the bike to the total nutritional calories the bike said that I had used. Interestingly, the nutritional calories it gave me credit for were equal to nearly five times the integrated mechanical work I'd done on the bike! Apparently, the bike assumes that we, as heat engines, are only about 20% efficient ... which is probably about right; most of the calories we use in exercise goes into heating up our bodies, heating up the equipment, evaporating sweat, and making noise.

Anyway, to get back to your question: I think the machines that I've used are pretty credible on calories expended. If I were you, I'd just log what the machine said and forget about it.

I see what you mean. Does the elliptical that you use have heart-rate sensors (usually metal plates in the handles)?

From what I've read, the only way to be sure is to wear a heart-rate monitor, and I don't own one of those. Calories burned has to be a strong function of how you use the device -- there's a lady who's usually on the machine a few down the line from me, and she's usually on there for an hour, and kind of goes slowly, reading the paper, and is never sweaty or breathing hard. Yet, if she told MFP she'd done 60 minutes on the elliptical, I suppose it would credit her with some vast number of calories. Outside of wearing a HRM and logging accordingly, I'm not sure what the best answer is.