Wondering how accurate these calorie counts are...

Bashsmom

I am just wondering how accurate the "calories burned" numbers are on MFP. Some of the time it seems like you input an amount of time and it gives you a "calories burned" number. Doesn't the intensity of the exercies make a difference? For instance, I put in that I was on the elliptical trainer for 30 minutes. I could be on the lowest or highest setting but I am getting the same number of calories burned?