I tried some online calculators and they state that I burn an average of 100 calories per hour sitting at a desk.

I tried some online calculators and they state that I burn roughly 100 calories for each mile I traverse when traveling at speeds between 3mph to 6mph.  I couldn't find a pedometer that would let me calculate calories burned traveling at 1mph or slower.  But if I extrapolate the information gained from the 100 calories per mile, then traveling at 1mph would mean I burn 100 calories in 1 hour.

But that doesn't make sense to me intuitively because it would suggest that sitting and reading burns the same amount of calories as walking at 1mph?  How do you calculate calories burned traveling at speeds less than 1mph?