Recording activity from work

Here's a question I've been trying to figure out. In the course of my job I spend a large proportion of my workday lifting bags of varying weights (1 lb to 50 lbs) and pushing cages of heavy bags. I'd like to be able to record an exercise value for this activity to get a better sense of calories expended, but I'm at a loss how to arrive at a decent figure. To this point I've not recorded anything. Any thoughts on how to arrive at an appropriate value? Or is it too difficult to track?