calories burned?

I need some advice on how to count calories burned... I do a lot of swiming, mainly breast stroke. A while ago I injured the muscle in my shoulder. Rather than giving up swiming for a while, I now use a kick board and "swim" only with my legs. Great leg work out! I wonder if anyone knows how to count calories burned. Do I burn about the same considering I work out my shoulders/arms less (nothing) but my legs more or do I burn less because I don't use my arms and shoulders.
I appreciate your advice.