Determining calories burned?

YorriaRaine

I am just starting out on myfitnesspal and I was wondering where are good calculators for estimating calories burned?

Currently my primary cardio exercise is doing jumping jacks on a fitness trampoline (I do it on a trampoline because of a bad knee). I'm not sure how doing it on a trampoline would effect the amount of calories vs. doing it on a floor.

I was also mildly wondering if maybe I should pick up the polar ft4 to help me estimate my calories. Would like some opinions.