Accurate way to track calorie burn aside from HRM?

Hi Everyone,

I need to pick everyone's brains. I have a Polar F4 that I absolutely love. But I'm not going to lie....sometimes I get real lazy with going through the motions of putting it on every stinkin' time I want to exert myself a little. Having to wet the sensors....put it on....put on the wristwatch.....then afterward take it all apart, rinse the sensors......sometimes I just want to work out. Period. But I cannot believe how grossly OFF some of the calorie burnage estimations are on MFP. I'll wear my HRM and work out, then go log it in MFP - had I not used the HRM, it would have estimated that I burned WAY fewer calories than I actually did!!! So my question is - is there ANY way, using ANY online calculator or otherwise, that I can quickly get a much better estimation of the calories my body burns without having to wear my HRM every time?

Thanks for your help all!!
Lisa