I use a heart rate monitor as well and have found it to be much more on the money than machines or even the sites that have calculators on them
Mine uses body weight, resting heart rate, height and sex as well as my actual heart rate. Its good because it takes into account my increasing fitness (lowering weight AND decreasing resting heart rate) - the fitter you are the harder you have to work to burn those pesky calories lol. Your body becomes more efficient which is why sites generally aren't all that accurate or machines in the gym.
I am in the market to buy a new one for myself cause the above one is my partner's and he is getting back into running and wants it back lol. Damn him :P
but if you are after a site try www.calorieking.com.au
and use their tools. Its alright for a general number out.