Heart Rate Monitor

I have just bought my first heart rate monitor that also calculates calories burned.
Need some advice and this may be a really dumb question, but I have to ask.
When I use the calculators on different websites, I have to enter the activity, duration AND my WEIGHT to get an estimate.
Now, my monitor has a stopwatch to time the activity and even without using a chest strap it seems to have the heart rate fairly accurately.
I have updated it with my age and sex, but it doesn't give me the oportunity to update it with my weight.
Is it still going to give an accurate measure of calories burnt without the weight info?
I'm sure as I get lighter and fitter, my heart rate will slow, so I'm assuming the heart rate itself is a good guide for caloric consumption, but does anyone know from experience?