Basal metabolic rate calculations - why do they vary so much from site to site?

[swedemonkey]

Been working out and exercising, hit a slow patch where I wasn't loosing weight so I did some internet searching to get myself thinking differently and keep up the motivation. Found some sites which calculate your basal metabolic rate(BMR).

 

The problem is these sites give very different BMRs. I entered the same info at each site, and got BMRs ranging from 1800 to 3100.

 

My weight loss program is doing ok, I just now want to track calories much more closely, and couple this with BMR and exercise. Of course, I'll get very different results if I use a BMR of 1800 vs. a BMR of 3100. (I'll have another Big Mac with that milk shake please, got to keep the furnace burning )

 

Does anyone have an accurate formula for BMR, or a site that will give you an accurate BMR?

 

thanks,

[honey2005]

Most of the sites I look at give you ranges, so I don't know the exact.