Thread Subject: calculate the bit rate of jpeg comression

hi, everybody
    I use matlab command imwrite to convert the bmp into jpeg. In this
command, I can set the quality factor(0-100). In many papers, they
often use bit rate with the unit of 'bpp' to indicate the quality of
compressed images. But I don't know how to compute it. Does it equal
to the result of the total compressed file size divided by the number
of pixels of original image? Or is there other methods?
   Thank you.