heat transfer coefficient reference in fix heat/gran

Dear all,
In fix heat/gran description has mentioned that:
h_c,i-j = 4 * k_pi * k_pj / (k_pi + k_pj ) * sqrt (A_contact,i-j)
However, based on the references, it seems that h_c,i-j related to the radius of contact circle. Thus, I think the formula should be like the following:
h_c,i-j = 4 * k_pi * k_pj / (k_pi + k_pj ) * sqrt (A_contact,i-j / pi )
Can you kindly tell me that I am right or is there another explanation?
Best regards,
Mohammad