Here is the formula for you, from the Newbie FAQ.

How do you calculate total protection on a unit with natural protection (thick skin) and armor? According to Johan O.:
"Pb = base protection, intrinsic, from stone skin etc.
Pa = protection from armor
Pt = total protection
Pb and Pa are always restricted to 40 or less. (This quote has been changed because the forum Boards cannot handle greater than and less than symbols - Cherry)

Pt=Pb+Pa -(Pa*Pb)/40
So a unit with 10 intrinsic 10 protection and a total of 18 in equipment armor would have an effective armor of: 10+18-(10*18)/40 = 28-4.5 = 23.5, rounded up to 24"