I haven't attempted to measure the diameter of my penis with a caliper and don't intend to try, but I can give you some info regarding shape and taper, if you are that interested.
I have always been rather tree-trunk shaped with a mild taper from distal shaft to near the base, with greatest girth right at the base. Looking down on the dorsal aspect from above I know I am very slightly greater than 2" in diameter right at the base. If I was perfectly cylindrical, this would correlate well with my maximal girth of just over 6 1/4" at the base (probably around 6 3/8" or just over 16 cm). The minimal diameter of the shaft is right around 1 3/4" right below the glans.
At the glans I am just slightly under 5 1/2" (13.5 cm) in diameter measuring the maximal circumference of the glans in a plane perpendicular to the shaft. If I attempt to measure around the perimeter of the corona at it's point of greatest girth I am a bit over 5.5".
Through the shaft I am around 5 5/8" (just over 14 cm) just below the glans, about 5 3/4" at mid-shaft (14-15 cm) and about 16 cm at the base as stated above.
But looking down from above, the diameter of the shaft looks pretty uniform (about 1 3/4") except for a bit of outward flare right at the base. Looking from the side in a mirror, however, I can see that the corpus spongiosum gets a bit thicker as one goes from tip toward base, and this is what is responsible for the increase in girth.
So measuring the diameter of my penis in a horizontal plane would not really give an accurate indication of girth, as was pointed out by Paige, who appears to know a thing or two about penises.