I've added a close up of me measuring the width of my penis. It clocks in at 2" or 5cm. Is that wide? I would do circumference but I don't have a tape measure, only a flexi ruler.
Take a strip of paper, wrap it around your dick when hard, matching end to end. Mark it with a pencil. Take it off and measure it with a straight ruler, then you'll have the circum.
I doubt you're able to accurately estimate the diameter by eyeballing it, but if you did accurately peg it at 2 inches, then the circumference is simply diameter x pi (3.14), or 6.28 inches for your estimated 2 inch diameter.
Usually it's wider than it is deep, though how much difference there is varies. As well, it's very hard to get an accurate measure of diameter with a ruler, because of parallax. The paper strip method for finding circumference gives a more accurate measure of thickness.