1" of diameter for every 5 feet of boat. You need 4" minimum.
I'm curious- where did you find that formula? I'd think it depends most on where you tie up. That formula might work okay for a lake boat, but I use 6-inchers on my 21' Pro-Line. I have to contend with lots of wind and wave action at the bulkhead, and my boat is not a lightweight.
I may be buying a 24' Chris Craft this Spring. That boat is heavy- over 5000 lbs dry. I'll keep 6-inchers aboard for raft-ups and transient docking, but "home port" will be set up with 8 inchers.
Jolin,
you ask where the formula came from for fenders.
here it is............
http://www.boattest.com/Resources/view_news.aspx?NewsID=2872
this is capt.Steve from boattest.com scroll down to lesson #19 title requirements,supplies,lines and fenders.
lot of good info in short videos.
woosterken