Sorry, been in the shop making sawdust. I think the "I", moment of intertia, for bending calculations is the second moment. The formula for a simple rectangular cross section is 1/12 bh3 - 1/12th of the thickness times the height cubed. That is why the height of a joist makes so much difference (as has been mentioned). My degree is in mechanical engineering. I don't really do any design work but I remember how to use the equations (but I have to look them up).