I am working on a project and I am having difficulty understanding a concept I have to analyze a rectangular fin in 2 cases (Adiabatic tip AND Convective tip) and I am having difficulty understanding which surface area to use. For the first case, I want to find Afin which according to my book is: Afin = 2wL + wt -> why does it not include the lateral sides (2*L*t)? It's just the top, bottom and front surface. Schematic: http://imgur.com/YgfHmUw Same thing with the corrected length Lc. The area with the corrected length is Afin=2wLc -> but this now only includes the top and bottom surface (i get that I don't include the tip because it's adiabatic) but still no lateral sides. My professor taught me this formula: Afin = P*L where P is perimeter. This formula includes all 4 sides and then we can add the tip cross section if needed. Someone please explain the difference.