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Penny size is bullshit. Penny size applied to different nail types means a different length! Read this - [1]
Building code defines a 10D nail as a .148 nail, 3" long [2]
1" is 2D [3]. Note that penny size came historically from cost of the nails in pennies. So the length is not proportial to D size. [4]. The only thing we know is that the larger the D number, the larger the nail.
10D is 3" long - [5]
Joist hanger nails do not seem to have D size attached to them - this one is 1.5" [6]
This 8D is 1.5" long [7]. Is that an error? Chart says 8D is 2.5".
3.5" common nail, galvanized. 16D [8]

Penny Size of Nails