3.25 inches
So, a 2d nail is 1 inch long. More common nail sizes: a 4d nail measures 1.5 inches, a 6d nail is 2 inches long, an 8d nail is 2.5 inches long, a 12d nail is 3.25 inches long, and 16d nails are 3.5 inches long. Every increase or decrease in number means the length change in 1/4 inch.