Nail Size Converter

Nail sizes are measured in “penny” (d), an old English system.

The “d” in nail sizes stands for “denarius” (a Roman coin). Originally, it was the cost in pennies for 100 nails.

Common nail sizes:

The practical catch with the penny number is what it does and doesn’t tell you. It sets the length only, not the thickness. An 8d nail is always about 2.5 inches long, yet its gauge (diameter) varies between common, box, and finish styles, which are made thicker or thinner for different jobs.

So “8d” tells you how deep the nail reaches but not how much it will split the wood or how well it will hold. Common nails are thick and strong for framing; box nails are thinner to reduce splitting in lighter stock; finish nails are thin with a tiny head meant to be hidden in trim. Choosing the right type matters as much as the length, which is why a framing nailer and a finish nailer take completely different nails even at the same penny size.

Diameter by type, in inches. Wire gauge runs backwards, so a smaller gauge number is a thicker nail. Building codes specify nails by diameter and length rather than by penny size for exactly this reason.

The two numbers worth committing to memory are 8d common = 0.131 inch by 2.5 inches and 16d common = 0.162 by 3.5, because those are the nails structural tables are written around. A shear wall specified for 8d commons at 4 inches on centre does not get the same performance from 8d box nails, which are 14% thinner and carry noticeably less lateral load.

Most framing today is not done with true 16d commons. Coil and strip nailers are loaded with sinkers, which are shorter and thinner than the nominal size: a “16d sinker” is typically 0.148 by 3.25 inches rather than 0.162 by 3.5, with a thin coating to ease driving. That is close enough for most prescriptive framing, and it is why a gun-nailed wall and a hand-nailed wall are not quite the same wall. If a drawing calls for 16d common, check what is actually in the gun.

How deep should it go? For thin stock, the rule of thumb is that the nail should be about three times the thickness of the board you are fastening, so a 3/4-inch board wants a 2 to 2.5-inch nail, which is a 6d or an 8d. That leaves roughly two thirds of the nail in the piece underneath.

Do not extend that ratio to framing. Three times 1.5 inches would be a 4.5-inch nail, and nobody frames with 30d. Structural connections are governed by the code nailing schedule, not by a ratio: a 16d through a 1.5-inch plate leaves 2 inches in the stud, and that is the standard, approved, universally used connection despite being under two thirds. The practical floor is that the nail should reach into the second member by comfortably more than the thickness of what it is holding.

Splitting is a diameter problem, not a length problem. A thick nail near the end of a board levers the fibres apart. Three fixes, in order of how often they work: move the nail further from the end and the edge, switch to a box nail of the same penny size, or blunt the point with a couple of taps so it crushes its way through rather than wedging. Blunting a nail sounds like folklore and genuinely works, which is why carpenters have done it for two hundred years.