What is a 16d nail? Why is it called a "penny"? Complete guide to nail sizing.
The penny system (abbreviated as "d") is the standard way to measure and name nails in construction. The letter "d" comes from an old British monetary unit — historically, nails were priced by the penny, and the name stuck.
When you see a label like "16d nail", that number directly correlates to the nail's length. But it's not as simple as "one penny = one nail length." Let's decode it.
| Penny Size | Length | Diameter | Common Uses |
|---|---|---|---|
| 2d | 1" | 0.072" | Trim, finish work, paneling |
| 4d | 1½" | 0.080" | Light framing, molding |
| 6d | 2" | 0.099" | General framing, sheathing |
| 8d | 2½" | 0.131" | Joist hangers, general construction |
| 10d | 3" | 0.148" | Framing lumber, deck construction |
| 16d | 3½" | 0.165" | Heavy framing, structural |
| 20d | 4" | 0.192" | Structural framing, beams |
While the penny system doesn't follow a perfectly linear formula, there's a loose pattern:
But this breaks down for very small and very large sizes. The relationship is historical, not mathematical.
As nails get longer, they also get thicker (larger diameter). This makes sense from an engineering perspective:
The penny system applies to several types of nails:
Thicker shaft, large head. Used for framing, structural work, and general construction. These are what most people think of as a "nail."
Same lengths as common nails but with a slightly thinner shaft. Used when you're trying to minimize wood splitting or need lighter fastening.
Tiny head, thinner shaft. Not measured in the penny system — usually labeled by gauge and length instead (like "1½\" 15-gauge finishing nail").
Special grooves help them grip better. Still measured in penny system. Used where holding power is critical.
The same penny size can refer to different nail types:
Always specify which type you need when buying, or the hardware store might give you the wrong one.
The penny system is traditional and standardized across North America. Modern alternatives (like metric sizing) exist but haven't replaced it in construction. Contractors and builders know what a "16d nail" means, so the system persists.