What the penny system means
Nails in the United States are still sold by a size called the penny, written with a lowercase d, as in 8d or 16d. The d is an abbreviation for denarius, an old Roman coin, and the exact origin of the naming is debated. Some explanations relate to the price per hundred nails in medieval England, but the history is uncertain, so treat any single explanation with caution.
What matters in practice is simpler. A higher penny number means a longer nail, and usually a thicker one. The penny number is a label, and it does not tell you the price today.
Common length chart
These are the standard lengths for the common sizes. Nails sold with the same penny number should match these lengths, though diameter varies by nail type.
| Penny size | Length |
|---|---|
| 2d | 1 inch |
| 3d | 1-1/4 inches |
| 4d | 1-1/2 inches |
| 5d | 1-3/4 inches |
| 6d | 2 inches |
| 7d | 2-1/4 inches |
| 8d | 2-1/2 inches |
| 9d | 2-3/4 inches |
| 10d | 3 inches |
| 12d | 3-1/4 inches |
| 16d | 3-1/2 inches |
| 20d | 4 inches |
| 30d | 4-1/2 inches |
| 40d | 5 inches |
| 50d | 5-1/2 inches |
| 60d | 6 inches |
Very small nails, such as brads, are usually sold by length in inches and by wire gauge rather than by penny size. Air-driven fasteners are also labeled by length and gauge, which you will see if you use a pneumatic nailer.
Nail types matter as much as size
Two nails of the same length can behave very differently.
- Common nails have a thick shank and a wide, flat head. They are strong and typical in framing.
- Box nails are thinner than common nails of the same length. They are less likely to split thin boards, with somewhat less holding strength.
- Sinker nails are slim, often with a coating, and their heads sit slightly below the surface when driven. They are popular for framing with power nailers.
- Finish nails have small heads that can be set below the surface and filled, for trim and moldings.
- Casing nails are slightly thicker finish nails with a modest head, often used on door and window casings.
- Roofing nails have a wide head and are usually galvanized to resist rust.
- Ring-shank and spiral-shank nails have ridges that resist pulling out better than a smooth shank.
Diameters differ for the same penny size across these types, so compare packaging if strength matters.
How to pick length for the job
A common rule of thumb is that a nail should be about three times as long as the thickness of the piece it passes through, so that most of the nail sits in the piece it is being driven into. Another version says at least about two-thirds of the nail should end up in the receiving piece. These are rough guides, not standards. For structural work, follow the nailing schedule in your plans or local building code.
A worked example, with hypothetical numbers: you want to nail a board that is 3/4 inch thick to a framing member. Two-thirds of the nail should end up in the framing member. If the nail is 2-1/2 inches long (an 8d), then 3/4 inch passes through the board, leaving 1-3/4 inches in the framing member. That is 1-3/4 divided by 2-1/2, which is 0.7, or about 70 percent in the receiving piece. That meets the two-thirds guideline. A shorter 6d nail, at 2 inches, would leave 1-1/4 inches in the framing, which is 62.5 percent. That is a little under the guideline, so the 8d is the better pick.
A nail also should not be so long that it pokes out the back. Check what is on the other side, such as wiring, plumbing or a finished surface. Pointing the nail at an angle, known as toenailing, lets a nail join two pieces meeting at a right angle, but the nail needs to be long enough to reach through and set firmly.
Diameter and splitting
Thick nails hold well but can split wood, especially near the end of a board or in dry hardwood. If splitting is a problem, drill a small pilot hole, use a thinner nail such as a box or finish nail, or move the nail away from the edge. Some people blunt the tip slightly, because a blunted point crushes fibers instead of wedging them apart, though sharp points drive more easily in softwood.
Coatings and material
Bare steel nails rust in damp conditions and can stain wood. Galvanized nails resist rust better outdoors, and stainless steel nails resist it more again. Pressure-treated lumber can corrode ordinary steel nails over time, so look for fasteners rated for that use. The article on fastener corrosion covers the details.
Choosing the right hammer
A hammer’s weight affects driving power and fatigue. For general framing with larger nails, a heavier claw hammer is typical, while trim work with small nails calls for a lighter hammer. The topic of hammer weight is covered in another article.
Safety
Wear safety glasses when driving nails, because nails can bend and fly, and hammer faces can chip. Keep your free hand out of the strike path, and hold small nails with pliers or a nail holder. Pull or bend over exposed nails in scrap wood. This is general guidance, so for structural work follow local building codes or ask a qualified professional.