Nail Size Table
| Penny size | Length (inches) | Length (mm) |
|---|---|---|
| 2d | 1 | 25 |
| 3d | 1 1/4 | 33 |
| 4d | 1 1/2 | 38 |
| 5d | 1 3/4 | 44 |
| 6d | 2 | 51 |
| 7d | 2 1/4 | 57 |
| 8d | 2 1/2 | 64 |
| 9d | 2 3/4 | 70 |
| 10d | 3 | 76 |
| 12d | 3 1/4 | 83 |
| 16d | 3 1/2 | 89 |
| 20d | 4 | 102 |
| 30d | 4 1/2 | 114 |
| 40d | 5 | 127 |
| 50d | 5 1/2 | 140 |
| 60d | 6 | 152 |
| Type | What distinguishes it |
|---|---|
| Common | Thickest shank at a given size; disk head 3-4x shank; structural default |
| Box | Thinner shank than common of the same size; splits thin stock less |
| Finish | Head barely wider than shank; countersink and putty; trim duty |
| Casing | Slightly larger head than finish; flooring and casing work |
| Duplex | Second head for easy pull; concrete forms and scaffolding |
| Cut | Machine-made square shank; masonry and historical restoration |
Nail sizes are counted in pennies, written d: an 8d nail is read eight-penny and is two and a half inches long. The unit is a price, not a length - it goes back to medieval England, where a hundred nails cost as many pennies as the Roman denarius count, and the abbreviation outlived the accounting system it came from. The table lists the full US penny line from 2d (one inch) to 60d (six inches), the sizes stocked by every lumberyard, in both inches and millimeters.
There is a mental-math shortcut hiding in the first half of the table: from 2d through 10d, length in inches equals the penny number plus two, divided by four - 4d gives 1.5 inches, 8d gives 2.5, 10d gives 3. The rule is exact for that run and then bends: 12d lands at 3.25 rather than 3.5, 16d at 3.5 rather than 4.5, and 60d stops at 6 inches rather than 15.5. Long nails get stockier instead of proportionally longer, because driving forces scale with embedment, not with the penny count.
How to use
- Buy by penny size, not by ruler memory: 16d (3.5 in) is the framing spike of record, 8d and 6d cover sheathing and trim, and the table doubles as the label decoder when a box only says the d number.
- Match the type to the wood, not just the length: box nails run a thinner shank than common nails of the same size, so they split thin stock less; common nails carry the thicker shank for structural grip; finish nails take a countersink and putty.
- Choose the coating by exposure: hot-dip galvanizing deposits rough, thick zinc that survives pressure-treated lumber and acidic wood; electrogalvanized is smooth and thin - fine indoors, outclassed outdoors.
Frequently asked questions
What does the d in nail sizes mean?
It abbreviates denarius, the Roman penny - the coin that gave England its word for the unit. In medieval England nails were sold by the hundred, and the price of a hundred nails of a given size was so many pennies: a four-penny nail cost 4 pence per hundred and was written 4d, using the old coinage abbreviation for denarius. The pricing system died centuries ago; the size names never did. So the d is frozen commerce, not geometry - which is why the penny number correlates with length but never multiplies out to an exact scale, and why the table, not the name, is the specification.
Is there a formula to convert penny size to length?
For the small half of the line, yes: length in inches = (penny + 2) / 4. Test it against the table - 2d gives 1 inch, 6d gives 2, 10d gives 3 - and it holds without exception from 2d through 10d. Then it drifts: 12d should be 3.5 but is 3.25; 16d should be 4.5 but is 3.5 (the single most-used framing nail on the continent, and the rule's most famous casualty); 60d should be 15.5 but stops at 6. The divergence is engineering, not sloppiness: a nail long enough to bend the rule gains shank thickness instead of length, because the driving and withdrawal forces that matter scale with embedment depth and shank area, not with the label. Use the formula as a memory scaffold for the small sizes; use the table past 10d.
Common vs box vs finish nails - what is the actual difference?
Shank thickness and head size, in that order. Common nails are the structural default: the thickest shank of the three at a given penny size, with a disk head about 3 to 4 times the shank diameter - maximum grip, maximum splitting risk in thin or brittle stock. Box nails are the same length class with a thinner shank, the everyday choice for crates, sheathing and thin wood where splitting matters more than peak holding power. Finish nails shrink the head to barely wider than the shank so it can be countersunk below the surface and hidden with putty - the trim and casing nail. Casing nails sit between finish and common with a slightly larger head for flooring work, and drywall nails add a thin, broad head to hold gypsum board without pulling through. Pick by what you are fastening, then confirm the length in the table.
Which coating should I buy - and does 16d always mean the same nail?
Two different questions, both common traps. On coatings: hot-dip galvanizing runs the nail through molten zinc and leaves a rough, thick coat that is the only finish rated for the acids in pressure-treated lumber and for weather exposure; electrogalvanizing plates a smooth, thin coat - neater to drive, but its lighter zinc budget is indoor duty, and it can look bright and new while corroding outside. On 16d: the penny system only fixes length, and the shank varies by type - a 16d common nail is a different object from a 16d box or finish nail - same name, different shank, head and duty. That is why framing schedules specify both: penny size for length, type and coating for everything else, and why two boxes labeled 16d are interchangeable only when the rest of the label agrees too.