Fastener Sizing: Penny Sizes, Gauges and Counts

What the d in 16d actually means

The letter is 'd', for the Latin denarius, and the widely repeated explanation for where it comes from traces to old English pricing, where a size of nail was once quoted by its cost per hundred. The historical pricing link is folklore by now — nobody buys nails by that reckoning anymore — but the letter and the length-to-number relationship it settled into survived the pricing system it came from, and 'penny size' is still the standard way length is specified on a box of common or box nails.

The penny-size table

• 2d — 1 in
• 3d — 1-1/4 in
• 4d — 1-1/2 in
• 5d — 1-3/4 in
• 6d — 2 in
• 7d — 2-1/4 in
• 8d — 2-1/2 in
• 9d — 2-3/4 in
• 10d — 3 in
• 12d — 3-1/4 in
• 16d — 3-1/2 in
• 20d — 4 in
• 30d — 4-1/2 in
• 40d — 5 in
• 50d — 5-1/2 in
• 60d — 6 in

This table fixes length. It says nothing about the nail's diameter, which is a separate spec that changes with the nail's type at the same penny size — a common nail and a box nail both sold as 16d are the same length and different thicknesses, which is covered further down.

Gauge runs backward between nails and screws

Wire gauge, which is what a nail's shank diameter is often specified in on a technical spec sheet even though it's rarely printed on the box, runs the way most people expect from sheet metal: a higher gauge number means a thinner wire. A framing nailer's fasteners run in the low teens; a fine finish nailer's pins run into the high teens, noticeably thinner.

Screw gauge is a different numbering system that happens to share the word 'gauge,' and it runs the other direction: a higher gauge number means a thicker screw shank. A #6 wood screw is meaningfully thinner than a #10, and a #10 is thinner again than a #14. Someone used to nail wire gauge who assumes screw gauge works the same way will guess the wrong screw diameter every time — this is the single most common gauge mix-up in the fastener aisle, and it goes exactly one direction, never the other.

Nails per pound, and why it falls off a cliff with length

• 8d common — roughly 106 nails per pound.
• 12d common — roughly 69 nails per pound.
• 16d common — roughly 49 nails per pound.
• 16d sinker — roughly 60 nails per pound.

The count per pound drops faster than length alone would suggest, because a nail's weight scales with both its length and roughly the square of its diameter, and penny size increases both together — 16d isn't just longer than 8d, it's also noticeably thicker. Going from 8d to 16d is a 40% increase in length, but the count per pound falls by more than half, which is why a rough 'longer nail, proportionally fewer per pound' estimate consistently under-orders on a box of larger fasteners. The nail calculator uses the real per-pound figures for exactly this reason rather than scaling a single reference size.

Common, box and sinker: three shanks at the same penny size

These three nail styles can share the same penny-size length and still behave differently on the job. A common nail has the thickest shank and the largest, flattest head of the three, meant for general framing where raw holding power and head bearing matter most. A box nail is thinner in the shank at the same length, which reduces the risk of splitting thinner or more brittle stock, at some cost to holding power compared to a common nail of the same length. A sinker carries a thin vinyl or resin coating and a slightly smaller, angled head designed to set flush without tearing the surface — the coating adds friction as it's driven, which partly offsets the thinner shank sinkers are typically made with compared to a common nail.

Because sinkers run thinner and lighter than common nails of the same penny size, their per-pound count is meaningfully higher — the 16d sinker figure above, 60 per pound against a common 16d's 49, is the same length nail weighing noticeably less per piece.

Screw length: the two-thirds rule

The general rule of thumb for wood screw length is that roughly two-thirds of the screw's total length should land inside the piece it's actually gripping — the receiving member — rather than passing through the piece being attached. Attaching a 3/4 inch board to a stud with a 2 inch screw puts about 1-1/4 inches into the stud, which is comfortably past that two-thirds mark and gives the screw real material to hold onto rather than just enough to bite.

This is guidance, not a fixed spec — a screw that's too long for its application can blow through the far side of a thin receiving member entirely, and one that's too short for a heavy, load-bearing connection under-grips regardless of how well it's driven. Where a connection is genuinely structural rather than a general fastening job, the specific structural calculator for that connection — the header size calculator or the deck footing calculator, for instance — takes precedence over a general rule of thumb.