Nail Gauge and Penny Size Identifier

A free tool for putting a name to a loose nail. The old penny system labels a nail with a number and the letter "d" — a leftover from when "8d" meant the price per hundred — and today that figure maps straight to length: a 2d nail is one inch, a 16d nail is three and a half. Lay the nail along a calibrated rule or a printed scale, and the tool reads its length, names the penny size and gives the shank gauge.

Identifying a nail step by step

1
Measure tip to head

Lay the whole nail along the length scale, from the point to the underside of the head — that overall length sets the penny size.

2
Read the penny number

The tool snaps the length to the nearest penny size and shows the "d" label printed on the box.

3
Gauge the shank

Rest the shaft across the thickness scale to read the wire diameter; thinner wire at the same length means a box or finishing nail.

4
Check head and point

Compare the head: a broad flat head is a common nail, a small head a box nail, a tiny dimpled head a finishing nail for trim.

Translate a nail's length into its penny (d) size

What this gauge does

Length-to-penny conversion
Reads the overall length and returns the penny size from 2d (1") up to 20d (4"), the figures used on every North American nail box.
Common, box and finishing
Flags the nail type from its head and shank — a flat-headed common, a thinner-wire box nail, or a near-headless finishing nail of the same length.
Shank wire gauge
Measures the shaft thickness and reports it as a Steel Wire Gauge number, the spec that decides splitting risk and holding power.
Pilot-hole guidance
Turns the measured shank into a sensible pilot-drill size for hardwoods and trim, where driving a bare nail would split the wood.
Inch and millimetre length
Shows length in both inches and millimetres, since penny sizes are an inch-based system but most rulers today read in millimetres.

Why match the penny size at all

Buy a box labelled only "8d common" and a length alone will not tell you whether it will hold a stud or split your skirting board — the penny size, the nail type and the wire gauge together do. This tool reads all three from one nail without a caliper, so you can replace a fastener exactly, choose between a common and a finishing nail for the job, and drill the right pilot before the wood ever splits.

Nail sizing questions

Why are nails sized in "pennies" with a d?

It is a centuries-old English convention. The "d" is the symbol for an old penny, and the number once meant the cost per hundred nails of that length. The pricing is long gone, but the labels stuck, so 8d, 10d and 16d now simply denote set lengths.

How does penny size relate to length?

For common sizes the rule is roughly length-in-inches = (penny ÷ 4) + 0.5, so a 6d nail is 2" and a 10d is 3". The relationship holds up to about 16d; above that the steps widen, which is why the tool matches to a table rather than a formula.

What is the difference between common, box and finishing nails?

At the same penny size they share a length but differ in the shank and head. A common nail has the thickest wire and a wide flat head for framing; a box nail uses thinner wire to resist splitting; a finishing nail has a tiny head you can sink and fill for trim work.

What does the shank gauge tell me?

The gauge is the wire thickness on the Steel Wire Gauge scale — a higher number is thinner. Thicker shanks hold better but split wood more readily, so trim and hardwood call for a higher-gauge box or finishing nail, or a pilot hole.

Do nail sizes work outside the US?

Many countries skip pennies and sell nails purely by length and shank diameter in millimetres, for example 75 × 3.4 mm. The tool gives that metric length and shank alongside the penny size so the same nail is recognisable either way.

When should I drill a pilot hole?

Drill when the wood is hard, the nail is thick relative to the board, or you are nailing near an edge or end. A pilot a little smaller than the shank lets the nail hold without wedging the grain apart; the tool suggests a starting bit size.

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