Nail sizes In most countries, nails are described by their dimensions in millimeters, for example “50 × 3” indicates a nail 50 mm long(not including the head) and 3 mm in diameter. In the United States, however, the length of a nail is designated by its penny size, written with a number and the abbreviation d for penny, a system that originated in England centuries ago. A larger penny number indicates a longer nail. Most nails are sized by length and wire gauge. For the wire gauge, the bigger the number, the thinner the nail.

Nail sizes – length of steel nails
(Penny-Inch-Millimeter equivalents)

• 2d = 1″ = 25mm
• 3d = 1-1/4″ = 32mm
• 4d = 1-1/2″ = 38mm
• 5d = 1-3/4″ = 44mm
• 6d = 2″ = 51mm
• 7d = 2-1/4″ = 57mm
• 8d = 2-1/2″ = 65mm
• 9d = 2-3/4″ = 70mm
• 10d = 3″ = 76mm
• 12d = 3-1/4″ = 83mm
• 16d = 3-1/2″ = 89mm
• 20d = 4″ = 102mm
• 30d = 4-1/2″ = 115mm
• 40d = 5″ = 127mm
• 50d = 5-1/2″ = 140mm
• 60d = 6″ = 152mm

Nail sizes – diameter of steel nails
(Gauge-Inch-Millimeter equivalents)

• 7/0 = 0.4900 = 12.45mm
• 6/0 = 0.4615 = 11.72mm
• 5/0 = 0.4305 = 10.93mm
• 4/0 = 0.3938 = 10.00mm
• 3/0 = 0.3625 = 9.2076mm
• 2/0 = 0.3310 = 8.407mm
• 0 = 0.3065 = 7.785mm
• 1 = 0.2830 = 7.188mm
• 1½ = 0.272 = 6.91mm
• 2 = 0.2625 = 6.668mm
• 2½ = 0.253 = 6.43mm
• 3 = 0.2437 = 6.190mm
• 3½ = 0.234 = 5.94mm
• 4 = 0.2253 = 5.723mm
• 4½ = 0.216 = 5.49mm
• 5 = 0.2070 = 5.258mm
• 5½ = 0.200 = 5.08mm
• 6 = 0.1920 = 4.877mm
• 6½ = 0.184 = 4.67mm
• 7 = 0.1770 = 4.496mm
• 7½ = 0.170 = 4.32mm
• 8 = 0.1620 = 4.115mm
• 8½ = 0.155 = 3.94mm
• 9 = 0.1483 = 3.767mm
• 9½ = 0.142 = 3.61mm
• 10 = 0.1350 = 3.429mm
• 10¼ = 0.131 = 3.33mm
• 10½ = 0.128 = 3.25mm
• 11 = 0.1205 = 3.061mm
• 11½ = 0.113 = 2.87mm
• 12 = 0.1055 = 2.680mm
• 12½ = 0.099 = 2.51mm
• 13 = 0.0915 = 2.324mm
• 13½ = 0.086 = 2.18mm
• 14 = 0.0800 = 2.032mm
• 14½ = 0.076 = 1.93mm
• 15 = 0.0720 = 1.829mm
• 15½ = 0.067 = 1.70mm
• 16 = 0.0625 = 1.588mm
• 16½ = 0.058 = 1.47mm
• 17 = 0.0540 = 1.372mm
• 17½ = 0.051 = 1.30mm
• 18 = 0.0475 = 1.207mm
• 18½ = 0.044 = 1.12mm
• 19 = 0.0410 = 1.041mm
• 19½ = 0.038 = 0.97mm
• 20 = 0.0348 = 0.884mm

Apart from nail sizes, nails are classfied according to heads, shanks, points, materials, finishes. Please click here to check.