Metric and imperial lumber, fasteners and drill bits

Why nominal and actual sizes differ

A US 2 × 4 is not 2 inches by 4 inches. The name is the rough-sawn size before the board is dried and planed smooth. The finished board is 1-1/2 by 3-1/2 inches, or about 38 by 89 mm. Canada uses the same nominal system, and the metric dimensions are often printed as 38 × 89 mm. In Europe, Australia and much of Asia, lumber is sold by its real size, such as 45 × 95 mm, and the numbers on the label match a tape measure.

When you design with US lumber, use the actual size. A frame built from three 2 × 4s side by side is 4-1/2 inches deep, not 6. Mixing nominal and actual sizes is the classic cause of gaps and misfits.

Lengths and sheet sizes

US lumber is sold in 2-foot steps: 8, 10, 12 and 16 feet, which are 2.44, 3.05, 3.66 and 4.88 metres. Metric lumber is sold in 0.3 m steps such as 2.4, 3.0, 3.6 and 4.8 m. A US 8 foot board is therefore 2.438 m, a few millimetres longer than a 2.4 m board, so the two are not interchangeable in a precise layout.

Plywood and drywall come in 4 × 8 foot sheets in North America, which is 1,219 × 2,438 mm. Elsewhere the standard is usually 1,200 × 2,400 mm, about 1.6 percent smaller on each side. Thicknesses are 1/4, 1/2, 5/8 and 3/4 inch (6.4, 12.7, 15.9 and 19.1 mm) in North America and 6, 12, 15 and 18 mm in metric countries. A "3/4 inch" plywood sheet is often a little thinner, about 23/32 inch or 18.3 mm, so measure before cutting joints.

Board feet and cubic metres

US hardwood and some softwood is priced by the board foot, which is a volume of 144 cubic inches: a piece 1 inch thick, 12 inches wide and 1 foot long. One board foot is 2.36 litres, and one cubic metre is about 424 board feet. To find board feet, multiply thickness in inches by width in inches by length in feet and divide by 12. A 2 × 4 that is 8 feet long, using the nominal size, is 2 × 4 × 8 / 12 = 5.33 board feet. Metric suppliers sell by the cubic metre or by the linear metre.

Wood screws: gauge numbers

US wood screws use a gauge number, not a diameter. The diameter in inches is 0.060 plus 0.013 times the gauge. A #8 screw is therefore 0.060 + 0.104 = 0.164 inch, or 4.2 mm. Metric screws are named by diameter in millimetres, such as 4.0 mm, and the nearest match is close enough for most woodwork.

Length is the part that trips people up: US screws are measured in inches (a 1-1/2 inch screw is 38 mm) and metric screws in millimetres (a 40 mm screw is 1.57 inches). The two are a close swap but not an exact one, so check the depth of the hole before you substitute.

Nails and penny sizes

US nails are sold by "penny" size, written with a d, such as 16d. The number refers to length, not weight. Metric nails are sold by length in millimetres and diameter.

Bolts, nuts and thread pitch

Imperial bolts are named by diameter and threads per inch, such as 1/4-20. Metric bolts use the letter M and the diameter in millimetres, such as M6, with the pitch (the distance between threads) in millimetres. The two systems are not interchangeable. A 1/4 inch bolt is 6.35 mm and an M6 is 6.00 mm, so one can start in the other's nut and then jam or strip.

To compare pitches, divide 25.4 by the threads per inch. A 1/4-20 bolt has a pitch of 25.4 / 20 = 1.27 mm, which is not the 1.0 mm of an M6 coarse thread. Fine threads (UNF in imperial, a smaller pitch in metric) also exist, so check both the diameter and the pitch before you buy.

Wrench sizes follow the same split. Metric heads are sized in millimetres (M8 uses a 13 mm wrench, M10 a 16 or 17 mm). Imperial heads are in fractions of an inch (a 1/4 inch bolt uses 7/16 inch, a 3/8 inch bolt uses 9/16 inch). The sizes sometimes sit within half a millimetre of each other, which is how bolts get rounded off.

Drill bit sizes

Drill bits come in four sets: fractional inches in 1/64 or 1/16 steps, number bits (#1 to #80) and letter bits (A to Z) for in-between sizes, and metric bits in 0.1 mm steps. A 1/4 inch bit is 6.35 mm, and the nearest metric bit is 6.5 mm, which is 0.15 mm larger.

Pilot and tap drill sizes

A pilot hole should be slightly smaller than the screw's core so the threads still bite. For a #8 wood screw, a 7/64 to 1/8 inch (2.8 to 3.2 mm) pilot works in hardwood, and a clearance hole of about 11/64 inch (4.4 mm) in the top piece lets the screw pull two boards tight. Threaded holes in metal need a tap drill matched to the thread, as the table shows.

Common mistakes

• Designing with nominal lumber sizes instead of actual sizes.
• Mixing metric and imperial bolts, which can cross-thread and damage both parts.
• Assuming a metric bit equals the same-sized fractional bit. A 1/4 inch hole is 6.35 mm and a 6 mm bit makes a tighter fit.
• Forgetting that a US "8 foot" board is 2.438 m, not 2.4 m.
• Choosing a screw length without allowing for the thickness of the first board.

For the units behind these sizes, see imperial vs metric.

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