A piece of timber sold as a two-by-four measures 1.5 by 3.5 inches, which is 3.81 by 8.89 cm. The label is a nominal size — a name inherited from the rough sawn dimension before the wood was planed and dried — and converting the name rather than the measurement is the most common way a metric build goes wrong by a centimetre.
Nominal sizing is not confined to timber. Screen sizes, pipe bores, wire gauges and paper weights all carry labels that once described something and now identify a product. A converter will translate any of them faithfully, which is exactly the problem.
Which timber sizes are nominal?
All of the common ones. Softwood loses roughly half an inch in each direction to planing, and the loss is proportionally larger on the small dimension.
| Sold as | Actual inches | Actual centimetres |
|---|---|---|
| 1 × 6 | 0.75 × 5.5 | 1.91 × 13.97 |
| 2 × 4 | 1.5 × 3.5 | 3.81 × 8.89 |
| 2 × 6 | 1.5 × 5.5 | 3.81 × 13.97 |
| 2 × 8 | 1.5 × 7.25 | 3.81 × 18.41 |
| 4 × 4 | 3.5 × 3.5 | 8.89 × 8.89 |
Note the 2 × 8. The second dimension loses three quarters of an inch rather than a half, because the planing allowance grows on wider boards. There is no single rule to apply — the sizes are a table to look up, not a formula.
How big is a 55-inch television?
The 55 inches is the diagonal of the panel, so a 16:9 screen at that size is 47.94 by 26.96 inches, or 121.8 by 68.5 cm. The number that matters for a wall or an alcove is 121.8 cm wide, not the 139.7 cm you get by converting 55 inches directly.
| Diagonal | Width | Height |
|---|---|---|
| 43 in | 95.2 cm | 53.5 cm |
| 50 in | 110.7 cm | 62.3 cm |
| 55 in | 121.8 cm | 68.5 cm |
| 65 in | 143.9 cm | 80.9 cm |
| 75 in | 166.0 cm | 93.4 cm |
| 85 in | 188.2 cm | 105.8 cm |
Add the bezel and the stand for the real footprint. Two panels with the same diagonal from different makers can differ by a couple of centimetres in overall width, which is enough to matter in a recess.
Where else does the label stop being a measurement?
In several places you will meet on the same job.
- Pipe. A half-inch pipe is named for an approximate bore, and neither its inside nor its outside diameter is half an inch.
- Paper. A4 is 210 × 297 mm, which is 8.27 × 11.69 in — close to US Letter at 8.5 × 11 in, and different enough that a full-bleed print will crop.
- Wire gauge. The number falls as the wire gets thicker, and the scale is not linear.
- Shoe sizes. UK, US and EU scales share no common origin, so conversion is a lookup table rather than arithmetic.
What should you actually do?
Measure the thing rather than converting its name. Where you cannot measure it — because it has not been delivered yet — find the manufacturer stated dimension in millimetres and work from that, treating the nominal label as a part number.
The habit that saves the most rework is writing both figures on the drawing: the name you will order by, and the dimension you will build to. A cut list that says "2 × 4 (38 × 89 mm)" cannot be misread by whoever picks it up next.
How do you order timber in a metric country?
By the planed size in millimetres, which is the honest number and the one the yard will quote. The metric equivalent of a two-by-four is sold as 38 × 89 mm, and that is what arrives — no allowance to subtract and no nominal label to decode.
The two markets have converged on nearly the same stick from opposite directions. A US two-by-four at 1.5 × 3.5 in is 38.1 × 88.9 mm, so it is within 0.2 mm of the European 38 × 89. They are interchangeable in practice and not in name, which is the whole shape of the problem.
Where it stops being interchangeable is spacing. Stud centres are 16 in in North America, which is 406.4 mm, against 400 or 600 mm in metric construction. Sheet goods are cut to match the local spacing, so a 1220 mm sheet lands correctly on 400 mm centres and awkwardly on 406.4 mm ones. Convert the spacing before ordering the boards, not after.
Questions people ask
Why 1.5 by 3.5 and not something rounder? The rough sawn board really was close to 2 by 4 inches. Kiln drying shrinks it and planing takes the rest, and the finished sizes were standardised around what was left.
Is metric timber nominal too? Less so. Timber sold as 38 × 89 mm is the planed size, so the label is the measurement. The same wood sold as a two-by-four is not.
Does the same apply to plywood? Sheet goods are closer to honest — an 8 by 4 sheet really is about 2440 by 1220 mm — but thickness is often nominal, and an 18 mm sheet can arrive at 17.5 mm.
Is A4 ever the same as Letter? No. A4 is 6 mm narrower and 18 mm taller. Printing one on the other either crops the edges or leaves a band.
Convert measurements, not names. Inches to centimetres and centimetres to inches handle the numbers once you have real ones, and feet to metres covers the room the timber has to fit in.