“3 × 1” is the traditional imperial designation for timber formerly supplied at 3 inches by 1 inch. Since the UK’s move to metric, timber is specified in millimetres.
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Nominal imperial size: 3" × 1" (≈ 76.2 × 25.4 mm)
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Nominal metric reference: 75 × 25 mm
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Typical finished/regularised size (after drying and planing): ≈ 19–22 mm × 70–75 mm
(Actual dimensions vary by mill, grading, moisture content, and whether the section is sawn or planed.)
The “3×1” label remains common in trade use, but orders, drawings, and cutting lists should be based on the metric finished size. For tight tolerances or when matching existing work, measure the actual section supplied. Lengths are supplied in metres.