Timber sizes explained (nominal vs actual)

Learn why a timber's stated size is never its true size, so your measurements and cuts come out right.

TS-01Understand what 'nominal size' meansNominal size is the size timber is called and sold as, like '4x2' or '47x100mm'. It is not the size you will actually measure when the timber arrives. Merchants use this name because it is the size the wood was roughly cut to before it was finished.

TS-02Know why the actual size is smallerAfter rough sawing, timber is often planed (smoothed on a machine) to remove roughness and make it straight. Planing shaves off a few millimetres from each face. So a piece sold as '47x100mm' might actually measure 45x97mm once you check it with a tape measure.

TS-03Learn the common size differencesOld imperial names like '4x2' mean 4 inches by 2 inches nominal, which is roughly 100x50mm nominal, but the planed actual size is closer to 97x47mm. Modern metric-labelled timber, like '47x100mm CLS', usually loses 2-3mm per face too. Rough-sawn timber (unplaned) is the exception - it stays much closer to its stated size.

TS-04Always measure the actual piece before you plan cutsNever trust the label alone. Take a tape measure and check the width and depth of the actual piece of timber in your hands. Do this for every different batch, since sizes can vary slightly between suppliers and even between deliveries.

TS-05Use the actual size for all your calculationsWhen working out spacing, joint sizes, or how a shelf will sit in a bracket, use the real measured size, not the nominal name. A common beginner mistake is measuring a stud gap based on '4x2' meaning exactly 100x50mm, then finding the timber does not fit or leaves an unexpected gap.

TS-06Check for this trap when buying timberAt the merchant or DIY shed, timber is priced and labelled by its nominal size, so a '2x1' costs the same whether it is planed or rough-sawn, even though the planed one is physically smaller. If your project needs an exact finished size, ask whether the timber is planed (PSE - planed square edge) or sawn, since this changes the real dimensions you will get.

TS-07Check you have got it rightYou have understood this skill when you can pick up any piece of timber, ignore the label, and measure it yourself with a tape before using it in a calculation. If your finished join, shelf, or frame fits snugly with no unexpected gaps, you have accounted for the real size correctly. If things come out a few millimetres off, it is almost always because the nominal size was used instead of the measured one.

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