What size is 4x2 timber actually?
Why the timber you buy never measures what the label says, what the common sections really come out at, and how to plan a job around the real numbers.
- Difficulty
- beginner
- Time
- 6 min
- Read
- 3 min
Short answer
A 4x2 is nominally 100 by 50 millimetres, but the piece you carry out is smaller. Sawn carcassing is typically about 97 by 47 after drying and regularising, and the same piece in PAR is typically about 94 by 44. The nominal size is what it was cut at in the mill, green and rough; every stage after that takes material off.
Nothing about timber buying causes more confusion than this, and it is genuinely the merchant's fault for never saying it out loud. The imperial names have survived alongside metric sizes, and both refer to a piece of wood that no longer exists by the time it reaches the rack.
Step by step
- Understand where the loss comes from.Two stages. The board is cut to its nominal size when it is green, then it shrinks as it dries. It is then either regularised — machined to a consistent size while still rough — or planed all round, which removes more. Both losses come off every face.
- Learn the common carcassing sizes as they really arrive.Nominal 50 by 100 comes out around 47 by 97; 50 by 150 around 47 by 145; 50 by 200 around 47 by 195; 50 by 225 around 47 by 220. The 47 figure is remarkably consistent across UK structural stock.
- Learn the common planed sizes.Nominal 25 by 150 PAR comes out around 20 by 144; 25 by 100 around 20 by 94; 50 by 50 around 44 by 44; 50 by 100 around 44 by 94. Expect roughly 5mm off a nominal 25 and roughly 6mm off a nominal 50.
- Treat CLS separately.CLS is quoted at its finished size, not a nominal one — 38 by 63, 38 by 89, 38 by 140 and so on. That is one of its genuine advantages: what it says is what it is.
- Measure the actual stock before you cut anything.Tolerances vary between mills and between batches. Take a tape to the rack, measure a piece from the bundle you are buying, and write the real numbers on your drawing before you set out any joinery.
- Design from the real number, not the label.If a frame has to be a specific finished size, work from the measured section. This is where a job goes wrong quietly: three uprights each 6mm under adds up to a carcass 18mm narrower than the drawing.
Tips
- The 47mm figure matters for stud walls: a 47mm stud face gives you a comfortable target to screw plasterboard into, where a 38mm CLS stud is noticeably less forgiving.
- Sheet materials do not work this way. An 18mm sheet of MDF really is 18mm, and 12.5mm plasterboard really is 12.5mm. The nominal convention is a solid-timber thing.
- If you are matching into existing timber in an older house, measure that too — pre-metric stock was often a genuinely different size again.
Common mistakes
- Ordering by the nominal size for a gap that has to fit — A slot that needs a 44mm piece will not accept a piece you bought as 50mm sawn, which arrives at 47mm — and a slot cut for 50mm has a visible gap when you fill it with 44mm PAR.
- Mixing sawn and planed timber in one frame — They differ by several millimetres in both directions, so faces do not line up and everything needs packing. Pick one finish for the whole assembly.
Questions people ask
Why is it called 4x2 if it is metric?
Because the imperial names survived the metric changeover in trade use. 4x2 means nominal 4 inches by 2 inches, which was restated as 100 by 50 millimetres and then delivered at 97 by 47. Merchants and tradespeople still use both sets of words interchangeably.
Is the merchant allowed to sell it undersize?
Yes — it is not undersize. The nominal size is an established trade description of the sawn size, and the finished size is what you are buying. Structural design and span tables all work off actual sizes, not nominal ones.
Does the length lose anything too?
Barely. Timber shrinks almost not at all along its length, so a 2.4 metre length is 2.4 metres. Some stock is cut slightly over-length; that is a bonus rather than a rule you can rely on.