What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Statesyours
Same word, different thingnominal size
also 2x4, dimensional lumber
The name is imperial and pre-drying; the actual piece is smaller in both directions by an amount fixed in the grading rules, so the designation is an identifier rather than a measurement.
Canada
Same word, different thingnominal size
also 2x4, dimension lumber
As the United States, with the same nominal-versus-actual gap, and the actual sizes stated in millimetres alongside the imperial name.
United Kingdom
Same word, different thingsawn size
also nominal size, ex-size, regularised, PAR
British timber quotes the SAWN size, with the planed piece smaller — "ex 50 x 100" means sawn at that and delivered under it, which is a different convention from the American one, not a translation of it.
Australia
Same word, different thingnominal size
also sawn size, dressed size, DAR
Australian practice distinguishes sawn from dressed (DAR) sizes explicitly, so the same board has two published dimensions and which is quoted depends on how it is sold.
The governing standard, by market
United States
PS 20
American Softwood Lumber Standard — the nominal-to-actual size relationship.
United Kingdom
BS EN 1313-1
Round and sawn timber — permitted deviations and preferred sizes for softwood.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why is a 2x4 not two inches by four?
- Because it is named for its size before it was dried and planed. Timber is sawn oversized from green stock, then dried — which shrinks it — and then surfaced on all four faces, which removes more. The finished piece is smaller in both directions by amounts fixed in the grading rules, so every 2x4 from every mill arrives at the same actual size and the name survives as an identifier. The same applies to every nominal designation in the family. It matters because the DEPTH is what governs a joist or a rafter: bending stiffness rises with the cube of depth, so treating a nominal depth as an actual one over-estimates the member substantially, and span tables are written against the actual size for exactly that reason.
- What is the metric equivalent of a 2x4?
- There is not one, only a nearest size — and that is the honest answer rather than a cautious one. Metric sawn timber is produced to its own preferred sizes, which were not derived by converting the imperial ones, so the nearest metric section to a 2x4 differs from it in both dimensions by a few millimetres in whichever direction the local standard chose. A few millimetres of depth is a real change in stiffness, and a few millimetres of width changes how sheet material lands on the framing. So substituting across the systems is a design decision checked against a span table, not a conversion; and a detail drawn for one module — 16 or 24 inch centres against 400 or 600 mm — does not land on the other.
- What do ex-size, regularised, PAR and DAR mean?
- They are the British and Australian ways of saying how much machining has happened since the piece was sawn, and therefore which dimension is being quoted. "Ex" a size means sawn at that size and delivered smaller after processing. Regularised means machined to a consistent depth so that joists line up, with the width left sawn. PAR — planed all round — and the Australian DAR, dressed all round, mean surfaced on all four faces and therefore smaller again in both directions. Each has its own published tolerance from the sawn size. A quantity taken from a drawing in sawn sizes and ordered in planed ones is short in every dimension, which shows up as gaps at the sheathing rather than as an obvious error.