Timber and Lumber Sizes Around the World

A piece of framing timber is a “2×4” in Vancouver, a “38 × 89” in London, a “90 × 45” in Melbourne and a “38 × 114” in Johannesburg. Same job, four labels — and the differences are not cosmetic. They change what you order, what fits your joist hangers, and which span table you are allowed to read.

This guide maps the systems onto each other: why a 2×4 is not 2 by 4, what the UK, Europe, Australia, New Zealand and South Africa actually sell, how to read a span table in any of them, and how to convert nominal to actual in your head. Sizes and grades vary by country and mill — treat every number as typical, and read the stamp on the timber.

Nominal vs Actual: Why a 2×4 Isn’t Two by Four

North American softwood lumber is sawn green — wet, rough and full-size — at the nominal dimensions: a 2×4 starts life genuinely close to 2 in by 4 in (51 × 102 mm). Then it is kiln-dried, which shrinks it, and planed smooth on all four faces, which removes more. The piece that reaches the yard finishes at 1½ × 3½ in — about 38 × 89 mm. The nominal size survives as the trade name; the actual size is what the tape says.

That is the key mental shift. In North America the number on the sticker is a name, and the measurement lives in a sizing table. In most metric countries it is the opposite: the label is approximately the size. A UK sawn 47 × 100 joist really is about 47 × 100 mm, and planed “regularised” stock runs a few millimetres under its sawn name. Ask for a “two-by-four” in a metric country and you will be politely resold in millimetres.

The pattern holds across the family: 1× boards finish at ¾ in (19 mm) thick, a 4×4 post at 3½ in (89 mm) square, a 6×6 at 5½ in (140 mm). Fractional actuals are the norm — the millimetre equivalents, whole and round, are easier to work with.

The North American Ladder

Framing lumber comes in a standard ladder of depths, spaced by two inches nominally. Every 2× board is 1½ in (38 mm) thick; the depth is where the sizes step:

Species and grade ride on the stamp: SPF (spruce-pine-fir) is the everyday framing group, Douglas fir-larch and southern pine are regional heavyweights, and grades such as No. 1 and No. 2 choose the strength row of the span table. Pressure-treated versions of the same sizes go anywhere near weather, and spacing is spoken in inches on centre — 16 in OC, about 400 mm, is the default.

United Kingdom and Europe

British and Irish merchants sell softwood carcassing in a 47 mm-thick ladder: 47 × 75, 47 × 100, 47 × 150, 47 × 200 and 47 × 225 for joists, with deeper sections usually to order. The 47 mm thickness is the metricated descendant of a 2 in nominal, and the widths step at 25 mm. Strength is carried by grade classes — C16 and C24 — stamped on the piece; the class, not the species name, is what the span table reads.

Studwork is where the systems meet. CLS — Canadian Lumber Standard — is the metricated copy of the North American actuals: 38 × 89, 38 × 140, 38 × 184 and 38 × 235, planed with eased edges. Merchants also stock studwork at 45 × 90 and similar; on the continent the same C-classes apply, with sections in similar metric ranges that vary by country and mill.

Australia and New Zealand

Structural pine in Australia and New Zealand is sold by its dressed (planed) size, written depth first: 90 × 45, 140 × 45, 190 × 45, 240 × 45. The everyday wall stud is 90 × 45 MGP pine — machine-graded pine, with MGP10, MGP12 and MGP15 as the rising grades and the older F-grades (F7 and up) still in circulation. Radiata pine dominates, usually treated against termites where the region demands it. Written 45 × 90, it is the same stick.

South Africa

South Africa builds its roofs from SA pine — plantation softwood graded S5, S7 and S10 — on a ladder that steps differently: 38 × 76, 38 × 114, 38 × 152 and 38 × 228 are the common sections, the last within 7 mm of a 2×10’s 235 mm depth. Trusses, rafters, purlins and battens all come from this ladder. See Home Project Basics for South African Homes for the masonry-and-millimetres world it sits in.

Everywhere Else

One metric system, many ladders. Japan’s post-and-beam framing runs its own sections, with common posts around 105 × 105 or 120 × 120 mm; Scandinavia, Latin America and the Gulf each have their own ranges. The universal habit is that the label is the actual size — but grades and sections still vary by country and mill.

The Conversion Table

Put the systems side by side and the mapping is close enough to plan with. The rows are the North American ladder; the columns are what each region commonly sells for the same job. These are nearest equivalents, not identities — grades differ, and the SA ladder brackets rather than matches some depths.

Read the table with the grade stamp in mind. A 47 × 225 C24 joist is not automatically interchangeable with a 2×10 No. 2 — the span tables decide — and the Australian and SA columns assume their own native grades (MGP, S-grades). Match the size first, then the grade, then the span.

How to Read a Span Table

Every system publishes span tables, and they all ask the same five questions. Learn them once and you can read a table from any country:

1. Species and grade. SPF No. 2, Douglas fir-larch, C24, MGP12, S7 — the stamp on the timber chooses your row. Higher grade, longer span, same size.
2. Spacing. The centres between members. North America speaks 12, 16 and 24 in on centre (≈ 305, 405 and 610 mm); metric tables use 300, 400, 450 and 600 mm. Tighter spacing buys span.
3. Loads. Dead load (the structure itself) plus live load (people and furniture), in kPa. Interior floors are commonly designed around 1.5 kPa live load; decks and balconies carry higher requirements.
4. Bearing. How much of the joist end sits on its support. Tables assume a minimum; short-change it and the printed span is not yours.
5. The footnotes. Wet service, deflection limits, installation details. The fine print is where a span number becomes honest.

Fix any three of species, spacing, load and span, and the table hands you the fourth. That is the whole skill — plus the discipline to use the table your country publishes, because the grades and load assumptions behind it are local.

Planning Across Systems

The practical rule: convert to actual size, then plan. A “2×10” joist is roughly a 47 × 235 mm timber in metric countries — close enough to sketch, not close enough to order by. Order by the actual millimetre section your merchant sells.

Hardware forces the same discipline. Joist hangers, framing anchors and bolt patterns are sized to actual dimensions: a hanger for 38 mm-thick North American stock is a loose fit on a 47 mm UK joist, and the reverse grips nothing. When systems mix on one job, the fittings pick the fight first.

One caveat from this site: the Deck Lumber Calculator works from the North American IRC span tables, in nominal lumber sizes. It is a sound rough-planning tool anywhere — the framing logic is universal — but outside North America treat its member sizes as a first pass and confirm against your local span tables and grades. The Deck Lumber Guide covers the details.

Converting in Your Head

Four rules cover most of it:

1. Thickness: every 2× board finishes 1½ in (38 mm) thick; 1× boards finish ¾ in (19 mm); 4× posts finish 3½ in (89 mm).
2. Depth: subtract ½ in (13 mm) from the nominal depth through 2×6; subtract ¾ in (19 mm) from 2×8 upward.
3. Inches to millimetres: multiply by 25.4 — or skip the arithmetic and memorise the ladder: 89, 140, 184, 235, 286.
4. The way back: match the metric depth to the ladder — 235 mm is a “2×10 job”, and 45 mm-thick Australian pine is a 2× in thickness. When the label is already metric, stop converting: it is the actual size.

Common Mistakes

• Ordering by nominal abroad. Ask for a 2×4 in a metric country and you will be resold as 47 × 100 or 38 × 89 — specify millimetres.
• Comparing sizes without grades. C24 is not No. 2 is not MGP10 is not S7; the span table knows the difference.
• Mixing hardware across systems. Hangers and anchors sized for 38 mm stock do not fit 47 mm joists.
• Trusting a foreign span table. Load assumptions and grades are local; use your country’s tables.
• Forgetting planed versus sawn. Regularised and dressed timber runs under its quoted sawn size.

Size Deck Framing With the Deck Lumber Calculator

Related MyHouseLogic Guides

• Home Project Basics for South African Homes — the masonry-and-millimetres primer for SA projects.
• Deck Lumber Guide — joist sizing, spacing and span tables in deck practice.
• Planning a Renovation in a Compact Japanese Home — metric planning in tight quarters.
• Homes Around the World — the collection of regional measurement and planning guides.
• Field Guide library — every MyHouseLogic guide, each tied to a calculator.

The bottom line: North America names timber for what it was sawn at; most of the world sizes it for what it is. Convert to actual — a 2×4 is about 38 × 89 mm, a 2×10 roughly a 47 × 235 joist — then let a local span table finish the decision.