Why isn’t a "2×4" actually 50 × 100 mm?

Nominal size is the rough size from the mill before planing. After drying the wood shrinks, then it’s planed for smooth faces and straight edges. That leaves a finished product 10-15 mm smaller each way than the name suggests. "2×4" means "ordered as 2×4 inch rough stock", not "measures 2×4 inches finished". Same tradition across the Nordics, US, Canada and UK.

Is 1 inch always exactly 25.4 mm?

Yes. The international inch was defined as exactly 25.4 mm in 1959, used in all countries except a few old special standards. So 4 inches = 101.6 mm exactly. Confusion only comes when you mix nominal lumber size with inches; "2×4" no longer refers to physical inches.

Can I order timber in mm instead of inches?

Yes, all major lumberyards label in mm. Order e.g. "48 × 98 mm C24 construction timber in 4.8 m length" directly. Many pros prefer that because it’s unambiguous, especially since NS-EN 336 and NS-EN 1313 allow small variations. Shelf labels usually show inches as customer aid but mm is the accurate measure.

Inches to mm and Lumber Sizes (2x4, 48x98)

Nominal	Actual (mm)
1×2	19 × 38
1×4	19 × 89
1×6	19 × 140
2×2	38 × 38
2×3	38 × 64
2×4	38 × 89
2×6	38 × 140
2×8	38 × 184
2×10	38 × 235
2×12	38 × 286
4×4	89 × 89

Nominal

Actual (mm)

1×2

19 × 38

1×4

19 × 89

1×6

19 × 140

2×2

38 × 38

2×3

38 × 64

2×4

38 × 89

2×6

38 × 140

2×8

38 × 184

2×10

38 × 235

2×12

38 × 286

4×4

89 × 89

How it works

One inch is exactly 25.4 mm. Enter inches or millimetres and the other value is calculated automatically.

About this tool

The lumber converter switches between inches and millimeters for planks, boards and other timber sizes. The Norwegian building and lumber trade still uses inch measurements (2×4, 4×4 etc.) even though lengths are metric. The tool also explains the difference between nominal dimension (what the plank is called, e.g. "2×4") and actual planed dimension (finished plank is smaller after planing). Useful for material lists, drawings and to understand what you’ll actually get off the shelf.

How to use it

Enter inches to convert to mm, or mm to get inches.
See the reference table for common sizes (2×4, 2×6, 4×4, etc.).
Use the "actual size" column when planning cutouts, nailing or material calcs.
Copy the value into a drawing or material list.

Examples

Standard 2×4 stud

InputNominal 2×4 inches (2" × 4").

Output

Nominal: 50.8 × 101.6 mm. Actual planed size in Norwegian timber: 36 × 98 mm (48 × 98 mm is also common, depending on supplier and standard).

Norwegian lumber standard NS-EN 336 gives slightly different actual sizes than US. A "2×4" in the US is 38 × 89 mm, in Norway usually 36 × 98 mm or 48 × 98 mm. Ask the lumberyard for "C24 construction timber" if you need a guaranteed load rating.

4×4 post for a deck

InputNominal 4×4 inches (4" × 4").

OutputNominal: 101.6 × 101.6 mm. Actual planed: typically 95 × 95 mm or 98 × 98 mm.

For deck posts use impregnated timber in class NTR-A (ground contact) or NTR-AB (above ground). Actual size matters when choosing steel post shoes, which come in fixed mm sizes (91, 95, 98, 101).

Common use cases

Planning stud walls, floor joists or roof structure.
Material lists before ordering from Byggmax, Maxbo or local lumberyard.
Sizing post shoes, nail plates and other fittings sold in mm.
Reading US or UK drawings (imperial units).
Teaching kids or apprentices the nominal vs actual timber concept.

Frequently asked questions

Why isn’t a "2×4" actually 50 × 100 mm?: Nominal size is the rough size from the mill before planing. After drying the wood shrinks, then it’s planed for smooth faces and straight edges. That leaves a finished product 10-15 mm smaller each way than the name suggests. "2×4" means "ordered as 2×4 inch rough stock", not "measures 2×4 inches finished". Same tradition across the Nordics, US, Canada and UK.
Is 1 inch always exactly 25.4 mm?: Yes. The international inch was defined as exactly 25.4 mm in 1959, used in all countries except a few old special standards. So 4 inches = 101.6 mm exactly. Confusion only comes when you mix nominal lumber size with inches; "2×4" no longer refers to physical inches.
Can I order timber in mm instead of inches?: Yes, all major lumberyards label in mm. Order e.g. "48 × 98 mm C24 construction timber in 4.8 m length" directly. Many pros prefer that because it’s unambiguous, especially since NS-EN 336 and NS-EN 1313 allow small variations. Shelf labels usually show inches as customer aid but mm is the accurate measure.

Technical background

Conversion: 1 inch = 25.4 mm exactly. All conversions multiply/divide by 25.4 directly, no precision loss in JavaScript for typical lumber sizes. Reference table is drawn from standard Norwegian practice (NS-EN 336 for construction timber) and US lumber standard ALSC PS 20-20. Norwegian actual sizes vary somewhat between mills; the table shows the most common. Nominal in inches, actual in mm as the trade uses them.

Common sizes (2x4 system, planed)