The Challenge
A 2×4 is not 2 inches by 4 inches. It never was. The nominal size is what the lumber was called before drying and planing — the actual size you get off the shelf is 1.5×3.5 inches, or 38×89mm. This trips up every first-time builder and causes real problems when mixing US lumber specs with metric hardware, European joinery plans, or CNC cutting files. The gap between nominal and actual isn't random. Rough-sawn lumber shrinks during kiln drying and loses more material during surface planing. A nominal 2-inch dimension becomes 1.5 inches actual (38mm). A nominal 4-inch becomes 3.5 inches (89mm). Nominal 6-inch and above lose half an inch — a 2×6 is actually 1.5×5.5 inches (38×140mm). Beyond 6 inches, the actual dimension is always nominal minus 0.5 inches.
Lumber Nominal vs Actual Size: Inches and Millimeters
| Nominal Size | Actual Size (inches) | Actual Size (mm) | Common Use |
|---|---|---|---|
| 1×2 | 0.75 × 1.5 in | 19 × 38 mm | Furring strips, trim |
| 1×3 | 0.75 × 2.5 in | 19 × 64 mm | Trim, battens |
| 1×4 | 0.75 × 3.5 in | 19 × 89 mm | Shelving, trim, siding |
| 1×6 | 0.75 × 5.5 in | 19 × 140 mm | Shelving, paneling |
| 1×8 | 0.75 × 7.25 in | 19 × 184 mm | Shelving, paneling |
| 1×10 | 0.75 × 9.25 in | 19 × 235 mm | Wide shelving |
| 1×12 | 0.75 × 11.25 in | 19 × 286 mm | Wide shelving, fascia |
| 2×2 | 1.5 × 1.5 in | 38 × 38 mm | Nailers, blocking |
| 2×3 | 1.5 × 2.5 in | 38 × 64 mm | Non-structural framing |
| 2×4 | 1.5 × 3.5 in | 38 × 89 mm | Wall framing, general construction |
| 2×6 | 1.5 × 5.5 in | 38 × 140 mm | Wall framing, floor joists |
| 2×8 | 1.5 × 7.25 in | 38 × 184 mm | Floor joists, rafters |
| 2×10 | 1.5 × 9.25 in | 38 × 235 mm | Floor joists, headers |
| 2×12 | 1.5 × 11.25 in | 38 × 286 mm | Stairs, large headers |
| 4×4 | 3.5 × 3.5 in | 89 × 89 mm | Posts, columns |
| 4×6 | 3.5 × 5.5 in | 89 × 140 mm | Beams, posts |
| 6×6 | 5.5 × 5.5 in | 140 × 140 mm | Heavy posts, pergolas |
| 6×8 | 5.5 × 7.5 in | 140 × 191 mm | Structural beams |
| 8×8 | 7.5 × 7.5 in | 191 × 191 mm | Heavy timber posts |
Plywood and Sheet Goods: Nominal to Actual Thickness
| Nominal Thickness | Actual Thickness (inches) | Actual Thickness (mm) | Typical Use |
|---|---|---|---|
| 1/4 in | 7/32 in (0.219) | 5.56 mm | Cabinet backs, underlayment |
| 3/8 in | 11/32 in (0.344) | 8.73 mm | Sheathing, light shelving |
| 1/2 in | 15/32 in (0.469) | 11.91 mm | Subflooring, wall sheathing |
| 5/8 in | 19/32 in (0.594) | 15.09 mm | Roof sheathing, flooring |
| 3/4 in | 23/32 in (0.719) | 18.26 mm | Cabinets, furniture, flooring |
| 1 in | 31/32 in (0.969) | 24.61 mm | Heavy shelving, workbenches |
Why Nominal and Actual Lumber Sizes Differ
North American softwood lumber is named by its rough-sawn green size — the dimensions immediately after the log is cut, before any drying or finishing. Kiln drying removes moisture and causes the wood to shrink. Surface planing then removes additional material to produce a smooth, uniform surface. The result is a finished board consistently smaller than its name suggests. The American Softwood Lumber Standard (PS 20), published by the National Institute of Standards and Technology, defines the minimum actual sizes for each nominal size. A 2×4 must measure at least 1.5×3.5 inches actual under PS 20. This standard applies to pine, fir, spruce, and other common framing softwoods sold in the US and Canada.
Converting a US Framing Plan to Metric
- Identify whether each dimension in the plan is a nominal lumber callout (e.g. '2×4 stud') or an actual measurement (e.g. '16 inches on center')
- For nominal lumber callouts, look up the actual size in the chart above — do not multiply the nominal number by 25.4
- For actual measurements and spacing dimensions, multiply directly by 25.4 to get millimeters
- Cross-check critical structural dimensions against local building code — metric equivalents may require rounding to standard metric lumber sizes available in your region
Nominal Sizes Will Break Your Build
- Using nominal instead of actual dimensions in a CNC cut list will produce parts 25–33% oversized
- Dado blades and router bits sized for '3/4-inch plywood' are ground to 18mm or 23/32in — not 19.05mm
- Structural load calculations based on nominal cross-sections overestimate capacity — always use actual dimensions for any load-bearing design
- Hardware rated for '4×4 posts' is designed around 89mm actual — ordering metric hardware at 100mm will not fit
US Nominal Lumber vs European Metric Timber
Step-by-Step Workflow
Enter the nominal inch dimension — what's printed on the label or plan
Select lumber or general conversion — lumber mode applies actual-size adjustment automatically
Get both actual inch size and millimeter equivalent for use in metric plans or hardware specs
Specifications
- 2×4 actual size
- 38 × 89 mm (1.5 × 3.5 in)
- 2×6 actual size
- 38 × 140 mm (1.5 × 5.5 in)
- 2×8 actual size
- 38 × 184 mm (1.5 × 7.25 in)
- 4×4 actual size
- 89 × 89 mm (3.5 × 3.5 in)
- 1×4 actual size
- 19 × 89 mm (0.75 × 3.5 in)
- Formula (over 2in nominal)
- actual = nominal − 0.5 in, then × 25.4
- Formula (1in nominal)
- actual = 0.75 in = 19.05 mm
Best Practices
- Always use actual dimensions in structural calculations — nominal sizes will produce dangerously wrong load figures
- European timber is sold in actual sizes — a 38×89mm European stud matches a US 2×4 exactly
- Hardwood lumber follows different rules: a hardwood 1×4 is often closer to 0.875×3.75 inches actual
- Plywood thickness: nominal 3/4in = actual 23/32in = 18.26mm in most sheet goods
- CNC and laser cutting files must use actual dimensions — import nominal sizes and your cuts will be wrong
Frequently Asked Questions
Why is a 2x4 not actually 2 inches by 4 inches?
The 2×4 name comes from the rough-sawn green lumber size before processing. After kiln drying (which causes shrinkage) and surface planing (which removes more material for a smooth finish), the actual size is 1.5×3.5 inches (38×89mm). This sizing convention has been standard in North America since the 1960s and is codified in American Softwood Lumber Standard PS 20.
What is a 2x4 in millimeters?
A 2×4's actual dimensions are 1.5×3.5 inches, which converts to 38.1×88.9mm — rounded in practice to 38×89mm. The nominal 2×4 name is used for ordering and specification, but 38×89mm is what you physically measure on the lumber.
What is a 4x4 post in millimeters?
A 4×4 post is actually 3.5×3.5 inches = 88.9×88.9mm, standardized as 89×89mm. This matters when specifying post base hardware, post caps, or through-bolt lengths — all hardware rated for '4×4' is designed around the 89mm actual dimension.
Do European timber sizes match US lumber sizes?
European structural timber is sold in actual (finished) sizes and uses metric. A common European stud is 45×95mm — close to a US 2×4 (38×89mm) but not identical. European CLS timber at 38×89mm is an exact match to US 2×4 actual size and is sold specifically for compatibility with North American framing methods.
What is 1-inch lumber in millimeters?
Nominal 1-inch softwood boards have an actual thickness of 0.75 inches = 19.05mm, commonly listed as 19mm. So a 1×6 board is actually 19×140mm (0.75×5.5 inches). The full 25.4mm thickness only applies to rough-sawn or hardwood lumber sold to specific custom dimensions.
How do I convert a lumber plan from inches to millimeters?
First confirm whether the plan uses nominal or actual inch dimensions — most construction plans use nominal sizes for member callouts ("use 2×4 studs") but actual dimensions for spacing and gap measurements. Convert actual dimensions by multiplying by 25.4. Never multiply nominal lumber sizes directly — a nominal 2-inch dimension is 38mm, not 50.8mm.
Is plywood sold by nominal or actual thickness?
Plywood is sold by nominal thickness but the actual thickness is consistently less. Nominal 3/4 inch plywood is typically 23/32 inch = 18.26mm. Nominal 1/2 inch is 15/32 inch = 11.91mm. Nominal 1/4 inch is 7/32 inch = 5.56mm. For cabinet joinery and dado cuts, always measure the actual sheet before cutting — thickness varies slightly by manufacturer and species.