Use nominal dimensions to identify stock and measured dimensions to design the joints. A US dry dressed 2×4 commonly measures 1½ by 3½ inches. That does not mean you should subtract the same amount from every wood product you buy. Rough hardwood, metric panels and dressed softwood follow different naming conventions.
NIST explains why the 2×4 name differs from the dressed size. Put both the purchased stock and finished-part dimensions on your drawing; they answer different questions.
Nominal vs actual lumber size chart
| Nominal US size | Dry dressed thickness × width | Converted to mm |
|---|---|---|
| 1×2 | ¾ × 1½ in | 19.1 × 38.1 mm |
| 1×3 | ¾ × 2½ in | 19.1 × 63.5 mm |
| 1×4 | ¾ × 3½ in | 19.1 × 88.9 mm |
| 1×6 | ¾ × 5½ in | 19.1 × 139.7 mm |
| 1×8 | ¾ × 7¼ in | 19.1 × 184.2 mm |
| 1×10 | ¾ × 9¼ in | 19.1 × 235.0 mm |
| 1×12 | ¾ × 11¼ in | 19.1 × 285.8 mm |
| 2×2 | 1½ × 1½ in | 38.1 × 38.1 mm |
| 2×3 | 1½ × 2½ in | 38.1 × 63.5 mm |
| 2×4 | 1½ × 3½ in | 38.1 × 88.9 mm |
| 2×6 | 1½ × 5½ in | 38.1 × 139.7 mm |
| 2×8 | 1½ × 7¼ in | 38.1 × 184.2 mm |
| 2×10 | 1½ × 9¼ in | 38.1 × 235.0 mm |
| 2×12 | 1½ × 11¼ in | 38.1 × 285.8 mm |
These rows cover common US dry surfaced softwood sizes in NIST PS 20-25. The millimeter column converts the inch dimensions and rounds to 0.1 mm; it is not a separate metric-stock specification. Measure delivered stock before cutting joints. These are familiar US retail sizes, not a conversion table for every country or product. Wet stock, rough-sawn stock, specialty millwork and subsequent machining can differ. The AHEC/NHLA grading guide covers hardwood purchasing conventions; those are not interchangeable with a US dressed-softwood size table.
Work the assembled dimension backward
Consider a simple cabinet whose top fits between two sides. This is an arithmetic example, not a joinery specification. You want an outside width of 600 mm and your sides measure 18 mm each. The between-sides piece is 600 − 18 − 18 = 564 mm. If the sides actually measure 17.5 mm, the same outside width requires 565 mm. Repeating an assumed thickness through several cabinets can make a visible installation error.
Write the relationship beside the drawing: outside width equals left side plus clear opening plus right side. If a dado, rabbet or face frame changes the joint, redraw the relationship rather than applying that subtraction unchanged. A panel that enters two grooves requires the two engagement depths as well as the clear opening.
Give each number a job
- Stock label: what you order, such as 4/4 maple or nominal ¾-inch plywood.
- Measured stock: what arrives, checked at more than one location.
- Rough blank: a deliberately oversize piece for your milling process.
- Finished part: the dimension after flattening, trimming and fitting.
- Assembled envelope: the space the completed object occupies.
Do not silently replace one category with another in a cut list. If a plan calls for a finished 19 mm part, buying rough stock whose thinnest place is already 19 mm leaves no material for flattening. Conversely, increasing every part “for safety” without updating the drawing changes the joints and overall size.
Check the hardware before committing the batch
Measure a representative offcut and build one joint sample. Fit the actual hardware to it. Drawer runners, inserts and connectors often care about thickness in a more specific way than a shelf label does. Write the tested dimension on the sample and keep it beside the cutting list.
If you use both inches and millimeters, choose one working unit for each drawing. Convert once, keep enough precision until the final measurement, and do not round every intermediate number independently. Three times a rounded panel thickness is not always the same as a rounded total.
For repeated parts, record the settled dimension with a story stick or physical sample. For purchasing, keep volume calculations separate: a board-foot quotation describes a tally basis, not a promise that a nominal 1-inch board will finish at exactly 1 inch.
When the nominal-to-actual chart does not apply
| What you bought | What the label means | What to measure |
|---|---|---|
| Rough hardwood, such as 4/4 maple | A rough thickness class, not a guaranteed finished 1-inch part. | The usable thickness after flattening and planing. |
| Plywood or other sheet goods | A nominal thickness can differ from the panel in your hands. | Several points on the actual sheet; fit a test dado. |
| Green or specialty softwood | Moisture condition and product rules can change dressed dimensions. | Thickness and width now, plus the requirements for the intended use. |
| A plan written for metric stock | An 18 mm panel is not automatically a 3/4-inch panel. | Recalculate joints and overall dimensions with one consistent unit. |
Why is a 2×4 smaller than 2 by 4 inches?
The nominal name is the trade size; drying and surfacing produce the familiar dressed size. For a common dry dressed 2×4, use 1½ × 3½ inches as the reference, then confirm the board in hand. Do not apply a blanket “subtract half an inch” rule: a nominal 2×8 commonly has a 7¼-inch width.
Which way round are lumber dimensions written?
In this chart, thickness comes first and width second. Length is separate. A 2×4×8-ft board is not eight inches long, and an eight-foot label is not a substitute for checking whether both ends need trimming.
Before you copy a cut list
- Write the nominal purchasing size and the finished part size separately.
- Measure thickness and width at several places.
- Recalculate between-sides parts and groove engagement with the measured stock.
- Test one joint and the real hardware before cutting the batch.
- Keep one working unit through the calculation; round at the end.
Sources and specifications
Prepared for Unfinished Wood under an editorial pen name, with AI-assisted drafting and source review. Examples and diagrams are explanatory; they are not reports of hands-on product testing.
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