Nominal vs actual lumber size
Every lumber size you say out loud is wrong by a known amount. A 2×4 is 1½ × 3½ inches. A 1×6 deck board is ¾ × 5½. This is not a trick or a shrinking package — it is a standard, and it costs people money in exactly three places.
The table
| Nominal | Actual (inches) | Actual (mm) | Common use |
|---|---|---|---|
| 1×2 | ¾ × 1½ | 19 × 38 | Trim, lattice frame |
| 1×4 | ¾ × 3½ | 19 × 89 | Fascia, trim |
| 1×6 | ¾ × 5½ | 19 × 140 | Siding, deck board (5/4 is thicker) |
| 5/4×6 | 1 × 5½ | 25 × 140 | Deck board, standard |
| 2×2 | 1½ × 1½ | 38 × 38 | Balusters, lattice |
| 2×4 | 1½ × 3½ | 38 × 89 | Studs, rails, blocking |
| 2×6 | 1½ × 5½ | 38 × 140 | Joists, deck boards |
| 2×8 | 1½ × 7¼ | 38 × 184 | Joists, ledger |
| 2×10 | 1½ × 9¼ | 38 × 235 | Joists, beams |
| 2×12 | 1½ × 11¼ | 38 × 286 | Beams, stair stringers |
| 4×4 | 3½ × 3½ | 89 × 89 | Posts, rail posts |
| 4×6 | 3½ × 5½ | 89 × 140 | Beams, posts |
| 6×6 | 5½ × 5½ | 140 × 140 | Heavy posts, pergola |
| 8×8 | 7¼ × 7¼ | 184 × 184 | Timber posts |
The rule: below 2 inches nominal, subtract ¼ inch; from 2 to 6 inches, subtract ½ inch; above 6 inches, subtract ¾ inch. It holds for practically all softwood dimensional lumber sold in North America.
Why the sizes are smaller than the name
Rough-sawn lumber was once sold at full dimension and planed on site. As mills began surfacing boards before shipping, they kept the nominal name and reduced the dressed size so a 2×4 would still plane down from a 2×4 blank. The standard was formalised in the 1960s and has not changed since, which is why your grandfather's 2×4 really was two inches by four.
This matters when you are matching old work. A deck framed in 1955 has full-dimension joists; new material spliced onto it sits proud by half an inch.
The three places it costs money
- Deck board count. A 12 ft wide deck holds 26 boards at 5½ inches plus a ⅛ gap — not 24. Using 6 inches gives you 24 and a short deck.
- Baluster spacing. A 2×2 is 1½ inches, which changes the number of gaps per bay. Calculators that assume 2 inches produce layouts that fail the 4″ sphere rule.
- Stair stringers. A 2×12 gives 11¼ inches of depth, not 12, and the notch removes the riser height from that. The margin is ¾ inch thinner than the drawing suggests.
There is a fourth, quieter one: board feet. Lumber priced by volume is computed from nominal dimensions in most retail pricing, which is why a "1×6" board foot calculation uses 1 × 6, not ¾ × 5½. See the board foot calculator for that convention.
What this page does not cover
- Engineered lumber (LVL, I-joists, glulam), which has its own depth conventions.
- Hardwood, which is sold by quarters of an inch in rough thickness (4/4 = 1 inch rough, 13/16 surfaced).
- Metric equivalents beyond the table above — nominal sizes do not convert cleanly and should not be substituted.
- Timber above 8×8, where the surfacing allowance changes again.
Questions people actually ask
Why is a 2x4 not 2 inches by 4 inches?
Because it is planed down from a rough 2×4 blank. The finished size is 1½ × 3½ inches. The nominal name was kept for continuity when mills started surfacing lumber before sale.
What is the actual size of a 5/4 deck board?
One inch thick by 5½ inches wide. The 5/4 refers to quarters of an inch of rough thickness; after surfacing it is a full inch, which is thicker than a 1× board at ¾ inch.
How do I convert nominal to actual size?
Subtract ¼ inch below 2 inches nominal, ½ inch between 2 and 6 inches, and ¾ inch above 6 inches. So 1×6 becomes ¾ × 5½, and 2×10 becomes 1½ × 9¼.
Is a 4x4 post really 4 inches?
No, it is 3½ × 3½ inches. That half inch matters for baluster spacing and for post bases, which are sized to the actual dimension.