Nominal vs Actual Lumber Dimensions — 2×4, 2×6, 2×8, 2×10, 2×12 Sizes Explained
Every new woodworker asks the same question within their first trip to a lumber yard: "Why is a 2×4 not 2 inches by 4 inches?" The answer lies in PS 20-25, the American Softwood Lumber Standard — and misunderstanding this one fact causes more board-foot calculation errors, more lumber over-orders, and more confusion at the checkout counter than any other single detail in all of lumber estimating. This guide maps the full nominal-to-actual conversion table, explains why both systems coexist, shows how each changes your board foot total, and gives you a simple decision tree for which basis to use at your next purchase.
Why does nominal lumber not match its name?
The nominal size — 2×4, 2×6, 4×4, and so on — is the size the log is first cut to at the sawmill. At that stage the wood is green, meaning it still contains moisture from the living tree (typically 25–30% moisture content for most North American softwoods). Freshly cut boards at nominal dimensions are rough, uneven, and far too wet to use in construction — they will shrink as they dry, warp if not controlled, and cannot be joined with tolerances for framing, sheathing, or trim.
After sawing, softwood lumber is kiln-dried to bring its moisture content down to 19% or less (PS 20-25 dry specification) and then planed smooth on all four sides. Planing removes a consistent amount of material from each face — typically 3/8" total on thickness and 3/8" total on width, or about 3/16" per face. That is where the dressed (actual) size comes from: a nominal 2×4 starts at exactly 2.000" × 4.000" rough-sawn, then loses 0.125" per face during planing and finishes at 1.500" × 3.500" dry. This is the size you measure when you hold a caliper to a board at the lumber yard.
💡 Key takeaway: The gap between nominal and actual is not a mistake, not a typo, and not a scheme to sell less wood. It is a century-old standardization that lets architects, framers, sawmills, and lumber yards all speak the same language. Every PS 20-25 compliant board of nominal size X×Y will dress to the same actual size X'×Y' anywhere in North America.
Softwood vs hardwood — why only softwood uses nominal
The nominal system exists almost exclusively for North American softwood construction lumber. Hardwood is a completely different story — hardwood dealers sell by actual dressed dimensions, and thickness uses the "quarter" system (4/4 = 1", 5/4 = 1¼", 6/4 = 1½", 8/4 = 2"). A 4/4 cherry board is always 1" thick and whatever width you order, and its board-foot tally is calculated from those actual dimensions. Hardwood never uses nominal sizing — the PS standards for hardwood exist but they do not create a nominal-to-dressed gap the way the softwood standard does.
Why the two conventions? Softwood construction lumber is a commodity — millions of board feet of SPF, Douglas Fir, and Red Cedar move through the supply chain every year, and a universal pricing shorthand (nominal size × board feet) keeps everyone on the same page. Hardwood is sold in smaller volumes, often kiln-dried and surfaced two sides at the mill, with pricing tied directly to the physical dimensions you receive. There is no need for an intermediate "nominal" layer when the dressed board is the product being bought and sold.
How nominal vs actual changes your board foot total
Here is where the gap matters financially. The board foot formula is BF = (T × W × L) ÷ 12, where T and W are in inches and L is in feet. If you plug in nominal dimensions you get the billed volume. If you plug in actual dimensions you get the physical volume. For a single 2×4×8, these two numbers are:
Nominal basis (billing convention)
(2 × 4 × 8) ÷ 12 = 5.33 BF
This is what your supplier will quote
Actual basis (physical volume)
(1.5 × 3.5 × 8) ÷ 12 = 3.50 BF
This is the real wood you receive
That is a 52% difference — your supplier will bill you 5.33 BF for a 2×4×8, but the physical board only contains 3.50 BF of actual wood. This is not fraud. This is the convention. Softwood pricing is by the nominal board foot, and every supplier on the continent uses it the same way. The "extra" 1.83 BF you pay for is the historical pricing convention that keeps millions of commodity softwood transactions standardized.
When hardwood dealers quote board feet, they use actual dimensions — a 1.500" × 3.500" × 8' hardwood board gives exactly 3.50 BF and costs accordingly. This is why hardwood board-foot prices look so much higher than softwood prices at first glance — you are comparing apples to oranges unless you normalize for the dimension basis.
PS 20-25 nominal-to-actual conversion table
Every common softwood construction size under the current PS 20-25 standard, with actual dressed (dry) dimensions, nominal board feet at 8 ft, and actual board feet at 8 ft. The dressed sizes are for kiln-dried lumber at ≤19% moisture content. Green (unseasoned) lumber is slightly larger — roughly 1/16" more on each face — but board-foot calculations always use nominal regardless.
| Nominal size | Actual dressed (dry) | Nominal BF @ 8 ft | Actual BF @ 8 ft |
|---|---|---|---|
| 1×4 | 0.75" × 3.50" | 2.67 BF | 1.75 BF |
| 1×6 | 0.75" × 5.50" | 4.00 BF | 2.75 BF |
| 1×8 | 0.75" × 7.25" | 5.33 BF | 3.63 BF |
| 1×10 | 0.75" × 9.25" | 6.67 BF | 4.63 BF |
| 1×12 | 0.75" × 11.25" | 8.00 BF | 5.63 BF |
| 2×2 | 1.50" × 1.50" | 2.67 BF | 1.50 BF |
| 2×3 | 1.50" × 2.50" | 4.00 BF | 2.50 BF |
| 2×4 | 1.50" × 3.50" | 5.33 BF | 3.50 BF |
| 2×6 | 1.50" × 5.50" | 8.00 BF | 5.50 BF |
| 2×8 | 1.50" × 7.25" | 10.67 BF | 7.25 BF |
| 2×10 | 1.50" × 9.25" | 13.33 BF | 9.25 BF |
| 2×12 | 1.50" × 11.25" | 16.00 BF | 11.25 BF |
| 4×4 | 3.50" × 3.50" | 10.67 BF | 8.17 BF |
| 4×6 | 3.50" × 5.50" | 16.00 BF | 12.83 BF |
| 6×6 | 5.50" × 5.50" | 24.00 BF | 20.17 BF |
Source: PS 20-25 American Softwood Lumber Standard (2025 edition), American Lumber Standards Committee. Data cross-referenced with U.S. Forest Products Laboratory Wood Handbook, Chapter 1.
Board feet per linear foot — nominal vs actual side by side
Another way to see the gap: how many board feet you accumulate for every linear foot you buy. For softwood framing lumber, the nominal BF per LF is always higher than the actual physical BF per LF — and the bigger the board, the larger the absolute gap.
| Size | Nominal BF / LF | Actual BF / LF | Gap |
|---|---|---|---|
| 1×4 | 0.333 | 0.219 | +52% |
| 1×6 | 0.500 | 0.344 | +45% |
| 2×4 | 0.667 | 0.438 | +52% |
| 2×6 | 1.000 | 0.688 | +45% |
| 2×8 | 1.333 | 0.906 | +47% |
| 2×10 | 1.667 | 1.156 | +44% |
| 2×12 | 2.000 | 1.406 | +42% |
| 4×4 | 1.333 | 1.021 | +31% |
| 4×6 | 2.000 | 1.604 | +25% |
| 6×6 | 3.000 | 2.521 | +19% |
Decision tree: nominal or actual?
This is the question we get more than any other in the lumber calculator. Here is the simple rule set that covers 99% of purchases:
❓ Are you buying SPF, Douglas Fir, Red Cedar, or any construction softwood?
Use NOMINAL dimensions. Every big-box store and commodity lumber yard prices softwood framing lumber by the nominal board foot. Your estimate should match their invoice.
❓ Are you buying hardwood (Oak, Maple, Cherry, Walnut) from a hardwood dealer?
Use ACTUAL dimensions. Hardwood is sold by dressed size — a 4/4 board is 1" thick and that's what you plug into the formula.
❓ Are you buying rough-sawn lumber from a local sawmill?
Ask the mill. Some rough-sawn mills price by actual rough dimensions, others have their own board-foot convention. If you cannot confirm, measure the boards with a caliper and use actual.
❓ Is this a takeoff for a building permit or structural drawing?
Use actual dimensions for physical fit calculations (will a 2×6 actually fit in a 5.5" gap?). Use nominal for material takeoff board-foot tallies.
❓ Do you need the true physical volume (for weight, container loading, or cubic feet conversion)?
Always use ACTUAL dimensions. The physical wood in a 2×4×8 is 3.50 BF, not 5.33 BF, when you need real volume.
Five common misconceptions about lumber dimensions
- "A 2×4 is 2 inches by 4 inches." No — this is only true when the board is freshly sawn and wet. Every dried and planed 2×4 in North America measures 1½" × 3½" per PS 20-25. If you see a board described as 2" × 4", it means the NOMINAL size, not the physical size.
- "Lumber yards are short-changing me." No. The nominal-to-dressed gap is intentional and standardized across the entire supply chain. You are not getting less wood — you are getting the wood you paid for, under a naming convention that describes the rough-sawn starting size rather than the finished product.
- "I can use either basis, the calculator will fix it." Partially true — the board foot converter has a toggle, but you still need to know which basis your supplier uses. If you estimate 100 BF in nominal but your hardwood supplier quotes 100 BF in actual, you will end up paying 52% more wood than you planned.
- "Hardwood also uses nominal sizes." No. Hardwood dealers work exclusively in actual dressed dimensions. A 4/4 cherry board is 1" thick, period. There is no "nominal 1" that dresses to something smaller. The quarter-inch system (4/4, 5/4, 6/4, 8/4) is already the dressed thickness.
- "Green lumber is the right size." Green (unseasoned) softwood is slightly closer to nominal — a 2×4 green measures about 1 9/16" × 3 9/16" instead of 1½" × 3½" dry. But green lumber is not suitable for construction and will shrink as it dries. All structural lumber you buy at a yard is dry, and all PS 20-25 sizes assume dry lumber.
Frequently asked questions
Why is a 2×4 not actually 2×4?+
Softwood lumber is rough-sawn at nominal sizes (2" × 4") and then kiln-dried and planed (dressed) to remove moisture and surface roughness. The standard dressed size for a 2×4 is 1½" × 3½" dry, established by PS 20-25, the American Softwood Lumber Standard. The gap accounts for about ¼" per face lost to planing.
Should I use nominal or actual dimensions for board foot calculations?+
Use nominal dimensions (2×4 as 2" × 4") when calculating board feet for softwood pricing — that is what suppliers quote by. Use actual dressed dimensions (1½" × 3½") for hardwood, rough lumber, or when you need true physical volume. If your supplier gives you a board-foot total, ask which basis they used — some mills mix them.
Does this apply to hardwood too?+
No. Hardwood lumber is almost always sold by actual (dressed) dimensions — a 4/4 hardwood board is 1" thick and whatever width you buy. Hardwood dealers work with dressed sizes throughout, so board-foot tallies and pricing use actual dimensions. The nominal system exists almost exclusively for North American softwood under PS 20-25.
What does PS 20-25 mean?+
PS 20-25 is the American Softwood Lumber Standard, updated periodically (2025 is the current edition). It establishes standard nominal and dressed sizes for all construction softwood species sold in the U.S. and Canada, along with grade rules, moisture content requirements, and marking standards. Every dimension lumber yard in North America follows this standard.
How much moisture content affects this?+
PS 20-25 specifies dressed sizes at ≤19% moisture content (dry lumber). Green (unseasoned) softwood is slightly larger — a 2×4 green measures 1 9/16" × 3 9/16" instead of 1½" × 3½" dry. Board-foot calculations always use nominal regardless of moisture; only the physical fit and weight change.
Is there a difference between 2×4 and 4/4 lumber?+
Yes. 2×4 is a nominal-dressed size under PS 20-25 with a standard dressed size of 1½" × 3½". 4/4 (say "four-quarter") is a hardwood thickness designation meaning 1" nominal thickness — hardwood uses the quarter-inch system (4/4 = 1", 5/4 = 1¼", 6/4 = 1½", 8/4 = 2") and is sold by actual dimensions.
Why does the lumber calculator have a Nominal/Actual toggle?+
Because the same physical board produces two different board-foot totals depending on which dimension basis you use. A 2×4×8 = 5.33 BF nominal but only 3.50 BF actual. The toggle lets you match your supplier's convention — most softwood suppliers use nominal, hardwood dealers use actual.
Put this into practice with the calculator
Now that you understand the nominal vs actual lumber dimensions gap and which basis your supplier uses, open the lumber calculator on the homepage and select the Nominal or Actual toggle to match your purchase. For a dedicated board-foot conversion tool with both bases side by side, try the Board Foot Converter. And for more foundation reading on how board feet actually work, browse the full guide library.
More guides you might need
Board Foot Prices by Species (2026 National Averages + What Actually Changes Them)
Reference prices for 12+ common lumber species per board foot — SPF $3.50, Oak $7.50, Walnut $14.00. Plus regional factors, grade impact, moisture content, and how to sanity-check local quotes.
Board Feet to Cubic Feet (and Cubic Meters): Complete Conversion Guide
One board foot equals 1/12 cubic foot = 144 cubic inches. Learn the math, the metric conversion (1 BF = 0.00236 m³), and when each unit is used in the lumber industry.
What Is a Board Foot? Definition, Formula, and How It Actually Works
A board foot is 144 cubic inches of lumber — here's the formula, why nominal vs actual sizes matter, and how board feet differ from linear and square feet.
Article last updated November 5, 2026 · Data sourced from PS 20-25 American Softwood Lumber Standard (2025 edition) and U.S. Forest Products Laboratory Wood Handbook.