詳細ガイド 近日公開
Lumber Dimension Converterの包括的な教育ガイドを準備中です。ステップバイステップの解説、数式、実例、専門家のヒントをお届けしますので、もうしばらくお待ちください。
The Lumber Dimension Converter handles the confusing reality that lumber labeled '2×4' is not actually 2 inches × 4 inches — the actual dimensions are 1.5 × 3.5 inches. This nominal vs actual difference (a residual of historical milling practices before kiln drying became standard) catches every beginner woodworker and DIYer. The calculator converts nominal sizes (1×2 through 6×6) to actual inches and millimeters, calculates board feet (BF, the standard lumber volume unit), converts length units (feet, inches, meters, centimeters), and computes total project costs based on price per board foot. The nominal-vs-actual lumber sizing system originated in the early 1900s when rough-sawn green wood was sold at its true dimensions. As kiln drying and finish planing became standard processes through the 1920s-1950s, wood shrunk during drying and lost dimension during planing — but the industry kept the original 'nominal' name for tradition, supply chain consistency, and marketing simplicity. A 2×4 was always called a 2×4 even after milling reduced it to 1.5×3.5 inches. The American Softwood Lumber Standard PS 20 formalizes these nominal vs actual relationships used across North American construction. Board feet (BF) is the standard volume unit in North American lumber commerce. One board foot equals 144 cubic inches (12″×12″×1″) or about 0.00236 cubic meters. Critically, board feet calculation uses NOMINAL dimensions, not actual — this is the convention regardless of physical accuracy. A 2×4×8 piece equals (2×4×8)/12 = 5.33 board feet, even though its actual volume is only (1.5×3.5×96)/144 ≈ 3.5 BF. This convention persists because traditional sawmill output and pricing has been based on nominal volumes since the 1800s. This calculator handles the practical reality of buying and building with lumber. Select nominal size from 17 common sizes (1×2 through 6×6), enter length and unit, quantity, and optional price per board foot. The calculator outputs total board feet, actual dimensions in inches and millimeters, length converted across all common units, and total cost. Use for: woodworking project planning, construction estimation, comparing pricing across retailers (always in board feet), and understanding what lumber will actually fit your design constraints.
Board Feet (BF) = (Nominal Thickness × Nominal Width × Length in feet) ÷ 12; Actual dimensions per US softwood standard table
- 1Step 1 — Select Nominal Lumber Size: Choose from dropdown of 17 common North American softwood lumber sizes: 1×2, 1×3, 1×4, 1×6, 1×8, 1×10, 1×12 (1-inch nominal thickness boards) and 2×2, 2×3, 2×4 (most common), 2×6, 2×8, 2×10, 2×12 (2-inch nominal framing lumber) plus 4×4, 4×6, 6×6 (timber/posts).
- 2Step 2 — Enter Length and Unit: Lumber length comes in standard 8, 10, 12, 14, 16 foot lengths (sometimes 6 or 20 foot). Enter your needed length and select unit. Calculator converts feet, inches, meters, or centimeters internally to feet for board feet calculation.
- 3Step 3 — Enter Quantity: How many pieces you need. Quantity is multiplied for total board feet and total cost.
- 4Step 4 — Enter Price per Board Foot: Optional but useful. Standard 2×4 SPF pine: $3-5/BF. Premium hardwood (oak, maple): $5-15/BF. Exotic hardwood (walnut, cherry): $10-30/BF. Reclaimed lumber: $4-12/BF. Check current local pricing.
- 5Step 5 — Calculator Outputs Board Feet: Board Feet per piece = (Nominal Thickness × Nominal Width × Length in feet) ÷ 12. For 2×4×8: (2×4×8)/12 = 5.33 BF. Total BF = per piece × quantity.
- 6Step 6 — Review Actual Dimensions: Calculator displays actual inches and actual millimeters. Critical for joinery planning — your 'two-by-four' shelf bracket actually needs to clear 1.5×3.5″ structural members, not 2×4.
- 7Step 7 — Total Cost Calculation: If price per BF entered, total cost = Total BF × Price/BF. Use for comparing retailers — always compare price per BF since lumber pricing varies by length, grade, and species.
Common quick-project quantity for framing or basic shelf
Standard framing project quantity. Each 2×4×8 contains 5.33 board feet (nominal calculation). At $3.50/BF (typical SPF pine pricing), total $93.33 for 5 boards. Actual physical dimensions per piece: 1.5″ × 3.5″ × 96″ (or 38 × 89 × 2438 mm). Plan your design around the actual dimensions, not nominal — bookshelf depths, cabinet face frames all depend on actual measurements.
Standard shelving and trim board
1×6 boards (actually 0.75″ × 5.5″) are common for open shelving, picture rails, baseboards, and trim work. 10-foot length is standard but they also come in 8 and 12 foot. Board feet = (1 × 6 × 10) / 12 = 5 BF. Note: shelf depth designs should use the 5.5″ actual dimension, not the 6″ nominal — many beginners design around 6″ depths only to discover their wood is shy ½″.
Standard fence post and structural post material
4×4 posts (actually 3.5″ × 3.5″) are the universal post material for fences, decks, pergolas, and small structures. 8-foot length is standard for fence posts with 2 feet buried in ground. Board feet = (4 × 4 × 8)/12 = 10.67 BF. Cedar or pressure-treated typically. Note actual cross-section is exactly the same as nominal 4×4 framing studs — useful for structural connection compatibility.
Metric length converted to imperial for board feet calculation
When working with metric lengths (common in Canada and most international locations), calculator handles conversion. 2.4 meters = 7.87 feet. Board feet still calculated using imperial convention. Useful for international shipping cost estimation or working from metric drawings with North American lumber sourcing.
Construction project estimating — knowing real lumber dimensions for layout and structural planning
Furniture making — calculating material requirements with accurate joinery measurements
Cost estimation for woodworking projects across multiple species and grades
DIY home improvement — fence building, deck construction, basic framing repairs
Comparing lumber prices across retailers using standardized board feet pricing
| Nominal | Actual (inches) | Actual (mm) | Common Use |
|---|---|---|---|
| 1×2 | 0.75 × 1.5 | 19 × 38 | Trim, edging, stir sticks |
| 1×4 | 0.75 × 3.5 | 19 × 89 | Shelving, trim, basic framing |
| 1×6 | 0.75 × 5.5 | 19 × 140 | Shelving, fence boards, siding |
| 1×8 | 0.75 × 7.25 | 19 × 184 | Wide shelving, casework |
| 1×12 | 0.75 × 11.25 | 19 × 286 | Stair treads, shelving |
| 2×2 | 1.5 × 1.5 | 38 × 38 | Furniture frames, hand rails |
| 2×4 | 1.5 × 3.5 | 38 × 89 | Wall studs, framing |
| 2×6 | 1.5 × 5.5 | 38 × 140 | Floor joists, wall plates |
| 2×8 | 1.5 × 7.25 | 38 × 184 | Floor joists, headers |
| 2×10 | 1.5 × 9.25 | 38 × 235 | Larger floor joists, headers |
| 2×12 | 1.5 × 11.25 | 38 × 286 | Deep beams, large headers |
| 4×4 | 3.5 × 3.5 | 89 × 89 | Posts, decks, fences |
| 4×6 | 3.5 × 5.5 | 89 × 140 | Beams, larger posts |
| 6×6 | 5.5 × 5.5 | 140 × 140 | Heavy posts, timber framing |
Why is 2×4 actually 1.5×3.5 inches?
Original 2×4 was rough-sawn from green wood at exactly 2×4 inches. Drying shrinks wood by about 1/8 inch per inch of width, and surfacing (planing for smooth surfaces) removes another ~¼ inch per side. By the time lumber reaches retail, dimensions are reduced to 1.5×3.5 inches. The industry kept the original 'nominal' name for tradition, marketing simplicity, and supply chain consistency. The American Softwood Lumber Standard PS 20 formalizes these nominal vs actual relationships.
Is there a metric equivalent?
Yes. Most US 2×4s are sold as 38mm × 89mm internationally — these are the actual measurements. Canada uses the same imperial-based actual dimensions but often labels in millimeters. Some European countries use slightly different sizing (45 × 95mm or 50 × 100mm). When importing/exporting lumber, the metric measurement is the actual size, not nominal — there's no European equivalent of '2×4' labeling.
Why use board feet instead of cubic feet?
Board feet measures 'square foot of 1-inch lumber' which matches how lumber is sold historically. 1 BF = 144 cubic inches = 1' × 1' × 1". This is intuitive for woodworkers buying boards — visualize a 1-foot square of 1-inch thick wood. Cubic feet (1728 cubic inches) would require dividing by 12 mentally for thinking about typical lumber. Board feet has persisted because it matches actual lumber commerce since the 1800s.
Do board feet calculations use nominal or actual?
ALWAYS nominal. Industry convention universally uses nominal dimensions in board feet calculations regardless of physical accuracy. A 2×4×8 is always 5.33 BF in commerce even though actual volume is closer to 3.5 BF. When pricing lumber, you're paying for the nominal volume. This convention is firmly established and not changing — accept and use it.
Why are some grades cheaper than others?
Lumber grades indicate appearance and structural integrity. Common softwood grades: Select (top, knot-free, premium pricing), #1 Common (some small knots, mid-tier), #2 Common (typical framing, knots and minor defects, lowest framing grade), Stud (specifically for vertical wall studs, may have crook tolerance). Hardwood grades: FAS (First and Seconds, top), Select, #1 Common. Higher grades cost 20-100% more for cosmetic improvement.
What's the difference between dimensional and hardwood lumber?
Dimensional lumber (softwoods like pine, fir, spruce, cedar) is sold by nominal sizes (2×4, etc.) at standard lengths (8, 10, 12, 14, 16 feet). Hardwood lumber (oak, maple, walnut, cherry) is typically sold by exact dimensions in board feet — a hardwood plank might be 4/4 (1 inch nominal thickness), 6/4 (1.5 inches), 8/4 (2 inches), at random widths and lengths. Hardwood sold as 'quarter sawn' or 'plain sawn' to indicate grain orientation.
How do I plan a project knowing nominal vs actual?
Always design around actual dimensions, not nominal. For example: A bookshelf with 8-inch deep shelves should use 1×10 lumber (actual 9.25 inches deep, allows room) not 1×8 (actual 7.25 inches). For 24-inch interior width between studs, plan for 22.5 inches actual interior (24 inch on-center stud spacing minus 1.5 inch stud thickness). Mistakenly using nominal for design causes painful discoveries during construction when measurements don't fit.
プロのヒント
For tight-fitting joinery, measure actual lumber dimensions, not nominal. A 2×4 dado joint planned for 2-inch depth will only need 1.5-inch dado — adjusting calculations to actual dimensions prevents misfitting joints and wasted material. Always plan from actual dimensions; label communications by nominal (since that's how lumber is purchased and labeled at retailers).
ご存知でしたか？
The 'inch' difference between nominal and actual lumber dimensions wasn't formalized until 1964 when the American Softwood Lumber Standard PS 20 was first published. Before that, regional variation existed — Pacific Northwest mills produced slightly different actual dimensions than Southern Pine mills. The standardization was driven by mass production housing (post-WWII suburb construction) requiring consistent material sizing across regions. The 'shrinkage' allowance was calibrated to typical kiln-drying losses plus standard planing reductions, but the math is approximate — actual dimensions can still vary by 1/16 inch between mills and lots.