Why is a 2x4 not actually 2 inches by 4 inches?

Because lumber is dressed (S4S — surfaced four sides) after milling. The rough-sawn 2×4 comes off the mill at 2″ × 4″, but planing both faces loses 0.25″ each surface, so dressed actual = 1.5″ × 3.5″. This standard hasn't changed since the early 1900s (American Lumber Standards Committee). Rough-sawn lumber still matches nominal (2×4 = 2″ × 4″ actual) and is available for traditional + heavy-timber framing, but big-box stores only sell dressed.

What is the actual size of a 2x6 board?

Dressed 2×6 = 1.5″ × 5.5″ actual (38 mm × 140 mm). Rough-sawn 2×6 = 2″ × 6″ actual (51 × 152 mm). Big-box stores (Home Depot, Lowe's) sell dressed only. Lumber yards may stock rough on request. The dressed dimension is what matters for framing calcs: joist span tables, blocking math, and load capacity all use the dressed actual dimension, not the nominal.

How do I calculate board feet?

Board feet = (nominal thickness × nominal width × length in ft) ÷ 144. Uses NOMINAL not actual. Examples: 2×4 × 8 ft = (2 × 4 × 8) / 144 = 0.444 bf. 2×8 × 12 ft = 1.333 bf. 2×10 × 16 ft = 2.222 bf. 4×4 × 8 ft = 0.889 bf. 6×6 × 10 ft = 2.5 bf. Lumber yards price by $/bf — multiplying your bf count by the species rate gives you total cost.

How much does a 2x10 weigh?

Depends on species + length. A 2×10 × 12 ft (1.667 bf): PT pine = 5.7 lb (1.667 × 3.4). Douglas fir = 5.2 lb. SPF = 4.5 lb. Cedar = 3.7 lb. IPE = 9.7 lb (1.667 × 5.8). Cumaru = 9.0 lb. Tigerwood = 8.7 lb. For wet PT pine fresh from the lumberyard (treated with copper-based preservative), add 30-50% — wet PT can hit 8-9 lb on a 12 ft 2×10.

What's the actual size of a 4x4 post?

Dressed 4×4 = 3.5″ × 3.5″ actual (89 mm × 89 mm). Rough-sawn 4×4 = 4″ × 4″ actual. Most deck posts at big-box stores are dressed 4×4 PT pine. For taller posts (over 7 ft span) or load-bearing, use 6×6 (actual 5.5″ × 5.5″) for adequate stiffness. The IRC R507.4 deck-post sizing rules use the actual dressed dimensions when checking adequacy.

How do I convert lumber dimensions to metric?

Inches to mm: multiply by 25.4. Inches to cm: multiply by 2.54. Feet to meters: multiply by 0.3048. Examples: 1.5″ = 38.1 mm = 3.81 cm. 5.5″ = 139.7 mm = 13.97 cm. 12 ft = 3.658 m. The calculator surfaces all three units automatically.

What's the difference between dressed and rough-sawn lumber?

Dressed (S4S = Surfaced Four Sides) has been planed smooth on all four sides after rough milling. Loses ~0.25-0.5″ per surface from the planing. Standard at big-box stores. Used for finish carpentry, deck framing, most residential construction. Rough-sawn comes straight off the saw — matches nominal exactly. Used for traditional barns, timber-frame construction, rustic aesthetic. More expensive (less common) and rougher to the touch.

Is IPE really 2.5× heavier than cedar?

Yes — IPE specific gravity 0.92 vs cedar 0.35 (also 2.6×). Per board foot at 12% MC: IPE 5.8 lb/bf vs cedar 2.2 lb/bf. Practical impact: a 5/4×6 × 12 ft IPE deck board weighs 8.7 lb vs same cedar 3.3 lb. A 192 sqft deck (~120 boards) in IPE = 1,040 lb of decking vs 400 lb in cedar. The heavier IPE means more deck dead-load on framing — use the Deck Load Calculator to confirm framing handles it.

What length is most common for deck lumber?

12 ft is the deck-framing sweet spot (covers most joist spans + minimizes cuts). 8 ft used for short joists. 10, 14, 16 ft for longer spans. 20 ft available for premium decks + composite boards but adds 30-50% cost premium. 16 ft is the universal max for composite decks (Trex/TimberTech/Fiberon all stock at 12, 16, 20 ft). For board lengths over 20 ft, splice mid-joist with a butt joint + flashing.

Why is PT pine heavier than the chart says?

Pressure-treated pine absorbs water + preservative chemicals during the treatment process. Fresh-from-supplier PT pine can be 30-50% heavier than the dried-12%-MC chart values (4.5-5.0 lb/bf vs the 3.4 chart). As PT dries over 4-8 weeks after install, weight drops to chart values. For truck load math + crane lifts, use the WET PT figure. For finished-deck dead-load math, use the dried figure.

Utility · nominal-to-actual + board feet + weight

Lumber Dimensions Calculator

Converts 14 nominal lumber sizes (1×4 through 8×8) to actual dressed dimensions in inches + mm + cm, plus board feet, species-specific weight (lb + kg), and cost at reference $/bf pricing. The killer feature: it surfaces ALL the lumber math in one place — most users guess at nominal-vs-actual (2×4 is actually 1.5″ × 3.5″, not 2″ × 4″), at board-feet calculations (uses nominal not actual, by US convention), and at weight estimates (IPE is 2.5× heavier than cedar at same dimensions). 9 species head-to-head: PT pine, Douglas fir, SPF, Hem-fir, redwood, western red cedar, IPE, cumaru, tigerwood. Dressed (S4S) vs rough-sawn toggle — rough matches nominal exactly. Metric conversion (mm + cm + m) for international users.

14 sizes9 speciesmm + cm + inBoard feetWeight + cost

14·Nominal sizes

9·Species

lb + kg·Weight math

$/bf·Reference cost

Inputs

Nominal size14 standard sizes

Lengthft

Quantitypieces

Speciesdrives weight + cost

Conditiondressed (big-box) vs rough-sawn (lumber yard)

Dressed (S4S) = surfaced 4 sides — what big-box stores sell. Actual size loses 1/4-1/2″ per surface. Rough-sawn = matches nominal exactly (2×4 = 2″ × 4″).

Lumber · 2x8 Pressure-treated SYP (Southern Yellow Pine)

1.5″ × 7.25″38.1 × 184.15 mm · dressed (S4S)

nominal 2″ × 8″12 ft × 1 pcs1.333 bf total

Board feet each

1.333 bf total

Weight each

2.06 kg · 4.53 lb total

Cost each

$1.85/bf

Total cost

1 pcs reference price

Dimensions in all units

Nominal (US convention)

2″ × 8″

Length

12 ft · 3.658 m · 3658 mm

Actual (in)

1.5″ × 7.25″

Actual (mm)

38.1 × 184.15

Actual (cm)

3.81 × 18.42

Volume

1566 in³ each

Advisories

2x8 Pressure-treated SYP (Southern Yellow Pine) — dressed (S4S)

WCLIB + SPIB grading rules

Nominal 2″ × 8″ → actual 1.5″ × 7.25″ (38.1 × 184.15 mm). Standard big-box dimensional reduction applied (planed both surfaces).

Pressure-treated SYP (Southern Yellow Pine)

Most common deck framing in eastern US. Treated with copper-based preservative. Heavier than untreated (chemical absorption). 3.4 lb/bf typical dried.

Dressed dimensions per WCLIB + SPIB grading rules. Species weight per USDA Wood Handbook (FPL-GTR-190) at 12% MC. PT pine weight is dry; wet (fresh from supplier) can be 30-50% heavier. Reference $/bf pricing varies regionally — confirm with local supplier.

How to use

Three steps. Permit-ready output.

Pick nominal lumber size

14 standard sizes: 1×4, 1×6, 1×8, 1×10, 1×12 (decking + trim). 2×4, 2×6, 2×8, 2×10, 2×12 (framing — joists, beams, blocking). 4×4, 4×6, 6×6, 8×8 (posts + heavy beams). Nominal is what you order; actual is what you get. Calculator surfaces both.

Enter length

Standard lumber lengths: 8, 10, 12, 14, 16, 20 ft. Custom lengths require special order at +30-50% premium. For deck framing, use the length your span calc recommends — overspanning is the #1 cause of bouncy/saggy decks.

Pick species (drives weight + cost)

PT pine = $1.85/bf, 3.4 lb/bf — standard eastern deck framing. Douglas fir = $2.10, 3.1 — West Coast structural standard. SPF = $1.65, 2.7 — interior + light outdoor. Hem-fir = $1.80, 2.8. Redwood = $5.20, 2.5 — premium West Coast. Western red cedar = $4.10, 2.2 — lightest common. IPE = $8.50, 5.8 — hardest tropical. Cumaru = $6.80, 5.4. Tigerwood = $5.90, 5.2.

Pick condition (dressed vs rough-sawn)

Dressed (S4S — surfaced 4 sides) is what big-box stores sell. Standard dimensional reduction: 2×4 = 1.5″ × 3.5″, 2×6 = 1.5″ × 5.5″, 2×10 = 1.5″ × 9.25″. Rough-sawn matches nominal exactly (2×4 = 2″ × 4″ actual). Used for traditional + heavy timber framing. Rough lumber is harder to find + more expensive.

Enter quantity

Number of pieces. Calculator multiplies for total board feet, total weight, total cost. Useful for: ordering full lifts of lumber, calculating truck weight ratings, estimating crane lifts on rooftop decks.

Read actual dimensions + math

Top of results: actual width × thickness in inches, mm, cm. Below: board feet per piece + total, weight per piece + total, cost per piece + total at reference $/bf pricing. Plus metric length conversion.

How we calculate

The math, fully transparent.

The Lumber Dimensions Calculator converts 14 nominal lumber sizes (1×4 through 8×8) to actual dressed dimensions in inches + mm + cm, plus board feet, species-specific weight (lb + kg), and cost at reference $/bf pricing. The killer feature: it surfaces ALL the lumber math in one place — most users guess at nominal-vs-actual (2×4 is actually 1.5″ × 3.5″, not 2″ × 4″), at board-feet calculations (uses nominal not actual, by US convention), and at weight estimates (IPE is 2.5× heavier than cedar at same dimensions). 9 species: PT pine, Douglas fir, SPF, Hem-fir, redwood, western red cedar, IPE, cumaru, tigerwood. Dressed (S4S) vs rough-sawn condition toggle — rough-sawn matches nominal exactly, dressed loses 1/4″ to 1/2″ on each surface from planing. Metric conversion (mm + cm + meters) for international users.

IRC references

WCLIB Standard Grading Rules — dressed (S4S) dimensional tolerances
SPIB Standard Grading Rules — Southern Pine S4S dimensions
USDA Wood Handbook FPL-GTR-190 — species specific gravity + weight tables
ASTM D245 — establishing structural grades for lumber

Dressed (S4S) dimensions per West Coast Lumber Inspection Bureau (WCLIB) and Southern Pine Inspection Bureau (SPIB) standard grading rules — 2026-Q1 current spec. Species weight + specific gravity from USDA Forest Products Lab Wood Handbook (FPL-GTR-190) at 12% moisture content. Reference $/bf pricing from 2026-Q1 wholesale + retail surveys — confirm with local supplier.

Nominal to actual (dressed)

actual = nominal - 2 × surfacing_loss

Surfacing loss = ~0.25-0.5″ per surface depending on size. 1× boards lose 0.25″/surface (1″ nominal → 0.75″ actual). 2× boards lose 0.25″/surface (2″ → 1.5″). 4× posts lose 0.5″/surface (4″ → 3.5″). 6× and 8× lose 0.5″ (6″ → 5.5″, 8″ → 7.5″). Width follows: 4″ → 3.5″, 6″ → 5.5″, 8″ → 7.25″, 10″ → 9.25″, 12″ → 11.25″.

Board feet (US convention)

bf = (nominal_thick × nominal_width × length_ft) ÷ 144

Board feet uses NOMINAL dimensions, not actual — this is a US lumber industry convention that confuses people but enables consistent volume comparison. A 2×4 × 12 ft = (2 × 4 × 12) / 144 = 0.667 bf. A 2×8 × 12 ft = (2 × 8 × 12) / 144 = 1.333 bf. Lumber priced by $/bf uses this formula.

Weight per piece

weight = bf × species_lb_per_bf

Species weight at 12% moisture content from USDA Forest Products Lab. PT pine 3.4 lb/bf (wet can be 4.5+). Douglas fir 3.1. SPF 2.7. Redwood 2.5. Cedar 2.2. IPE 5.8 (2.5× cedar). Cumaru 5.4. Tigerwood 5.2.

Metric conversion

mm = in × 25.4 · cm = in × 2.54 · m = ft × 0.3048

Standard SI conversions. Actual 2×8 dressed = 1.5″ × 7.25″ → 38.1 mm × 184.15 mm or 3.81 cm × 18.42 cm. 12 ft length = 3.658 m or 3658 mm.

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Board Foot Calculator

Convert dimensioned lumber (thickness × width × length × quantity) to board feet across 13 species. Multi-row BoM mode, weight (lbs), regional $/bf cost range. 2026 retail.

Deck Material Calculator

Calculate exact decking boards, joists, beams, posts, footings, and fasteners for your deck — IRC 2021 compliant.

Cut List Calculator

Full project lumber order. Joists + beams + posts + decking + railing + stairs. Stock board count + total LF + estimated cost. IRC-aware warnings (composite OC, riser height, guard rail).