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Lumber and material quantities

Almost every material mistake on a deck traces back to one of three things: buying nominal when you measured actual, pricing by the wrong unit, or forgetting that the lumber in the stack is not the width the label says. This section is about getting the quantity right before anything is cut.

Three ways lumber is sold, and they are not interchangeable

Unit	What it measures	Where you meet it
By the piece	One board of a named size and length	Dimensional softwood at a home centre — "2×8×12, ten of them"
Board foot	Volume: 144 cubic inches, or 1″ × 12″ × 12″	Hardwood, thick stock, and anything sold from a mill
Linear foot	Length only, with the profile fixed by the product	Decking, fascia, railing, trim, siding

The trap is that a price looks comparable when the units are different. A 2×8×12 at $18 is $18 per piece, which is 16 board feet, which is $1.13/bf — but it is also 12 linear feet. Three numbers, one board. If you compare it to a hardwood quote at $6/bf you will conclude the softwood is cheaper by a factor of five, and you will be right, but for a reason that has nothing to do with the numbers on the tags.

Nominal is not actual

Softwood is sawn oversize and then planed on four sides, so a "2×4" arrives at 1½″ × 3½″. The gap is not a rounding error: a nominal 2×4 has 5.25 in² of cross-section against 9 in² nominal, which is 42% less material than the name suggests. Every quantity calculation on this site uses actual dimensions, and this is the table it uses.

Nominal	Actual (in)	Notes
1×4	¾ × 3½	Boards, trim, fascia
1×6	¾ × 5½	Fascia, fence pickets
5/4×6	1 × 5½	The standard wood deck board
2×4	1½ × 3½	Rails, blocking, light framing
2×6	1½ × 5½	Decking on 24″ centres, joists
2×8	1½ × 7¼	Joists, rim, stair stringers
2×10	1½ × 9¼	Joists, built-up beams
2×12	1½ × 11¼	Joists, beams, stringers
4×4	3½ × 3½	Guard posts, pergola posts
6×6	5½ × 5½	Structural posts, beams

Deck boards deserve their own line because the difference is invisible. A 5/4×6 is 5½″ across the face, not 6″. Using 6 in the arithmetic overstates coverage by 9%, which on a 12 ft wide deck is two rows — roughly four boards on a 16 ft deck.

Board feet, in one paragraph

A board foot is thickness (in) × width (in) × length (ft) ÷ 12, using nominal dimensions. A 2×8×12 is 1½ × 7¼ actual, but it is priced as 2 × 8 × 12 ÷ 12 = 16 bf. That inconsistency is deliberate: hardwood is graded and priced on nominal volume because that is how the log was measured, and the surfacing loss is baked into the price. It means board feet are a purchasing unit, not a physical measurement of the board in your hand — the actual volume is about 30% less for a 2×.

Moisture is a quantity problem, not a detail

Pressure-treated lumber arrives wet. A 5/4×6 deck board at 25% moisture content will shrink in width as it dries, opening the gaps you carefully set at ⅛″ into gaps of ¼″ or more. This is why treated decking is usually installed with a slightly tighter gap than the finished look suggests, and why composite — which moves far less — specifies its own gap in the installation manual, not in a rule of thumb.

Weight is the other consequence. Water is heavy, and a treated 2×8×12 that weighs 32 lb at the yard may weigh 26 lb six weeks later. If you are calculating a trailer load, a roof load or a beam's self-weight, the moisture state matters as much as the species.

Everything in this section

Lumber weight calculatorWeight per board and per order from actual dimensions, species density and moisture content. Includes a reference table of densities from cedar to ipe. Wood species density and behaviourDensity, movement and rot resistance for 18 deck and framing species, with the weight of a standard 2×8×12 so the numbers are comparable at a glance. Board feet to linear feetConvert board feet to linear feet and back. The conversion only works once you fix the cross-section, so this tool makes you state it and shows both directions. Plywood sheet calculatorHow many plywood or OSB sheets cover a given area, at 4×8, 4×9, 4×10 or 4×12, with waste and a layer multiplier for double-skin floors. Deck paint and stain coverageGallons for a deck floor and railing, by coating type and coats, plus primer and realistic coverage for weathered or porous wood. Deck fastener guideWhich fastener for treated wood, cedar, ipe and composite: corrosion classes, screw length rules, hanger nails versus deck screws, and how many a deck consumes. Composite vs wood deckingA comparison built on numbers rather than preference: installed cost per square foot, surface temperature, span limits, and what happens when a board is damaged. PVC vs composite deckingPVC decking is not composite with a better cap. Differences in expansion, stiffness, heat and cost, with the installation limits that catch people out. Cedar vs pressure-treated lumberWestern red cedar and treated pine compared on durability, weight, fastener compatibility and how each takes a finish, plus the grade question buyers miss. Hardwood decking: ipe, cumaru and garapaDensity, movement, fastener rules and the installation cost of tropical hardwood decking. Why the material price is not the number that surprises people. Nominal vs actual lumber sizeA 2x4 is 1-1/2 by 3-1/2 inches. The full table for dimensional lumber, why the sizes exist, and the three places the difference costs money. Lumber grades explainedSelect Structural, No.1, No.2, Stud and Utility: what each grade permits, which one a deck actually needs, and why the stamp matters more than the species. Deck board thicknessWhat thickness actually controls on a deck board: stiffness between joists, fastener hold, and how thickness and joist spacing have to be chosen together. Aluminum vs wood deck railingAluminum railing costs more per foot and needs fewer posts. A comparison of installed cost, post spacing, code compliance and what each system demands of the frame. Cable railing: what it costs and what it demandsStainless cable railing looks minimal and costs the most. The post spacing, tensioning and deflection rules that decide whether it will pass inspection. Deck boards cuppingCupped deck boards explained by the moisture gradient that causes them, which fixes work, which only postpone it, and when replacement is the answer. Deck squeaksWhat causes a squeaking deck, how to locate it precisely, and the four fixes ranked by how much decking they require you to remove. Deck rot repairFinding rot on a deck, judging whether it is cosmetic or structural, and the repair options from board replacement to sistering a joist. How often should you seal a deck?Sealing intervals by material and climate, the water bead test that tells you it is time, and why sealing too often is as bad as not sealing at all. Best stain for a deckChoosing a deck stain by species, exposure and existing finish. Why oil and water-based products are not interchangeable, and the two failure modes to avoid. How to clean composite deckingCleaning composite decking: what the cap will tolerate, how to remove the three common stains, and the tools that scratch a capped board permanently. Deck winterizingA winter checklist for a wood or composite deck: freeze-thaw damage, snow removal, ice, drainage and the tasks that should wait until spring. Deck cost per square footInstalled deck cost per square foot by material and height, what is and is not in the number, and why quotes for the same deck vary by 100%. Composite deck costWhat composite decking costs installed, the four extras that push the total up, and the honest calculation of whether the maintenance saving pays it back. Deck repair costWhat common deck repairs cost: boards, railing posts, joists, footings, stairs and staining. Plus the threshold where repair stops making sense.

What this section does not cover

Grading. Visual and machine grades (No. 2, Stud, Select Structural) determine allowable span, not just appearance. Two boards of the same species and size can differ by a factor on span because of grade alone — see how span tables work.
Engineered lumber. LVL, glulam and I-joists are specified by the manufacturer with their own span tables and their own units.
Metric. The site is imperial throughout, deliberately, because the products are sold that way in this market.

Questions people actually ask

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

Because it is sawn at 2″ × 4″ and then planed smooth on all four faces, which removes about ¼″ from each dimension. The nominal size survives as the name, not as a measurement. Any calculation that uses 2 and 4 instead of 1½ and 3½ overstates the material by 70%.

Is board feet the same as linear feet?

No. Linear feet measure length only; board feet measure volume. One 16 ft deck board is 16 linear feet and 7.3 board feet if it is a 5/4×6. They only coincide for a board exactly 1″ thick and 12″ wide.

Should I buy 10% extra lumber?

For decking, yes — 10% covers end cuts and defects on a simple rectangle, 15% for diagonal or a picture frame. For framing, waste is much lower because joists are cut to a plan, so 5% is usually enough unless the layout has many short pieces.

Does treated lumber shrink after installation?

Yes, in width more than in length. A nominal 6″ board can lose ⅛″ to ¼″ of face width as it dries from the mill's 25–30% moisture content down to equilibrium. That is one reason deck board gaps are set slightly tighter than the finished look suggests.

Why are hardwood prices quoted per board foot?

Because hardwood is sold from the mill by volume, in random widths and lengths, and grading rules are written in board feet. It lets a buyer compare a stack of mixed boards on one number rather than pricing each one.

Deck board calculatorQuantity in rows and linear feet, once you know the face width. Deck material guidePressure-treated, composite and hardwood compared on cost and maintenance. Board foot calculatorVolume and cost for hardwood priced by the board foot.