Deck Calculator

This deck material calculator works from five measurements: deck length, deck width, board size, joist spacing, and material. Two formulas do most of the work.

board rows = deck width ÷ (board width + gap)
joist count = (deck length ÷ joist spacing) + 1

Both use the board's dressed width, and both round up. You cannot buy nine-tenths of a row.

Length, width, and which one your joists span

Length runs along the house. Width is how far the deck projects out from the house, and it is the distance the joists span. Swap the two and the material list changes: at 16 × 12 ft this deck takes 26 rows of decking, and the same 192 sq ft turned the other way takes 35. Decking boards run parallel to the house, across the joists.

Square footage alone will not produce a board count, which is why this calculator asks for both dimensions rather than an area. A 192 sq ft (17.8 m²) deck can be almost any shape, and each stacks a different number of boards across its width. Area still matters for stain coverage and cost per square foot, and the calculator reports it from the two dimensions you enter.

Nominal size is a name, not a measurement

A 5/4 x 6 deck board measures 5½ inches wide and 1 inch thick. The nominal name is a label from before the board was dried and planed, and it survives on the price tag. Every quantity here uses the dressed dimension, published in the American Softwood Lumber Standard PS 20.

The gap between boards counts too. This calculator adds it to the board width before dividing, so the pitch of a 5/4 x 6 at a ⅛ inch gap is 5⅝ inches, not 5½. That gap is a default, not a rule: pressure-treated (PT) stock goes down tighter, at 1/16 inch, because it arrives wet and shrinks as it dries. Cedar, ipe and composite hold whatever gap you set.

What the calculator assumes, and what it does not decide

Four rows in the material list carry an ASSUMED badge: joists, beam, posts and concrete. To price a joist you have to pick a size, so this calculator picks the common one and tells you it picked it. Open the disclosure below the results and it states each one in full: joist size, beam plies, post spacing, and the footing dimensions behind the concrete estimate.

None of those is a span calculation. This site prices framing; it does not size it. A number here tells you what a deck of this size costs to build with typical framing, not what your deck's framing has to be — that depends on your species, your span, your snow load and your inspector.

Most deck calculators tell you how many boards you need. That number is not what you buy.

A 16 ft deck row can be one 16 ft board, or two 8 ft boards with a butt joint in the middle. A 10 ft row cut from 12 ft stock leaves 2 ft of offcut per row — 52 feet of waste across a deck. Buy the wrong lengths and you pay for lumber that goes in the scrap pile.

This calculator packs your rows into stock lengths and shows the result: buy 26 boards at 16 ft, no joints, no offcuts. For a 10 ft deck it will tell you to buy 13 boards at 20 ft and cut two rows from each, rather than 26 boards at 12 ft.

Take the cut list to the yard. It is the list you actually hand over.

A 16 × 12 ft deck covers 192 sq ft (17.8 m²) and comes to roughly $1,473 in materials. Here is where that goes.

Decking. The 12 ft width divided by a 5⅝ inch pitch gives 26 rows. Each row spans the 16 ft length, so the cut list is 26 boards at 16 ft — 416 linear feet, $666. One board per row, so no butt joints anywhere in the surface.

Framing. 13 joists at 16 in on-centre, each spanning the 12 ft width, plus a ledger against the house and a rim joist at the outer edge. A two-ply beam runs the length of the deck on 3 posts. Framing and beam together come to $347.

Footings and hardware. 13 bags of concrete for 3 footings, 13 joist hangers, a cap and a standoff base for every post, and 2 boxes of coated deck screws.

Waste. 5% on a straight rectangular layout adds 2 spare boards at $51.20.

That total excludes railings, stairs, the permit, delivery, tool rental, and tearing out whatever is there now. On a deck at 24 inches above grade, railings alone can add a material line comparable to the decking.

Stock lengths and where the butt joints land

Stock lengths for 5/4 x 6 decking run 8, 10, 12, 16, 20 ft. A run longer than the longest board needs a butt joint, and a butt joint has to land on a joist — two board ends, each with its own pair of screws, on the same joist. Stagger joints between adjacent rows rather than lining them up, or the deck reads as two decks meeting in a seam.

How much extra to buy, and why waste is two different things

Two different things get called waste. Offcut waste is the ends you cut off because stock lengths do not divide evenly into your deck, and the cut list has already priced it. Layout waste is the allowance for miscuts, splits, and boards you reject at the yard — 5% on a straight rectangular deck, rising to 15% on a 45° diagonal, where every board is a compound cut with two triangular offcuts. Add 5% if this is your first deck. Matching a board four weeks later is worse than storing a spare.

What to check before the pile leaves the yard

Sight down each board on edge and set aside anything with a pronounced crown, a twist, or bark left on a corner. Check the end tag on treated stock for a ground-contact rating if any part of the frame sits close to soil. Pressure-treated boards are usually still wet when they are sold, and they shrink across the face as they dry — which is why they go down at that 1/16 inch gap and open to roughly ¼ inch on their own.

Five materials cover almost every deck built in the United States: pressure-treated southern yellow pine, western red cedar, redwood, ipe, and wood-plastic composite. The calculator prices all five, and the spread is wide.

Pressure-treated pine is the default and the cheapest way to build a deck that lasts. It arrives wet, shrinks, checks and cups more than anything else here, and it wants cleaning and resealing on a schedule. Cedar and redwood are stable, light and pleasant to work, and both are soft — cedar sits at 350 on the Janka scale against pine's 690, so dog claws and dragged furniture leave marks. Their heartwood resists decay without treatment; the pale sapwood does not.

Ipe is roughly five times harder than pine and will outlast the house. It is also heavy, expensive, needs a pilot hole for every fastener, and takes hidden clips or plugged screws rather than face screws. Composite never needs staining, which is the whole pitch and a real one; it also costs two to three times as much per board, runs hot in direct sun, and cannot be sanded out when it scratches. The framing underneath composite is still pressure-treated wood and still ages at wood speed.

Fastener choice follows the material. Cedar, redwood and ipe are tannin-rich: coated steel corrodes against them and streaks the board black, so they take stainless. Composite and ipe take hidden clips, which set the side gap for you and leave no screw heads in the surface. The calculator switches the fastener line automatically when you change material.

This calculator does not cover five things, each of which has its own answer.

• Railings. Baluster count and gap spacing are their own calculation, governed by a hard limit on opening size.
• Stairs. Riser height, tread run and stringer layout have the tightest tolerances on a deck, and the total run changes where the footings go.
• Framing sizing. What size joist spans your width, and how far a beam carries between posts, come from published prescriptive residential deck guidance rather than from a material list.
• Footing sizing. Footing diameter follows the load over your soil's bearing capacity, and depth follows your local frost line. The concrete figure here assumes a size rather than deriving one.
• Ledger fastening. The deck-to-house connection is the one that fails catastrophically rather than gradually, and its fastener spacing is a code threshold.

Take the material list to a plans examiner with your dimensions. Prescriptive guidance is a set of pre-worked conservative cases, and your building department is the authority on which applies to your lot. Read how we work out these numbers for the source behind every figure on this page.