Deck Board & Material Calculator
Enter your deck's length and width in feet to get the square footage every decking order is priced against. The charts below turn that number into deck boards, joists, and screws.
Total area: 100.0 sq ft
How it works
Enter the deck surface as length by width in feet. Measure the decking footprint, not the framing below it, and use the Add area button for an L-shaped deck, a wrap-around, or a separate landing.
Do not deduct stair openings or the posts that pass through the surface. They are small relative to a board run, and the offcuts they create are rarely long enough to reuse elsewhere.
Take the total to the chart below and divide by the coverage for the board length you plan to buy. A 5.5 in board with a 1/8 in gap covers 0.47 sq ft per linear foot, so every 100 sq ft of deck takes about 213 linear feet of decking whichever length you order.
Reference chart
Deck boards needed by board length (5.5 in board, 1/8 in gap)
| Board length | Covers | Boards per 100 sq ft | Boards for a 16 × 20 ft deck |
|---|---|---|---|
| 8 ft | 3.75 sq ft | 27 | 86 |
| 10 ft | 4.69 sq ft | 22 | 69 |
| 12 ft | 5.63 sq ft | 18 | 57 |
| 16 ft | 7.50 sq ft | 14 | 43 |
| 20 ft | 9.38 sq ft | 11 | 35 |
Deck framing: joist spacing and how far a joist can span
Decking is only the visible half of the material list. Under it sits the frame — joists, beams, and posts — and the joist spacing is not a preference. It is set by the decking product going on top, so decide the surface material before you buy framing lumber.
Sixteen inches on center is the default for wood decking and for most composite boards laid perpendicular to the joists. Turn those same composite boards 45 degrees for a diagonal pattern and the unsupported distance grows, so nearly every manufacturer drops the requirement to 12 inches. Twenty-four inch spacing is wood territory only, and then only with thicker 2x decking rather than 5/4 stock.
Span is the other half of the framing question: how far a joist reaches between supports before it needs a beam. The figures below are the widely published Southern pine values at 16 inches on center under a 40 psf live load. Other species run shorter, and your local building department has the final say — check with them before ordering, because a joist that is one size light is not something you can fix after the decking goes down.
Joist spacing and joists needed for a 20 ft deck length
| Joist spacing | Joists for a 20 ft length | When it is required |
|---|---|---|
| 12 in on center | 21 | Composite laid diagonally; some PVC brands |
| 16 in on center | 16 | Standard for wood and most composite |
| 24 in on center | 11 | Wood only, and only with 2x decking |
Typical maximum joist span at 16 in on center (Southern pine, 40 psf live load)
| Joist size | Maximum span | Typical use |
|---|---|---|
| 2x6 | 9 ft 11 in | Ground-level decks and short runs |
| 2x8 | 13 ft 1 in | The common choice for most decks |
| 2x10 | 16 ft 2 in | Longer spans with fewer beams |
| 2x12 | 18 ft 0 in | Long spans and heavy loads |
Deck screws and hidden fasteners per square foot
Face screwing uses two screws per board at every joist it crosses. That makes the fastener count a function of joist spacing rather than deck size alone: tighter joists mean more crossings per board, so a 12 inch frame takes a third more screws than a 16 inch frame covering the same square footage.
Hidden fastener clips sit in the groove on the board edge and take one clip per crossing instead of two screws, which is why the clip column below is exactly half the screw column. Clips are sold by the square foot of coverage rather than by the piece, so match the box to your total area and check that the board you bought is actually grooved.
Buy about 10 percent over whatever the chart gives you. Screws strip, clips drop between joists, and running out mid-run means a second trip. Use stainless steel near salt water and with cedar or redwood, whose tannins corrode ordinary coated screws and leave black streaks around every hole.
Fasteners per 100 sq ft of decking by joist spacing
| Joist spacing | Face screws per 100 sq ft | Hidden clips per 100 sq ft |
|---|---|---|
| 12 in on center | 427 | 213 |
| 16 in on center | 320 | 160 |
| 24 in on center | 213 | 107 |
What deck materials cost per square foot
Two very different numbers both get called the cost of a deck. Materials only is what you spend at the lumberyard. Installed is what a contractor charges, and labor is usually the larger share — which is why an installed figure can run three times the boards-only figure for the same deck.
The boards-only column below covers the decking surface alone. Framing lumber, footings, joist hangers, and fasteners are on top of it, and on a typical deck they add roughly as much again as the decking itself. Railing is separate and expensive per linear foot, so price it as its own line rather than folding it into a per square foot number.
These are wide national ranges for 2026 and they are wide deliberately. Lumber prices move, the gap between builder-grade and premium lines of the same material is large, and a raised deck with stairs costs far more per square foot than a ground-level rectangle of the same area. Treat the table as a budgeting starting point, then get three local quotes.
Decking cost per square foot by material (2026 US national ranges)
| Decking material | Boards only, per sq ft | Installed, per sq ft | What to know |
|---|---|---|---|
| Pressure-treated pine | $2 – $5 | $15 – $25 | Cheapest; needs cleaning and sealing |
| Cedar | $4 – $8 | $20 – $35 | Naturally rot resistant; dents easily |
| Redwood | $6 – $12 | $25 – $40 | Best supply on the West Coast |
| Composite | $5 – $13 | $25 – $50 | No sealing; long warranties; gets warm |
| PVC | $8 – $15 | $30 – $60 | Lightest and coolest underfoot |
| Tropical hardwood | $8 – $16 | $30 – $60 | Very hard; every screw needs pre-drilling |
Frequently asked questions
How much material do I need for a 20x20 deck?
A 20 by 20 ft deck is 400 sq ft, which takes about 853 linear feet of decking — 72 boards at 12 ft or 54 at 16 ft, using 5.5 in boards with a 1/8 in gap. Framing at 16 in on center adds about 16 joists, and face screwing takes roughly 1,280 screws. Pressure-treated materials for a deck that size typically run $2,000 to $4,800.
What should a 12x24 deck cost?
A 12 by 24 ft deck is 288 sq ft. Installed, pressure-treated runs about $4,320 to $7,200 and composite about $7,200 to $14,400 at 2026 national rates. Doing it yourself removes the labor share, which is usually more than half the installed price. Raised decks and stairs push costs toward the top of both ranges.
What is the 3-4-5 rule for decking?
It is how you check that a deck frame is square before fastening it. Measure 3 ft along one side and 4 ft along the adjoining side, then measure between those two marks — it must read exactly 5 ft, because 3² + 4² = 5². Scale it up to 6-8-10 or 9-12-15 on a large frame for a more accurate check. An out-of-square frame shows up later as a tapering gap along the last deck board.
How many deck boards do I need per square foot?
Work in linear feet first, because that figure does not change with board length. A 5.5 in board with a 1/8 in gap covers 0.47 sq ft per linear foot, so every square foot of deck needs about 2.13 linear feet of decking. Divide the total by your board length to get a board count, then add 10 percent for cuts.
What gap should I leave between deck boards?
It depends on how wet the lumber is. Wet pressure-treated boards go down touching, because they shrink to roughly a 1/8 in gap as they dry. Kiln-dried lumber and cedar are already dry, so gap them about 1/8 in on installation. Composite and PVC follow the manufacturer's spec, usually around 3/16 in on the sides with a larger end gap in cold weather.
How far apart should deck joists be?
Sixteen inches on center covers wood decking and most composite laid perpendicular to the joists. Drop to 12 in for a diagonal pattern, where the boards cross the joists at 45 degrees and span further between supports. Only 2x wood decking should go on 24 in centers, and never composite — check the decking manufacturer's instructions, since the warranty depends on it.
Link to this calculator
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<a href="https://diymaterialcalculators.com/deck/">Deck Board & Material Calculator</a> — free online calculator