Deck Boards Calculator — Boards, Joists and Screws
Boards are sold by length, not by area, so the answer depends on which lengths you buy. This shows both, plus the joists and screws to go with them.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the deck
Length × width of the decked surface. A wrap-around or L-shaped deck is two rectangles — add a section for each.
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Use the actual board width
A 5/4×6 deck board is 5.5 inches wide, not 6. Using the nominal figure under-orders by about 9%, which on a big deck is several boards.
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Set the gap
1/8 in. for pressure-treated lumber bought wet, because it shrinks as it dries and the gap opens up on its own. 1/4 in. for kiln-dried lumber and for composite, which does not shrink and needs the drainage.
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Buy the longest boards that fit
A board that spans the whole deck has no butt joint. Butt joints have to land on a joist, they collect water, and they are the first place a deck starts to look tired. If a 16 ft board covers your span, the extra handling is worth it.
Decking by deck size
5.5 in. boards with a 1/8 in. gap, 10% waste for a straight layout.
| Deck | Area | Linear feet | 12 ft boards | 16 ft boards |
|---|---|---|---|---|
| 8 × 10 ft | 80 sq ft | 188 ft | 16 | 12 |
| 10 × 12 ft | 120 sq ft | 282 ft | 24 | 18 |
| 12 × 12 ft | 144 sq ft | 338 ft | 29 | 22 |
| 12 × 16 ft | 192 sq ft | 451 ft | 38 | 29 |
| 12 × 20 ft | 240 sq ft | 564 ft | 47 | 36 |
| 16 × 20 ft | 320 sq ft | 751 ft | 63 | 47 |
| 20 × 20 ft | 400 sq ft | 939 ft | 79 | 59 |
Longer boards mean fewer boards and fewer joints. Check they fit in your vehicle before ordering.
Joist spacing by decking type
| Decking | Spacing | Why |
|---|---|---|
| Wood, laid straight | 16 in. | The standard for 5/4 and 2× wood decking. |
| Wood, laid diagonal | 12 in. | The diagonal span between joists is longer. |
| Composite, straight | 12–16 in. | Check the maker — most composites need 16 in. or less. |
| Composite, diagonal | 12 in. | Almost universally required. |
| Any decking, stair treads | Per the tread | Support every tread at both ends and mid-span. |
Composite sags between joists where wood would not. The manufacturer's figure overrides everything here.
How many deck boards for a 12x16 deck?
A 12 × 16 ft deck is 192 sq ft. With 5.5 in. boards and a 1/8 in. gap that is 410 linear feet, plus 10% waste — about 451 feet.
In 16 ft boards that is 29 boards; in 12 ft boards, 38. Running 16 ft boards across the 16 ft dimension means no butt joints anywhere on the deck.
Why nominal sizes cost you boards
Lumber is named by the size it was before it was dried and planed. A 2×6 is 1.5 × 5.5 inches; a 5/4×6 deck board is about 1 × 5.5.
Work in nominal 6 inches and you will order roughly 9% short. On a 400 linear foot deck that is 36 feet missing, or three 12 ft boards — discovered at the far end on a Sunday afternoon.
Getting the gap right
Pressure-treated lumber from the yard is usually still wet. It shrinks across its width as it dries, so boards fitted tight at 1/8 in. end up with a comfortable 1/4 in. gap a few months later.
Kiln-dried lumber and composite do not shrink, so they need the full 1/4 in. from the start. Fit composite tight and it has nowhere to expand on a hot day — it buckles.
Butt joints have rules
Every butt joint must land on a joist, with both board ends fully supported. Doubling the joist at that point, or adding a sister alongside, gives each board its own bearing and enough room for a fixing that is not right on the end grain.
Stagger joints so they do not line up across the deck, and where you can, avoid them altogether by buying longer boards.
Adding a picture-frame border
A picture frame runs a border board around the deck perimeter, with the field boards cutting into it at 45 degrees or butting square against it. It hides the cut ends, which is the single biggest visual improvement you can make to a deck.
It costs material. The border itself is the deck perimeter in linear feet, and every field board now needs a cut at both ends rather than one — which is why the waste allowance jumps to 18%.
It also needs framing: blocking under the border line so both the border board and the field board ends have something to land on. Decide before the frame is built, because retrofitting the blocking afterwards means working from underneath.
How many joists and how far can they span
The joist figure above is an estimate for ordering. The real span depends on the lumber.
As a rough guide for pressure-treated southern pine at 16 in. centers: a 2×6 spans about 9 ft, a 2×8 about 12 ft, a 2×10 about 15 ft. Different species span differently, and the numbers drop at 12 in. centers carrying composite.
Above about 30 in. off the ground, most places need a permit and an inspection, and the inspector will want to see the span table you worked from. The American Wood Council's DCA 6 is the document to print out.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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area = length × width, summed over every sectionL-shaped decks total correctly. -
linear feet = area × 12 ÷ (board width + gap)Each board covers its own width plus one gap. -
× (1 + waste ÷ 100)10% straight, 15% diagonal, 18% with a picture-frame border. -
boards = linear feet ÷ board length, rounded upBoards are sold whole. -
joists = span × 12 ÷ spacing + 1Approximate — a real deck frame should be drawn out. -
screws = area × 2.2Two per joist crossing, plus a margin.
Common questions
How many deck boards do I need per square foot?
With 5.5 in. boards and a 1/8 in. gap, about 2.13 linear feet of decking per square foot of deck. Multiply your area by 2.13, then add waste.
What gap should I leave between deck boards?
1/8 in. for wet pressure-treated lumber, which shrinks to about 1/4 in. as it dries. A full 1/4 in. for kiln-dried lumber and composite, which do not shrink and need drainage.
How far apart should deck joists be?
16 in. on center for wood decking laid straight, 12 in. for diagonal or for most composites. Always check the decking manufacturer's figure — composite sags where wood would not.
Is a 5/4 board really 5/4 inches thick?
No. A 5/4×6 deck board is about 1 in. thick and 5.5 in. wide. Nominal sizes describe the rough-sawn lumber before drying and planing.
How many screws per square foot of decking?
About 2.2 — two at every joist crossing. A 200 sq ft deck needs roughly 440, so buy a 5 lb box.
Should I use hidden fasteners?
They give a clean surface with no screw heads and are standard with grooved composite boards. They cost more, take longer, and make replacing a single damaged board harder. Face screws are faster and easier to live with.
Which way should deck boards run?
Perpendicular to the joists, and usually parallel to the longest side of the house so the deck reads as an extension of it. Diagonal looks good but uses 15% more board and needs joists at 12 in.
How much extra decking should I order?
Ten per cent straight, fifteen diagonal, eighteen with a picture-frame border. Keep a couple of full-length boards back — replacing a damaged board later with a different batch never quite matches.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- American Wood Council DCA 6 — prescriptive residential deck construction, joist spacing and spans
- Trex and TimberTech installation guides — composite joist spacing and gapping requirements
- Softwood lumber standard PS 20 — actual versus nominal board dimensions
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.