Deck Calculator

Deck Calculator: How Many Boards and Joists Do You Need?

A deck is easy to picture and surprisingly easy to under-order. You measure the frame, guess a pile of boards, and only find the mistake when the last rows will not reach. This page answers how many deck boards do I need, how many joists belong underneath them, and how many screws hold the surface down. Enter your own deck size in the calculator below, in imperial or metric units, and check every result against the plain-English formulas further down before you buy anything.

Use it as your complete deck material calculator: enter the deck size and board size, and it works out the decking boards, frame timber, posts, footing concrete and screws in one go.

Quick answer by deck size

For decks built with 5.5 in wide boards laid straight, 12 ft boards, 10% waste included, and boards counted at full face width, the board counts for four common deck sizes are:

Deck size	Deck area	Boards needed
10 × 12 deck	120 sq ft	24 boards
12 × 16 deck	192 sq ft	39 boards
16 × 20 deck	320 sq ft	64 boards
20 × 20 deck	400 sq ft	80 boards

Those figures come straight from the board formula in “The formulas” below — deck area, plus waste, divided by the area one board covers — so double-check your own size with that formula before publishing an order to your supplier. Change the board length, the gap or the pattern and the count changes, which is exactly what the calculator is for.

Regional notes: in the US, decking is usually a 5/4×6 board, which is 5.5 in wide in actual size. UK deck boards are commonly 28×144 mm and are normally laid over joists at 400 mm centres. Australian boards are often 90×19 mm or 140×19 mm, in timbers such as merbau and spotted gum, and steel frames are common there instead of timber joists — the board maths stays the same, but follow local practice for the frame.

Quantities only — this calculator is not a structural design. Joist sizes, footings, ledger fixings and railings must follow your local building code; most decks need a permit.

The formulas

Every result above comes from five short formulas. None of them needs anything more than a tape measure, the sizes printed on the boards you are buying, and a pocket calculator to check.

• Boards = deck area × (1 + waste %) ÷ ((effective board width, in feet) × board length), rounded up. Effective width means the board width plus the gap, so a 5.5 in board with a 0.25 in gap covers 5.75 in of deck.
• Joists = the dimension the joists are spaced along, in inches, ÷ joist spacing, rounded up, plus 1. The extra joist closes the far end of the frame, the same way a fence needs one more post than it has sections.
• Screws = boards × joists × 2. That is two screws at every joist crossing, one into each side of the board.
• Fascia boards = deck perimeter ÷ board length, rounded up. The perimeter is the full trip around the outside edges that need facing.
• Decking linear feet = boards × board length. Use it to compare prices when suppliers sell by the linear foot instead of by the board.

The gap is the part people most often leave out, and it matters. Each board really occupies its face width plus one gap, because the space beside it is part of the pattern that covers the deck. Leave the gap out of the spacing plan and you undercount the space each board takes up: the rows will not land where the plan says, and a deck planned as if boards were laid tight has no room for drainage or for timber to expand. Boards laid tight also trap water between them. Count the gap, and the order matches the deck you are actually going to build.

Worked example

Take a 16 × 12 ft deck with 5.5 in × 12 ft boards, a straight layout at 10% waste, and joists at 16 in spacing. In the calculator, set the boards to run along the Width and the gap to 0 for this example, so the boards are counted at full 5.5 in face width.

• Step 1 — deck area: 16 ft × 12 ft = 192 sq ft.
• Step 2 — what one board covers: 5.5 in is 0.4583 ft, and 0.4583 ft × 12 ft = 5.5 sq ft per board.
• Step 3 — boards: add 10% waste, so 192 × 1.10 = 211.2, then 211.2 ÷ 5.5 = 38.4, rounded up to 39 boards.
• Step 4 — joists: the joists are spaced along the 16 ft length, which is 192 in, and 192 ÷ 16 = 12 spaces, plus 1 end joist = 13 joists.
• Step 5 — screws: 39 boards × 13 joists × 2 screws = 1,014 screws.

Those are exactly the numbers the calculator gives for the same inputs. One honest adjustment: the example counts boards at their full 5.5 in face width. Enter your real gap in the calculator — 0.25 in, say — and it adjusts the count, to 37 boards in this case, because each board then covers 5.75 in of deck width instead of 5.5 in.

Joist spacing: 12 in vs 16 in

16 in on-centre is the standard for wood boards laid straight. It keeps a 5/4 board feeling firm underfoot without paying for extra joists, hangers and fixings. It is the spacing this page assumes unless you choose otherwise.

Use 12 in on-centre for diagonal layouts and most composite decking. A board crossing the joists at 45 degrees effectively spans further between them, and composite boards are more flexible than timber, so both feel spongy at 16 in. For a rough guide to joist size by span: a 2×8 spans about 10 ft, a 2×10 about 14 ft, and a 2×12 about 18 ft. That is a rough guide only — check your local span tables and code before you order, because species, grade and load all change the answer.

Wood vs composite: what actually changes

The board-count maths is exactly the same for pressure-treated pine, cedar and composite. A 5.5 in board covers the same area whatever it is made of, so the formulas, the waste allowance and the worked example apply to all three.

What changes is everything around the board. Joist spacing usually tightens to 12 in for composite. The gap follows the board manufacturer rather than habit — many composite systems set it with their own clips or spacers, and it can differ from the 0.25 in used for timber. Fasteners change too: timber is normally face-screwed, two screws at every joist, while composite is often fixed with hidden clips. If you use clips, read the screw estimate as the number of fixing points and buy clip packs to the manufacturer's coverage.

The full materials list people forget

Boards, joists and screws are only the surface of a deck. The orders that stall on a Saturday morning are usually missing something from this list:

• Ledger board and flashing, to tie the frame to the house and keep water out of the wall.
• Joist hangers, plus the structural screws or bolts that belong in them.
• Concrete for the footings, and the tubes or formwork to pour it in.
• Posts and beams to carry the frame between footings.
• Railing — posts, rails, balusters and fixings — wherever the deck is raised.
• Fascia boards to face the outside edges (counted in the calculator).
• Stair stringers, treads and a handrail, if there are steps.

One honest limit: post and footing counts depend on beam spans and local code, not on deck area alone. This page estimates boards, joists and fasteners — it does not design your footing layout. Get that layout from span tables, your code, or someone qualified to check it.

Deck guides

Two longer guides go deeper on the money and the materials behind the numbers on this page:

• How Much Does It Cost to Build a Deck?
• Decking Boards Calculator — Boards, Joists, Beams & Posts

Frequently asked questions