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Decking calculator
Put your deck length, deck width, board width, board length and waste in: a 16 ft by 12 ft deck is 192 sq ft — 35 boards exact, 39 boards with 10% waste.
★ CONSTRUCTION · DECKS · BOARDS
Decking Calculator: 39 Boards Needed for 16x12 ft Deck
On the defaults — a 16 ft by 12 ft deck, 5.5 in by 12 ft boards, 10% waste — you need 39 boards of decking. This is the decking tool: measure the deck once, allow for the cuts, and order the boards once — a half-boarded deck waits for no second delivery.
What does decking needed 39 boards mean?
It means thirty-nine boards deck a 16 ft by 12 ft deck once cutting waste is allowed for. The deck itself is 192 square feet, since 16 times 12 is 192; with 10% waste that becomes 211.2 square feet of decking, and at 5.5 square feet per board — a 5.5 in board, 12 ft long — the exact count is 35 boards, rounded up to 39 whole boards with waste because boards are sold by the board, not by the fraction. The waste boards are not spare stock exactly — they are the boards the deck consumes in end cuts and breakage, so a deck boarded without waste runs short at the last row.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Deck area | length × width | 192 sq ft — 16 × 12. |
| Board area | (board width ÷ 12) × board length | 5.5 sq ft — (5.5 ÷ 12) × 12. |
| Boards | area with waste ÷ board area, rounded up | 39 boards — 211.2 ÷ 5.5, rounded up. |
Three things to check before you trust the number
First, check the board size you are actually buying: 5.5 in by 12 ft is the figure on the bundle, but a different product covers a different area, and the count follows the board, not the deck. Second, remember the count follows the area, not the length: a 16 ft by 12 ft deck is 192 sq ft, and a deck measured by one side under-orders. Third, judge the waste honestly: decking is cut at the ends of rows, so 10% fits a rectangular deck, but a deck with angles, stairs or a picture-frame border can want 15%, since every angle consumes boards. Joists, screws and railing are bought separately — see the joist calculator for the frame under the deck.
Worked example
16 ft long, 12 ft wide, 5.5 in boards, 12 ft long, 10% waste. Deck area: 16 × 12 = 192 sq ft. Board area: (5.5 ÷ 12) × 12 = 5.5 sq ft. With waste: 192 × 1.10 = 211.2 sq ft. Boards: 211.2 ÷ 5.5 = 38.4, rounded up to 39 boards. In practice that means buying 39 boards, and expecting the last boards to go on the end cuts and the breakage. If the same deck used 7.25 in boards instead, the need would fall to 30 boards with waste, but the deck boarded would be identical.
Every branch, checked
With a 20 ft long deck instead of 16 ft, the area rises to 240 sq ft and the need to 48 boards with waste. With a 16 ft wide deck, the area rises to 256 sq ft and the need to 52 boards with waste, since 281.6 sq ft still needs a fifty-second board. With 7.25 in boards, the area stays 192 sq ft but the need falls to 30 boards with waste. With 0% waste, the area stays 192 sq ft and the need falls to 35 boards. Deck size, board size and waste each move the count on their own, so a wrong board figure is as costly as a wrong measurement.
The reverses, worked backwards
The length behind 39 boards is 16 ft: at 12 ft the need is 29 boards (144 sq ft deck, 158.4 sq ft with waste, 5.5 sq ft per board, 10% waste); at 20 ft it is 48 boards (240 sq ft deck, 264 sq ft with waste, 5.5 sq ft per board, 10% waste). The width behind 39 boards is 12 ft: at 10 ft the need is 32 boards (160 sq ft deck, 176 sq ft with waste, 5.5 sq ft per board, 10% waste); at 16 ft it is 52 boards (256 sq ft deck, 281.6 sq ft with waste, 5.5 sq ft per board, 10% waste). The waste behind 39 boards is 10%: at 0% the need is 35 boards (192 sq ft deck, 192 sq ft with waste, 5.5 sq ft per board, 0% waste); at 20% it is 42 boards (192 sq ft deck, 230.4 sq ft with waste, 5.5 sq ft per board, 20% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension or one waste change.
What the defaults assume
The defaults assume a 16 ft by 12 ft rectangular deck, 5.5 in by 12 ft boards, and 10% cutting and breakage waste. They do not assume joists, screws, railing or stairs — a deck with stairs needs its stair boards counted separately, and the railing is bought by the foot, not by the board.
Who uses this
- A homeowner decking a backyard: the 39-board order settles the lumber-yard call before the first board is laid.
- A DIY builder replacing deck boards: the same maths on the deck alone gives the boards, not a whole house.
- A contractor quoting a job: deck area, board size, waste — the quote follows the same three steps, and the waste line shows the client why.
- A landlord budgeting a deck refit: the deck size off the plans, multiplied out, gives the board line on the refit budget.
Accuracy & limitations
The arithmetic is exact for the inputs entered; the judgement is the board size and the waste allowance. Real decks have stairs, angles and borders, and the board on the shelf overrides any planning figure; a deck measured gross over-orders, and a board figure read off a different product under-orders. Treat 39 boards as the planning order for a 16 ft by 12 ft deck at 5.5 sq ft per board, and raise the waste to 15% for a deck with angles or a picture-frame border.
Cite this calculator
Bright Side Kit, “Decking Calculator: 39 Boards Needed for 16x12 ft Deck”, https://brightoolbox.com/decking-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (16 ft × 12 ft deck, 5.5 in × 12 ft boards, 10% waste) so the 39-board result can be checked.
Frequently asked questions
How many deck boards do I need for a 16x12 ft deck?
39 boards at 5.5 sq ft per board with 10% waste. The area is 192 sq ft; with waste that is 211.2 sq ft, and 211.2 ÷ 5.5 = 38.4, rounded up to 39.
How do I work out decking boards from a deck size?
Multiply length by width for the area, add waste, then divide by the board area and round up. A 16 ft by 12 ft deck is 16 × 12 = 192 sq ft, or 39 boards at 5.5 sq ft with 10% waste.
How much waste should I allow for decking?
10% for a rectangular deck, 15% for a deck with angles, stairs or a picture-frame border. Waste is the end cuts and breakage, not spare stock.
Does this count joists too?
No — this calculator counts deck boards only. Joists are counted by spacing, and screws and railing are counted separately.
Should I buy the exact boards or round up?
Round up: boards are sold by the board, and the spare boards are the cutting stock. A deck boarded to the exact board leaves nothing for a miscut plank.
What if my deck is not a rectangle?
Split it into rectangles, work out each area, and add them before dividing by the board area. The square footage calculator handles the area step section by section.