How Much Decking Do I Need? (2026 Deck Board Calculator)
Ordering deck boards feels like guesswork until you know the formula, and guessing wrong is expensive: too few stalls the project, too many wastes money. This guide gives you the exact method and the free calculator.
How much decking do I need for a 16×12 deck?
A 16 by 12 foot deck needs 26 boards before waste and 29 boards with the standard 10 percent waste allowance, using 5.5-inch boards with a 0.125-inch gap. That is 464 linear feet of decking, costing $362.50 at $12.50 per board. Use the Deck Calculator to repeat this calculation for your own deck size.
How do you calculate how many deck boards you need?
Divide the deck width in inches by the board width plus the gap, then round up to whole boards, because you cannot buy part of a board. Add your waste percentage and round up again. Boards run along the deck length, so each board is one deck length long.
In full: effective width equals board width plus gap; boards without waste equals deck width in inches divided by effective width, rounded up; boards with waste equals that count times one plus waste percent, rounded up. Linear feet and cost follow by multiplying by deck length and price per board.
What size deck boards should I buy?
Most decks use 6-inch nominal boards, which actually measure 5.5 inches wide, and that is the calculator’s default. Four-inch nominal boards measure 3.5 inches, while 8-inch nominal boards measure 7.25 inches. Always enter the actual width, not the nominal size, or your count will come out low.
Narrow boards feel traditional and hide small alignment errors, while wide boards look modern but show uneven gaps more clearly. Wider boards need fewer fasteners but cost more each, so totals are usually similar.
What gap should I leave between deck boards?
Leave a gap of one eighth of an inch (0.125 inches) between boards, the standard default for wood and composite decking. The gap drains rainwater, lets boards expand in heat and humidity without buckling, and still feels solid underfoot. Some composite manufacturers specify a slightly wider gap, so check the guide for your exact board before fitting.
Gap size rarely changes the count: across 12 feet, a 5.5-inch board gives 26 rows with either a 0.125-inch or a 0.25-inch gap. Do not shrink the gap just to save a board.
How much extra decking should I buy for waste?
Buy 10 percent extra for a straight rectangular deck, and 15 percent for diagonal, picture-frame, or angled layouts. Waste covers boards that arrive warped or split, end cuts on every row, and the occasional cutting mistake. The calculator adds the percentage to the board count and rounds up again, so 26 boards plus 10 percent becomes 29 boards, not 28.
Skipping waste is the most common ordering mistake: a deck two boards short cannot be finished with offcuts, and a second delivery often comes from a different batch with a different shade. Leftover boards are useful for repairs anyway.
How much does it cost to deck a 16×12 area?
Boards for a 16 by 12 foot deck cost $362.50 at $12.50 per board, based on 29 boards including waste. That covers boards only: joists, beams, posts, fasteners, railing, and stain or sealant are separate costs, so budget for them on top of the board total.
Prices vary by material: pressure-treated pine is cheapest and is the basis of the $12.50 example, cedar and redwood cost more, and composite costs most up front but needs almost no maintenance. Enter your supplier’s actual per-board price for a total that reflects your material, not a national average.
Should deck boards run lengthwise or widthwise?
Run boards along the longer side of the deck whenever you can, because fewer butt joints means a cleaner look and less waste. The calculator assumes boards run along the deck length you enter, so enter the side your boards will run along as the length. Check fixed board lengths before ordering.
Boards usually run away from the house so water sheds off the deck, and the long lines make a small deck feel bigger. For a picture-frame border, use the 15 percent waste allowance, since the border boards run around the edge and add cuts.
What mistakes do people make when ordering decking?
The biggest mistakes are entering the nominal width instead of the actual width, forgetting waste, and pricing boards at the wrong length. A 6-inch nominal board is really 5.5 inches, and that half inch across 26 rows leaves you a full board short. Measure the actual board, add waste, and confirm the price is for your deck length.
Decks are rarely perfect rectangles: stairs, post notches, and angled corners all consume extra boards, so measure the widest and longest points and treat bump-outs as separate small rectangles. For a ground-level patio base, see the Concrete Calculator and Gravel Calculator, and the Tile Calculator for an adjacent tiled area.
Frequently Asked Questions
How many deck boards do I need for a 20×14 deck?
With 5.5-inch boards and a 0.125-inch gap, a 20 by 14 foot deck needs 30 boards before waste and 33 boards with 10 percent waste, giving 660 linear feet and a board cost of $412.50 at $12.50 per board. Try these numbers in the Deck Calculator.
Do I count the gap when calculating deck boards?
Yes, every row takes up one board width plus one gap, so the gap is part of the division. Ignoring the gap overstates the board count slightly, which is safe but wasteful, while using the nominal width instead of the actual width understates it, which leaves you short.
Can I use the calculator for a diagonal deck?
Yes, but set the waste allowance to 15 percent instead of 10 percent, because angled cuts waste more of every board. The board count formula is the same; only the waste figure changes for diagonal and picture-frame layouts.
What if my supplier only sells fixed board lengths?
If your deck is longer than the longest stocked board, each row needs a butt joint and a second piece, so count boards per row. For decks under 16 feet, stocked 16-foot boards fit the calculator’s assumption exactly.
Disclaimer: Estimates only, not professional advice. Counts assume a flat rectangular deck with boards running the full deck length; angled layouts, stairs, and fixed board lengths change the count. Prices are estimates, not quotes. Check local building codes and consult a qualified contractor for structural work.
