Deck Board Calculator
Decking boards run in parallel rows across your deck. Enter the deck size and board specs — the calculator counts the rows and converts to full-length boards.
Deck Board Calculator
About this deck board calculator
A deck is just rows of boards. Count the rows (deck width divided by board width), multiply by the deck length, and divide by the board length you are buying — that is your board count. The calculator does exactly this, then adds waste for end cuts and the occasional split board.
Board width deserves a real measurement. A nominal "6-inch" deck board is actually 5.5 inches wide, and that half inch compounds across 20+ rows into a meaningful difference. Composite boards vary by manufacturer — check the spec sheet.
Two things this count excludes: the picture-frame border (perimeter boards running perpendicular — measure the perimeter separately) and stairs (each tread is roughly one board width times stair width, times the number of steps). Add both manually.
Worked example: boards for a 16×12 deck
Say your deck is 16 feet long (the direction boards run) by 12 feet wide, and you are using 16-ft deck boards at the standard 5.5-inch width. First the rows: 12 ft = 144 inches; 144 ÷ 5.5 = 26.18, rounded up to 27 rows. Then the boards: 27 rows × 16 ft × 1.1 (10% waste) ÷ 16 ft per board = 29.7, rounded up to 30 boards.
Note that 16-ft boards on a 16-ft-long deck give clean full-length runs with no butt joints — the ideal case. If your deck were 14 feet long, those same 16-ft boards would each waste 2 feet, and 12-ft boards with a staggered butt joint every other row would be the smarter buy. Board length choice is where real money hides.
Buying tips
Buy all your decking from the same lot or batch when possible. Composite colors shift between production runs, and even pressure-treated pine varies — mixed batches show as visible banding across the deck. Order everything at once rather than topping up mid-project.
This calculator divides deck width by board width with no spacing gap subtracted, so the result runs slightly high — treat the difference as part of your waste allowance rather than a problem. And do not forget the fasteners: plan on 2 screws or clips per joist crossing per board, which adds up to hundreds of fasteners on a mid-size deck (see the FAQ below).
How to use
- Measure the deck length (the direction boards will run) and deck width in feet.
- Choose the board length you plan to buy — longer boards mean fewer butt joints.
- Enter the actual board width in inches (5.5 for nominal 6-inch boards).
- Press Calculate. Add border boards and stair treads separately.
Good to know
- Excludes picture-frame borders, stairs, fascia boards, and railing — all separate material lists.
- Assumes boards run straight and parallel; diagonal decking needs ~15% waste.
- Joists, beams, posts, and hardware are structural — size them per local code and span tables, not this calculator.
- No spacing gap is subtracted in the row math, so the count runs slightly high — consider the difference part of your waste allowance.
- Buy decking from a single batch/lot. Composite colors and even treated pine vary between batches, and mixed lots show as banding.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
With 5.5-inch boards running the 16 ft direction: 12 ft width ÷ 5.5 in ≈ 27 rows × 16 ft ÷ 16 ft boards = 27 boards, plus 10% waste ≈ 30 boards of 16 ft.
What size boards are used for decking?
Most residential decks use 5/4×6 (actual 1×5.5 in) or 2×6 (actual 1.5×5.5 in) boards in 8–20 ft lengths. Composite boards come in similar profiles.
How far apart should deck boards be spaced?
A 1/8 to 1/4 inch gap for drainage and expansion. The calculator's row count uses board width; the gap is absorbed in the waste allowance.
How do you calculate decking?
Rows = deck width ÷ board width; boards = rows × deck length ÷ board length; add 10% waste.
Should deck boards run parallel to the house?
Usually perpendicular to the house (running out from it) looks best and sheds water away, but either works structurally if joist spacing is correct.
What gap should I leave between deck boards?
1/8 to 1/4 inch for drainage, airflow, and expansion. Composite manufacturers specify the gap in their installation instructions — follow it, since the wrong gap can void the warranty. This calculator does not subtract gaps from the row count, so its result already covers them.
How many deck boards do I need for a 12x12 deck?
A 12×12 deck is 144 inches wide ÷ 5.5-inch boards = 26.18, rounded up to 27 rows. With 12-ft boards and 10% waste: 27 × 12 × 1.1 ÷ 12 = 29.7, rounded up to 30 boards. Enter your exact dimensions above for your deck.
Should I use 12 ft or 16 ft deck boards?
Use the longest board that spans your deck without waste — longer boards mean fewer butt joints, and butt joints are weak points that collect water and open up over time. 16-ft boards on a 16-ft deck give perfect full-length runs; on a 14-ft deck, 12-ft boards with staggered joints waste less.
How many screws or fasteners do I need?
Plan 2 fasteners per joist crossing per board. On 16-inch joist spacing, a 16-ft board crosses 13 joists = 26 screws; for 30 boards that is about 780 screws. Buy the 5-lb box — the small 1-lb boxes run out at the worst moment.