A Free Tool · Decking & Fasteners · Real Board Widths
How many deck boards do you actually need?
Enter your deck's length and width, pick a board profile, the gap between boards, and the
stock length you can buy. You'll get the number of decking boards to purchase, the total
linear feet, and the deck screws needed. The numbers use the real milled face widths
(a 5/4×6 is 5.5″ wide, not 6″) and a standard screw rule of about 350 screws
per 100 square feet.
Boards, linear feet & screws·Real face widths & gaps·Decking + fasteners only
Read this first
This estimates the decking surface and screws only — the boards you walk on
and the fasteners that hold them down. It does not size the joists, beams, posts, or
footings underneath; that framing is a separate calculation that depends on span, spacing,
and local code. Always confirm framing and spans with your local building department or a
structural pro. Buy a little extra for waste from cuts and defects, and check the actual
width and length of the boards in your hands.
The calculator
Estimate deck boards and screws
Enter your deck dimensions in feet, pick the board profile, gap, and stock length sold at your lumberyard, and you'll get boards to buy, total linear feet, and screws — with the row math shown.
Which way do the boards run?
Boards run parallel to the deck length; rows stack across the
width. Each board's run is the length, and the number of rows is set by
the width. Flip this if your boards run the other way.
Nominal sizes are bigger than the milled width; the face width is what counts.
Smaller gaps for dry boards; wider for wet pressure-treated lumber.
The length you can buy. Each row needs enough of these to span its run.
Board layout
Rows of decking
Boards per row
Total linear feet
Coverage per row
Deck area
Deck screws (~350 / 100 sq ft)
The math, honestly
How the board and screw counts are figured
It's all about rows. Each row of decking takes up its actual face width
plus one gap, so a 5.5″ board with a 1/8″ gap covers
5.5 + 0.125 = 5.625″ per row. To find the number of rows, take the
dimension the boards run across — the width, by default — convert it to
inches, divide by that coverage, and round up:
rows = ceil(width_ft × 12 / coverage_in).
Each row spans the run length (the deck length, by default). You can only buy boards in
fixed stock lengths, so boards_per_row = ceil(run_ft / stock_ft), and the
total is boards = rows × boards_per_row. The total
linear feet of decking is just rows × run_ft — useful
if your supplier sells by the foot.
Screws come from the deck area: deck_sqft = length × width,
then screws = ceil(deck_sqft × 3.5). The 3.5 is the
350-screws-per-100-square-feet rule — two screws at every point a board
crosses a joist, with joists 16″ on center. Tighter joist spacing uses more screws;
wider spacing uses fewer. The counts here are exact for the geometry you enter and round
boards-per-row up, but they do not add a waste margin — tack on 10–15%
yourself for cuts and defects.
Board face widths, gaps & the screw rule
The actual milled face widths the calculator uses, the common gap options, and the
screws-per-square-foot rule behind the fastener count. Nominal lumber names are always
larger than the real dimension — a "2×6" is not six inches wide.
Board profile
Actual face width
Coverage per rowwith 1/8″ gap
5/4×6
5.5″
5.625″
2×6
5.5″
5.625″
2×4
3.5″
3.625″
Coverage per row is face width plus the gap. Gap options in the tool are 1/8″ (0.125),
3/16″ (0.1875), and 1/4″ (0.25). Screw rule: about 350 deck screws per
100 sq ft (≈ 3.5 per sq ft), assuming two screws per joist crossing at
16″ on center.
Common deck sizes
Worked examples for a few common rectangular decks, all using 5/4×6 boards (5.5″
face), a 1/8″ gap, boards running along the length, and 12-foot stock — so these
match what the calculator gives you. Add your own 10–15% waste margin on top.
Deck (L × W)
Rowsacross width
Boards12-ft stock
Screws~350 / 100 sq ft
10 × 12
26
26
420
12 × 16
35
35
672
16 × 20
43
86
1120
20 × 24
52
104
1680
Rows = ceil(width×12 / 5.625). Boards = rows × ceil(length / 12). On the 16×20 and
20×24 decks the length exceeds the 12-ft stock, so each row needs two boards. Screws =
ceil(length × width × 3.5). These are exact geometry figures — add waste yourself.
Reading the result well
A board count is only useful if you act on it sensibly. Four things worth knowing before
you load the truck.
Add 10–15% for waste
The calculator gives the exact board count for your geometry and rounds each row's boards up, but it does not pad for waste. Cuts at the ends, boards with splits or bad crowns you have to reject, and offcuts too short to reuse all add up. Add about 10% for a plain rectangle and 15% or more for picture-frame borders, diagonals, or angled corners.
The gap quietly changes the count
The gap is added to every single row, so it compounds. Going from a 1/8″ to a 1/4″ gap on a wide deck can drop a row or two and shave boards off the total. Pick the gap for the lumber, not the count: tighter for dry kiln-dried boards that won't shrink much, wider for wet pressure-treated stock that will.
Board direction is a real choice
Which way the boards run sets which dimension drives the rows and which drives the per-board run. Flipping the direction usually changes the board count even though the square footage is identical, because the stock-length rounding lands differently. Set the toggle to match your actual layout — and remember running boards the long way means fewer end-to-end seams.
This is decking, not framing
The boards and screws here cover the walking surface only. The joists, beams, posts, and footings that hold the deck up are a separate structural calculation driven by span tables and local code. Never size your framing from a decking estimate; confirm it with your building department or a structural professional before you cut anything load-bearing.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
construction specifications — follow your local building code and any engineered plan
for the structural parts.
Face width (actual)
The real milled width of a deck board, which is narrower than the nominal name. A 5/4×6 and a 2×6 are both 5.5″ wide, and a 2×4 is 3.5″. The face width — not the nominal size — is what determines how many rows it takes to cover your deck.
Gap (spacing)
The deliberate space left between adjacent boards, typically 1/8″ to 1/4″, so water drains and air circulates instead of trapping moisture against the wood. Because the gap is added to every row, it directly affects how many rows and boards you need.
Row (course)
A single line of decking running the length of the run. Rows stack side by side across the deck. Each row "uses up" its face width plus one gap of deck width, which is why rows = ceil(width in inches / coverage per row).
Coverage per row
How much width one row consumes: the board's face width plus one gap. A 5.5″ board with a 1/8″ gap covers 5.625″ per row. Divide the deck width by this to get the row count.
Stock length
The fixed lengths decking is sold in — commonly 8, 10, 12, and 16 feet. Because you buy whole boards, each row needs ceil(run length / stock length) of them; a run longer than your stock length forces a seam and a second board per row.
Linear feet
The total running length of decking, rows × run length, regardless of how it's cut into boards. Some suppliers price decking by the linear foot rather than the board, so this is the figure you'd hand them.
Joist
A horizontal framing member that the deck boards screw down onto, usually spaced 16″ on center. Joist spacing drives the screw count: two screws at each board-over-joist crossing is the basis of the 350-per-100-sq-ft rule. Joists are part of the framing this tool does not size.
Screws per 100 sq ft
A planning rule of thumb: about 350 deck screws per 100 square feet (≈ 3.5 per sq ft), from two screws per joist crossing at 16″ on center. Tighter spacing (12″) uses more; wider (24″) uses fewer. Buy by the box or pound and round up.
Frequently asked
It depends on the deck size, the board's face width, and the gap. Each row covers its face width plus one gap, so a 5.5″ board with a 1/8″ gap covers 5.625″ per row. Divide the deck width in inches by that and round up to get the rows. For a 12×16 ft deck that's ceil(192 / 5.625) = 35 rows; with 12-ft stock each row is one board, so 35 boards and about 420 linear feet. Add a little extra for waste. Try it in the calculator.
A common gap is 1/8″ for dry, kiln-dried boards that will shrink as they dry, and 3/16″ to 1/4″ for wet pressure-treated lumber that will shrink further. The gap lets water drain and air move so boards don't trap moisture and rot. Composite boards have their own spacing guidance — follow the manufacturer. The gap matters for the count because it's added to every row: a wider gap means fewer rows and slightly fewer boards.
A common rule is about 350 deck screws per 100 square feet, roughly 3.5 screws per square foot. It comes from two screws wherever a board crosses a joist, with joists 16″ on center. For a 192-sq-ft deck that's about 672 screws. Buy them by the box or pound and round up so you don't run short. Joists at 12″ on center use more; 24″ spacing uses fewer.
Most decks use 5/4×6 or 2×6 boards, both with a face width of 5.5″. 5/4×6 is the lighter dedicated decking profile; 2×6 is a thicker dimensional board used for stronger or wider-spanned decks. 2×4 decking (3.5″ face) shows up on older or narrow decks and takes more rows and boards to cover the same width. Nominal sizes are always bigger than the milled dimension, which is why the face width — not the name — drives the count.
Yes. The number of rows is set by the dimension the boards run across, and the per-board run is set by the dimension they run along. By default boards run parallel to the deck length, so rows stack across the width. Flip the direction so boards run across the width, and rows now stack along the length while each board's run is the width — usually changing both the row count and how many stock boards each row takes. The square footage and screw count don't change, but the board count can, so set the direction to match your real layout. The calculator has a direction toggle.
Add about 10% for a simple rectangular deck and 15% or more if it has angles, a picture-frame border, a diagonal pattern, or many short cuts that leave unusable offcuts. This calculator gives the exact board count for the geometry you enter and rounds boards-per-row up, but it does not pad for waste, so add your own margin. A few extra boards are far cheaper than a second trip to the lumberyard or a dye-lot mismatch on composite decking.
No. This tool estimates the decking surface and fasteners only — the boards you walk on and the screws that hold them. It does not size the framing underneath: the joists, beams, posts, and footings that carry the load. Framing depends on span, spacing, deck height, and local code, and must be designed separately. Always confirm spans and connections with your local building department or a structural professional before you build.