Decking Calculator
Estimate the materials for a rectangular deck. Enter the deck length and width, pick the board width, gap and length, and set the joist spacing, and get the deck area, the number of deck boards to buy, total linear feet, joist count, a deck-screw estimate, and cost — with by-board-length and by-joist-spacing tables and a downloadable PDF.
| Length | Boards / row | Boards to buy | Cost |
|---|---|---|---|
| Enter a deck to compare | |||
| Spacing | Joists | Joist LF | Screws |
|---|---|---|---|
| Enter a deck to compare | |||
How to use the decking calculator
Enter the deck length and width, pick the board width, gap and length, set the joist spacing and a waste allowance, and get the deck area, the number of deck boards to buy, total linear feet, the joist count, a deck-screw estimate, and cost — plus reference tables for board length and joist spacing.
Enter the deck size
Length and width in feet. Boards run the length; rows stack across the width. Add a waste percent for cuts.
Pick the board
Tap 2×4, 5/4×6 / 2×6 or 2×8, set the drainage gap, and choose the board length you’ll buy.
Set joists & price
Choose 12, 16 or 24 inch joist spacing and a price per board to cost the order.
Read the count & save a PDF
See boards to buy, area, linear feet, joists and screws, compare options, and download the report.
How a decking estimate is calculated
It comes down to two counts: how many rows of board fit across the width, and how many boards make up each row along the length. Multiply, add waste, and you have the boards to buy — then the joists and screws follow from the framing.
Worked example — a 16 × 12 ft deck, 5½″ boards, ⅛″ gap, 16 ft lengths, 16″ joists
Area: 16 × 12 = 192 sq ft
Rows: roundup(144 ÷ (5.5 + 0.125)) = roundup(25.6) = 26
Boards per row: roundup(16 ÷ 16) = 1
Boards to buy: roundup(26 × 1 × 1.10) = 29
Joists: roundup(192 ÷ 16) + 1 = 13 · Screws: roundup(416 × 2 × 12 ÷ 16) = 624
29 sixteen-foot boards for 192 sq ft — 416 linear feet of decking over 13 joists, about 624 screws.
Decking coverage & material charts
The lookups that come up most — rows and linear feet by deck size, how board width changes the row count, joist and screw counts by spacing, and board face widths.
| Deck (L × W) | Area | Rows | Linear feet | Boards (16 ft, +10%) |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 22 | 220 ft | 25 |
| 12 × 12 ft | 144 sq ft | 26 | 312 ft | 29 |
| 16 × 12 ft | 192 sq ft | 26 | 416 ft | 29 |
| 16 × 16 ft | 256 sq ft | 35 | 560 ft | 39 |
| 20 × 12 ft | 240 sq ft | 26 | 520 ft | 58 |
| 20 × 16 ft | 320 sq ft | 35 | 700 ft | 77 |
Boards-to-buy assumes 16-foot stock. Note the 20-foot-long decks: a 16-foot board can’t span them in one piece, so each row needs two boards — which is why a longer board that matches the deck keeps the count down.
| Board | Face width | Per-row width | Rows |
|---|---|---|---|
| 2×4 | 3-1/2 in | 3.625 in | 40 |
| 5/4×6 · 2×6 | 5-1/2 in | 5.625 in | 26 |
| 2×8 | 7-1/4 in | 7.375 in | 20 |
Narrower boards mean more rows, more linear feet and more fasteners for the same deck — a 2×4 surface needs over 50% more rows than 5½″ boards. The per-row width is the board plus one gap, which is what the width divides by.
| Spacing | Joists | Screws | Typical use |
|---|---|---|---|
| 12 in OC | 17 | 832 | Composite, PVC, diagonal layouts |
| 16 in OC | 13 | 624 | Standard for most wood decks |
| 24 in OC | 9 | 416 | Only boards rated for it |
Joist count is for a 16-foot-long deck; screws assume two per board-joist crossing on a 192 sq ft surface. Tighter spacing means more joists and more screws — and is a structural requirement from the board’s span table, not a free choice.
| Board | Actual face width | Notes |
|---|---|---|
| 2×4 | 3-1/2 in | Narrow boards, tight look, most rows |
| 5/4×6 | 5-1/2 in | The standard dedicated decking board |
| 2×6 | 5-1/2 in | Same face as 5/4×6, thicker stock |
| 2×8 | 7-1/4 in | Wide boards, fewest rows and joints |
These are actual face widths, which is what the row math uses — a “2×6” is 5½ inches wide, not 6. 5/4×6 (five-quarter) is the dedicated decking profile; 2× lumber is thicker and used where boards also carry load.
Deck boards run the length in rows across the width, separated by a small drainage gap, and screw down to joists running perpendicular beneath. The row count drives everything — boards, linear feet, and the fasteners that follow.
- Count the gap in the rows — across 25+ rows a ⅛″ gap adds up to a full board’s width, so leaving it out under-counts.
- Match board length to the deck — a length that spans in one piece avoids butt joints and keeps offcut waste low.
- Joist spacing is structural — the board’s span table and code set it, not the look; composite and diagonal need 12″.
Everything the calculator works out
One set of measurements gives you the boards to buy, the area, the linear feet, the joist and screw counts and the cost — plus a comparison across board lengths and joist spacings, and a report for the yard.
The figures behind every deck
Built for anyone building a deck
A backyard platform, a composite rebuild, a client’s bid — the row-and-board math is the same, and getting the board count and the joist spacing right the first time keeps a build off a second supply run.
Building a first deck and wanting the board count, screws and a realistic shopping list before the first trip to the lumber yard.
- Match board length to the deck
- Leave a ⅛″ drainage gap
- Add 10% for cuts and defects
Re-decking over existing framing with composite boards, and checking the board count against the tighter joist spacing composite needs.
- Many composites need 12″ joists
- Use the clip system’s gap
- Switch to hidden-fastener counts
Pricing a deck package, comparing board lengths for the least waste, and handing the client a clear material takeoff.
- Compare lengths for waste
- Bump waste for patterns
- Save the PDF with the bid
7 tips for estimating decking without errors
The assumptions that quietly put a deck order short — or force a layout change mid-build — and the checks that catch them before the lumber arrives.
Decking FAQ
The board-count, spacing and material questions people ask most when planning a deck surface.
Work out how many rows fit across the width, then how many boards make each row, and multiply. Rows = width(in) ÷ (board width + gap); boards per row = deck length ÷ board length, rounded up; total = rows × per row, plus waste.
For a 16-by-12-foot deck with 5½-inch boards at a ⅛-inch gap in 16-foot lengths, that’s about 26 rows of one board, or 26 net, which comes to roughly 29 boards with 10% added for cuts and defects.
Divide the deck width by the on-center width of one board — the face width plus the gap — and round up. A 5½-inch board with a ⅛-inch gap takes 5.625 inches per row, so a 12-foot (144-inch) width is 144 ÷ 5.625 = 25.6, rounded up to 26 rows.
Round up because the last row is almost always a partial board ripped to fit, and you still buy a full board. The gap is easy to forget, but across 25-plus rows a ⅛-inch gap adds up to a board’s width, so leaving it out under-counts.
A ⅛-inch gap is the common default for drainage and airflow, measured after installation. Pressure-treated boards that are still wet are often laid tight or at 1/16 inch because they shrink to roughly ⅛ inch as they dry.
Composite and grooved boards are the exception — hidden-fastener clips set the gap, usually 3/16 to ¼ inch, so follow the manufacturer. Too tight traps water and debris; too wide and small items fall through and heels catch.
16 inches on-center is standard for most wood and many composite decks run straight. Composite and PVC often require 12 inches, and any board run diagonally needs 12 inches because the span between joists is longer at 45 degrees.
24-inch spacing is only for specific boards rated for it and is uncommon for the surface. Joist spacing is a structural requirement from the decking manufacturer’s span table and your local code, so confirm it there before framing rather than choosing by feel.
Plan on about 350 screws per 100 square feet at 16-inch joist spacing, using two screws where each board crosses a joist. Tighter 12-inch spacing raises that to roughly 460 per 100 square feet, since there are more joists to fasten to.
The calculator estimates screws from the installed linear feet and the joist spacing at two per crossing. Hidden-fastener and clip systems use their own coverage rate, so switch to the manufacturer’s figure if you’re not face-screwing. Buy a box extra — stripped and missed screws are routine.
Add about 10 percent for a straight board layout, which covers saw cuts, end trimming and the occasional warped board. Step up to 15 percent or more for diagonal decking and picture-frame borders, where angled cuts waste more.
Herringbone and complex patterns can push waste past 20 percent. The offcut from a board that doesn’t divide evenly into the deck length is the biggest hidden source of waste, so choosing a board length that divides cleanly is the easiest way to keep the overage low.
The common decking board is 5½ inches wide — that covers both 5/4×6 (five-quarter) and 2×6, since both finish at 5½ inches of face. A 2×4 is 3½ inches for a narrower look, and a 2×8 is 7¼ inches for a wider board with fewer rows.
Those are actual face widths, not nominal names, and face width is what the row math uses. Length runs 8 to 20 feet; a board that spans the deck in one piece avoids butt joints, which look cleaner and leave fewer gaps for water to sit in.
It’s mostly a look-and-waste decision for the surface, as long as the boards cross the joists at 90 degrees. Running boards the longer dimension means fewer rows and often fewer butt joints; running the shorter dimension can waste less if it matches the stock you can buy.
Running boards diagonally is a design choice that costs more material and needs 12-inch joist spacing to keep the span safe. Whatever direction the surface runs, the joists beneath must run perpendicular, so board direction and framing are decided together.
Estimation Only — Confirm Framing and Span With Your Code: This calculator estimates deck surface materials for a rectangular deck with boards running the length, rows stacking across the width at the board face width plus a gap. Board, joist and screw counts are material estimates and assume a straight layout face-screwed at two screws per board-joist crossing; picture-frame borders, diagonal or herringbone patterns, fascia, stairs and hidden-fastener systems all change the counts and need to be added separately. Joist spacing is a structural requirement set by the decking manufacturer’s span table and your local building code, not an aesthetic choice — this tool does not size joists, beams, posts or footings, and offers no structural guidance. Use the actual face width of your board. Prices shown are whatever rate you enter and are not quotes. For estimation purposes only; confirm framing, spans and fastening with your decking manufacturer and local building department before buying or building.