Tools
All guides

Deck Calculator

Area 1

Sample shape, not to scale.

No result yet. Enter length and width and gap between boards, joist spacing and board length.

Boards, joists and fasteners

This counts deck boards and fasteners for buying. It does not size joists, beams, posts, footings or the ledger, and does not check code. Ask your local building department or a structural engineer.

Each area is a rectangle of decking: its length runs along the boards, its width across them. For an L-shaped deck, add a second area.

5.5 in is the face width of 5/4 × 6 wood decking (ProWood) and of standard Trex, TimberTech and Fiberon Paramount boards. Some boards are a little narrower or wider, so check yours.

Fill in from a board maker's instructions

A chip fills in the gap, the joist spacing and the fasteners from one maker's instructions for its own boards, as a starting point. Gaps and fastening differ by product, so check the guide for your boards. For wood decking no one gap fits every wood: enter your own.

  • Trex, face screws: Trex's 2024 guide asks for at least 3/16 in between boards and two screws at every joist; its 2023 chart allows joists up to 16 in on center for 1 in boards on a home deck. The 2 extra screws per butt joint is this site's reading: each board end at a joint gets its own two.
  • TimberTech, CONCEALoc clips: TimberTech: 3/16 in between boards, one clip and screw at each joist, and two clips on each board end at a butt joint, so four at a joint instead of one; joists never more than 16 in on center (24 in only for its Advanced PVC Max board).

Required; 0 for boards laid tight. Trex asks for at least 3/16 in and never more than 1/2 in; TimberTech 3/16 in with its CONCEALoc clips. Other boards differ: use the guide for yours.

Your joists, center to center: it sets how many joists each row of boards crosses. The board maker sets the most it may be for its boards: Trex 16 in for 1 in boards on a home deck, TimberTech 16 in (24 in only for its Advanced PVC Max). No spacing fits every board, and this site does not size joists.

Board lengths

The length you buy. Trex and TimberTech sell 12, 16 and 20 ft boards. A row longer than one board joins end to end over a joist.

Starts at 0%: no board maker gives one figure. The count already buys a whole board for every row; add a few percent for boards you reject and cuts that go wrong.

Optional; enter fasteners per board at each joist to count screws or hidden clips. Extra per butt joint is what a joint takes beyond that: at a joint two board ends sit over the joist.

Optional. From the box or bag label, to count boxes.

Optional. Each price is multiplied by its own count; the box price needs fasteners per box.

Embed this calculator

Paste this code into your page. The calculator works inside the frame, with a link back to this page under it.

How the deck boards and fasteners are counted

Split the deck into rectangles and enter each as an area: its length runs along the boards, its width across them. Boards are laid across the joists, so the joists run along the width.

Rows of boards: each row is a board plus the gap beside it, and the last row is ripped to fit. Rows = (width + gap) ÷ (board width + gap), rounded up.

Boards per row: a row no longer than a board is cut from one board, and if a board is at least twice the row, it gives two rows or more. A longer row is made of boards end to end, joined over a joist, so each board is used up to the last joist it reaches: a 16 ft board on joists 16 in apart covers 16 ft, on joists 24 in apart also 16 ft, on joists 18 in apart 15 ft. Boards per row = row length ÷ that length, rounded up, and each row has one butt joint fewer than boards.

Joists under the deck: length ÷ joist spacing, rounded up, plus one for the far end. Each row crosses every joist, so crossings = rows × joists. Screws or hidden clips = crossings × fasteners per crossing, plus the extra ones at every butt joint.

The gap, the joist spacing and the fastening come from the instructions for the boards you buy, not from a single rule: they differ by maker and product. The chips fill in what Trex and TimberTech print for their own boards as a starting point. The count is for buying; joists, beams, posts, footings and the ledger are for your building department or a structural engineer.

Examples

A 16 × 12 ft deck in Trex boards with face screws: 16 ft × 12 ft

  1. Area: 16 ft × 12 ft = 192 sq ft = 17.84 m²
  2. Rows of boards: (12 ft + 0.1875 in) ÷ (5.5 in + 0.1875 in), rounded up = 26 rows
  3. Joists under the boards: 16 ft ÷ 16 in, rounded up, + 1 = 13 joists
  4. Boards: 26 rows × 1 board = 26 boards
  5. Crossings, board over joist: 26 rows × 13 joists = 338
  6. With 0% waste: 26 boards + 0%, rounded up = 26 boards
  7. Fasteners: 338 crossings × 2 = 676

26 boards no waste added. Deck area: 192 sq ft. Rows of boards: 26. Joists the boards cross: 13. Butt joints: 0. Fasteners: 676.

A 20 ft long deck in 16 ft boards, TimberTech clips, 5% waste and prices: 20 ft × 14 ft

  1. Area: 20 ft × 14 ft = 280 sq ft = 26.01 m²
  2. Rows of boards: (14 ft + 0.1875 in) ÷ (5.5 in + 0.1875 in), rounded up = 30 rows
  3. Joists under the boards: 20 ft ÷ 16 in, rounded up, + 1 = 16 joists
  4. Boards per row: 20 ft ÷ 16 ft, rounded up = 2 boards (16 ft: the part of a 16 ft board that ends on a joist)
  5. Boards: 30 rows × 2 boards = 60 boards
  6. Butt joints: 30 rows × (2 − 1) = 30
  7. Crossings, board over joist: 30 rows × 16 joists = 480
  8. With 5% waste: 60 boards + 5%, rounded up = 63 boards
  9. Fasteners: 480 crossings × 1 + 30 butt joints × 3 = 570
  10. Boxes of fasteners: 570 ÷ 90 per box, rounded up = 7 boxes
  11. Cost: 63 × $45.00 + 7 × $60.00 = $3,255.00

63 boards with 5% waste. Deck area: 280 sq ft. Rows of boards: 30. Joists the boards cross: 16. Butt joints: 30. Fasteners: 570. Boxes of fasteners: 7. Estimated cost: $3,255.00.

An L-shaped deck in two areas, 12 ft boards, boards only: Area 1: 16 ft × 12 ft; Area 2: 6 ft × 8 ft

  1. Area 1: 16 ft × 12 ft = 192 sq ft = 17.84 m²
  2. Area 2: 6 ft × 8 ft = 48 sq ft = 4.46 m²
  3. Total area: 192 sq ft + 48 sq ft = 240 sq ft = 22.30 m²
  4. Area 1: rows of boards: (12 ft + 0.1875 in) ÷ (5.5 in + 0.1875 in), rounded up = 26 rows
  5. Area 1: joists under the boards: 16 ft ÷ 16 in, rounded up, + 1 = 13 joists
  6. Area 1: boards per row: 16 ft ÷ 12 ft, rounded up = 2 boards (12 ft: the part of a 12 ft board that ends on a joist)
  7. Area 1: boards: 26 rows × 2 boards = 52 boards
  8. Area 1: butt joints: 26 rows × (2 − 1) = 26
  9. Area 1: crossings, board over joist: 26 rows × 13 joists = 338
  10. Area 2: rows of boards: (8 ft + 0.1875 in) ÷ (5.5 in + 0.1875 in), rounded up = 17 rows
  11. Area 2: joists under the boards: 6 ft ÷ 16 in, rounded up, + 1 = 6 joists
  12. Area 2: boards: 17 rows ÷ 2 rows per 12 ft board, rounded up = 9 boards (12 ft ÷ 6 ft, rounded down)
  13. Area 2: crossings, board over joist: 17 rows × 6 joists = 102
  14. Total boards: 52 + 9 = 61 boards
  15. With 0% waste: 61 boards + 0%, rounded up = 61 boards

61 boards no waste added. Deck area: 240 sq ft. Rows of boards: 43. Joists the boards cross: 19. Butt joints: 26.

Deck boards by deck size: 5.5 in boards, 3/16 in gap, joists 16 in on center, 16 ft boards, 2 screws a crossing and 2 extra per butt joint, no waste

Deck16 ft deck boards to buyDeck areaRows of boardsJoists the boards crossButt jointsFasteners
10 ft × 10 ft22 boards100 sq ft2290396
12 ft × 12 ft26 boards144 sq ft26100520
16 ft × 12 ft26 boards192 sq ft26130676
16 ft × 16 ft34 boards256 sq ft34130884
20 ft × 16 ft68 boards320 sq ft3416341,156
24 ft × 20 ft86 boards480 sq ft4319431,720

Every row gets whole boards, the last row ripped to fit. Rows no longer than a board share boards; longer rows join over a joist. Offcuts are not reused and no saw cuts are taken off: the waste is for that. Picture-frame borders, stairs and fascia are not counted.

Questions

How many deck boards do I need?

First the rows. Each row is one board wide plus a gap, and the last row is ripped to fit, so rows = (deck width + gap) ÷ (board width + gap), rounded up. A 12 ft deck with 5.5 in boards and a 3/16 in gap takes (144 + 0.1875) ÷ 5.6875 = 25.4, so 26 rows. Then the boards in each row. A deck no longer than one board takes one board a row, and a deck half a board long or less gets two rows out of each board. A longer deck joins boards end to end over a joist.

What joist spacing should I use?

The spacing of your framing, and no more than the board maker allows for the board you buy. Trex's guide allows 16 in on center for its 1 in boards on a home deck; TimberTech says never more than 16 in, or 24 in for its Advanced PVC Max board. Other boards have their own limits, so no spacing is filled in until you enter one or pick a maker. This site uses the spacing to count joists and fasteners; it does not size joists, beams, posts or footings.

How big a gap between deck boards?

What the instructions for your boards say. Trex asks for at least 3/16 in between boards and never more than 1/2 in; TimberTech asks for 3/16 in with its CONCEALoc clips. Other boards and fasteners differ, and no one gap fits every wood board, so enter your own for wood. Enter 0 for boards laid tight.

How many deck screws or hidden clips do I need?

One crossing is one board over one joist. Fasteners = rows × joists × fasteners per crossing, plus the extra ones at butt joints. Trex asks for two screws at every joist; TimberTech CONCEALoc uses one clip and screw at each joist. The first and last boards are often face-screwed instead of clipped; follow your board's guide for those.

How are butt joints counted?

Where a row is longer than a board, two boards meet end to end over a joist. Each board is used up to the last joist it reaches, so a 16 ft board on joists 16 in apart covers 16 ft and a 20 ft row takes two. Joints = rows × (boards per row − 1). At a joint two board ends sit over the joist, so they take more fasteners. TimberTech puts two clips on each board end, four in all instead of one, which is why its chip adds 3 per joint. Many makers also ask for a doubled joist or blocking there; ask your builder.

Does this tell me what joists, beams or footings I need?

No. It counts deck boards and fasteners for buying. It does not size joists, beams, posts, footings or the ledger, and does not check the building code. Those depend on the span, the load, the wood and your local rules; ask your local building department or a structural engineer.

What is not counted?

Picture-frame borders, fascia, stair treads and railings, joist tape and blocking. Offcuts from long rows are not reused for other rows and no saw cut is taken off each board, so add a little waste if your boards need trimming.

Related tools

Sources

Rows, joists, butt joints and crossings are plain arithmetic on your entries. The 5.5 in board width rests on Trex, TimberTech, Fiberon and ProWood, and the 12, 16 and 20 ft board lengths on Trex and TimberTech. The two maker chips fill in each maker's own gap, joist spacing and fastening from its installation documents, as starting values for its own boards only. The 0% starting waste has no source. The unit definitions are from NIST.

Methodology and sources

Last reviewed: October 9, 2026