How to use the deck calculator
Enter the deck's size, choosing which direction the boards will run, then the board width and gap. The calculator counts the rows of boards that fit across the deck, the boards per row at your chosen length, the joists underneath at your spacing, and the screws or hidden clips that hold it all down. The plan drawing shows board rows and joists to scale.
Boards usually run parallel to the house, across the joists. If the deck is longer than your boards, rows need a butt joint, which must land on a joist (often a doubled one); the calculator counts a full extra board for each joint.
The formulas
rows = ⌈(deck width + gap) ÷ (board width + gap)⌉
boards per row = ⌈deck length ÷ board length⌉
joists = ⌈deck length (in) ÷ spacing⌉ + 1
screws = rows × joists × 2
Worked example: a 16 × 12 ft composite deck
- Board plus gap: 5.5 + 0.1875 = 5.6875 in.
- Rows: (144 + 0.1875) ÷ 5.6875 = 25.4, so 26 rows.
- Boards: 16 ft boards cover each row in one piece, so 26 boards, 416 linear feet.
- Joists at 16 in on center: 192 ÷ 16 = 12 spaces, so 13 joists.
- Screws: 26 × 13 × 2 = 676 screws.
Gaps and spacing
| Item | Typical |
|---|---|
| Side gap, composite (Trex) | 3/16 in |
| End gap, composite (Trex) | 1/8 in above 40 °F, 3/16 in below |
| Side gap, wet pressure-treated wood | Tight to ⅛ in; it shrinks as it dries |
| Joist spacing, straight decking | 16 in on center (residential) |
| Joist spacing, diagonal decking | 12 in on center |
Always follow the installation guide for your specific board. Some composite and PVC lines allow 24 in spacing; many don't, and stair treads usually need closer supports.
Planning tips
- Pick board length to suit the deck. A 16 ft deck with 16 ft boards needs no joints. A 14 ft deck with 16 ft boards wastes 2 ft per row; 12 ft plus 4 ft pieces might suit better.
- Picture-frame borders look sharp and hide cut ends, but need blocking under the border and more linear footage.
- Hidden clips leave a clean surface and set the gap automatically; check that your board is grooved for them.
- The structure matters most. Ledger attachment, footings, beams and joist sizes are where decks fail. The AWC's DCA 6 guide is the standard prescriptive reference, and most decks need a permit.
Questions people ask
How many deck boards do I need for a 12x16 deck?
With boards running the 16 ft way, 5½ in wide with a 3/16 in gap, you need 26 rows across the 12 ft width: 26 boards 16 ft long. Running them the 12 ft way takes 34 rows of 12 ft boards.
How much gap between deck boards?
Trex's installation guide calls for 3/16 in side to side at all temperatures, and 1/8 to 3/16 in end to end depending on temperature. Wet pressure-treated wood is often laid tight or with a small gap, because it shrinks as it dries.
How far apart should deck joists be?
16 in on center is standard for residential decking laid straight; Trex specifies 16 in for residential and 12 in for commercial. Diagonal decking usually needs 12 in spacing. Check your board maker's span table.
How many screws do I need for decking?
Two screws at every place a board crosses a joist. A 12 × 16 ft deck with boards on 13 joists takes about 680 screws, so buy a 750 or 1,000 count box.
What width is a deck board?
A nominal 6 in deck board, wood or composite, is 5½ in wide. 5/4 × 6 wood decking and most composite boards are about 1 in thick; 2 × 6 wood is 1½ in thick.
Sources and assumptions
- Trex 2024 Decking Installation Guide: 3/16 in side gap, residential joists 16 in on center, two screws per joist
- American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide (framing, footings, ledger)
- Board counts assume full-length boards on each row; offcuts aren't reused, which also covers waste. Joist sizes and beam spans are not calculated here.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.