Which way do the boards run?
Deck boards run perpendicular to the joists. Enter the deck dimension parallel to the boards as length, and the other dimension as width. On a typical deck attached to a house, joists run out from the ledger, so boards run parallel to the house. The board direction is then the length along the house.
Diagonal decking and picture-frame borders use more material and usually need closer joist spacing (see below).
How boards and screws are counted
- Rows = deck width (in) ÷ (actual board width + gap), rounded up.
- Boards per row = deck length ÷ board length, rounded up. A 16 ft deck uses one 16 ft board per row, or two 8 ft boards.
- Boards = rows × boards per row × (1 + waste).
- Joists = deck length ÷ joist spacing + 1.
- Screws = 2 per board at every joist.
A "2×6" or "5/4 × 6" deck board is actually about 5.5 in wide, so use the actual width. The defaults (16 × 12 ft, 5.5 in boards, 3/16 in gap, 16 ft boards, joists 16 in on center) give 26 rows, 29 boards with 10% waste and about 676 screws.
Joist spacing for different decking
The decking material limits how far apart joists can be. The International Residential Code (Table R507.7) allows:
| Decking | Boards perpendicular to joists | Boards diagonal to joists |
|---|---|---|
| 5/4 in wood | 16 in on center | 12 in on center |
| 2 in nominal wood | 24 in on center | 16 in on center |
| Composite / PVC | Per manufacturer (Trex: 16 in residential) | Per manufacturer (Trex: 12 in) |
| Decking | Boards perpendicular to joists | Boards diagonal to joists |
|---|---|---|
| Wood, about 25–32 mm thick | 400 mm centres | 300 mm centres |
| Wood, 38 mm or thicker | 600 mm centres | 400 mm centres |
| Composite / PVC | Per manufacturer (Trex: 400 mm residential) | Per manufacturer (Trex: 300 mm) |
Closer spacing means more joists and more screws. On a 16 ft deck, going from 16 in to 12 in spacing changes the joist count from 13 to 17.
Picking board lengths
Boards commonly come in 8, 10, 12, 16 and 20 ft lengths. The best length matches the deck, or divides into it evenly, so you don't throw away long offcuts. Butt joints must land on a joist, often a doubled joist or blocking. For a 14 ft deck, 16 ft boards waste 2 ft per row, while 8 ft boards need a butt joint in every row. Compare the options:
| Deck length | Board length | Boards per row | Total boards (12 ft wide, no waste) |
|---|---|---|---|
| 12 ft | 12 ft | 1 | 26 |
| 14 ft | 16 ft | 1 | 26 |
| 14 ft | 8 ft | 2 | 52 |
| 16 ft | 16 ft | 1 | 26 |
| 16 ft | 8 ft | 2 | 52 |
| 20 ft | 20 ft | 1 | 26 |
| 20 ft | 10 ft | 2 | 52 |
| Deck length | Board length | Boards per row | Total boards (3.6 m wide, no waste) |
|---|---|---|---|
| 3.6 m | 3.6 m | 1 | 25 |
| 4.2 m | 4.8 m | 1 | 25 |
| 4.2 m | 2.4 m | 2 | 50 |
| 4.8 m | 4.8 m | 1 | 25 |
| 4.8 m | 2.4 m | 2 | 50 |
| 6 m | 6 m | 1 | 25 |
| 6 m | 3 m | 2 | 50 |
Staggering joints in a random pattern, or using a centered "breaker" board, avoids a visible line of joints across the deck.
Gaps between boards
The gap lets water drain and lets the boards move with moisture and temperature. Follow the manufacturer's installation guide for composite and PVC boards, since many specify side gaps and end gaps that depend on temperature at installation. Wet pressure-treated boards shrink as they dry, so they're often laid tight, and dry or kiln-dried wood usually gets a small gap. The calculator default of 3/16 in is a common figure, but change it to match your product.
Worked examples
Ground-level deck, 12 × 12 ft, 12 ft boardsGround-level deck, 3.66 × 3.66 m, 3.66 m boards
Deck length 12 ft · Deck width 12 ft · Board width 5.5 in · Gap between boards 0.188 in · Board length 12 ft · Joist spacing 16 in · Waste / overage 10 %Deck length 3.66 m · Deck width 3.66 m · Board width 14 cm · Gap between boards 4.8 mm · Board length 3.66 m · Joist spacing 40.6 cm · Waste / overage 10 %
- Deck boards (12 ft): 29Deck boards (3.66 m): 29
- Rows of boards: 26
- Joists (running across width): 10
- Deck screws (2 per joist crossing): 520
Composite deck, 20 × 14 ft, 20 ft boards, joists 12 in on centerComposite deck, 6.1 × 4.27 m, 6.1 m boards, joists 30.5 cm on center
Deck length 20 ft · Deck width 14 ft · Board width 5.5 in · Gap between boards 0.188 in · Board length 20 ft · Joist spacing 12 in · Waste / overage 10 %Deck length 6.1 m · Deck width 4.27 m · Board width 14 cm · Gap between boards 4.8 mm · Board length 6.1 m · Joist spacing 30.5 cm · Waste / overage 10 %
- Deck boards (20 ft): 33Deck boards (6.1 m): 33
- Rows of boards: 30
- Joists (running across width): 21
- Deck screws (2 per joist crossing): 1,260
Long deck, 24 × 10 ft, 12 ft boardsLong deck, 7.32 × 3.05 m, 3.66 m boards
Deck length 24 ft · Deck width 10 ft · Board width 5.5 in · Gap between boards 0.188 in · Board length 12 ft · Joist spacing 16 in · Waste / overage 10 %Deck length 7.32 m · Deck width 3.05 m · Board width 14 cm · Gap between boards 4.8 mm · Board length 3.66 m · Joist spacing 40.6 cm · Waste / overage 10 %
- Deck boards (12 ft): 49Deck boards (3.66 m): 49
- Rows of boards: 22
- Linear length of decking: 528 ftLinear length of decking: 160.9 m
What this calculator doesn't size
This tool counts decking, joists and screws. It doesn't size the structure: joist size and span, beams, posts, footings and ledger attachment. Those depend on the span, the species and grade of lumber, the loads and local code. The American Wood Council's DCA 6 guide gives prescriptive tables for decks built to its rules, and many building departments require a permit for attached or raised decks. For footings and piers, see the column and post hole calculators.
Common mistakes
- Using the nominal width (6 in) instead of the actual width (5.5 in). That undercounts rows by about 8%.
- Forgetting fascia, stair treads and picture-frame borders. Add them as separate line items.
- Using joist spacing that's too wide for composite or diagonal decking.
- Ordering exactly the calculated count. Boards can be bowed, split or the wrong colour, so keep the 10% allowance.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 12 ft wide deck, 5.5 in boards, 3/16 in gaps: rows of boards | 26 | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 3.66 m wide deck, 14 cm boards, 4.8 mm gaps: rows of boards | 26 | Hand calculation | Match |
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5 in boards and 3/16 in gaps, 26 rows of 12 ft boards: 26 boards before waste, or 29 with 10%.
How many screws per deck board?
Typically 2 at each joist. For a 16 ft board on joists 16 in on center, that's 13 joists × 2 = 26 screws.
What spacing should deck joists be?
16 in on center is common for 5/4 wood and most composite run perpendicular. Diagonal decking needs 12 in (IRC Table R507.7; Trex).
How much gap between deck boards?
Follow the manufacturer for composite. For wood, wet treated boards are often laid tight and dry boards get a small gap.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- International Residential Code Table R507.7 (decking span) and Section R507
- Trex: How to Space and Install Deck Joists
- American Wood Council: DCA 6 Prescriptive Residential Wood Deck Construction Guide
- NIST Voluntary Product Standard PS 20-25 (American Softwood Lumber Standard), Table 3: dressed sizes
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.