Decking calculator
This decking calculator tells you how many deck boards you need, how many linear feet that is, how many joists to put under them and how many screws to buy. Enter the deck size, the board width and gap, the board length and the joist spacing – 16 inches on center is the usual choice for wood decking. Two fasteners per board at each joist are counted.
Source: Terragarden/Habisreutinger, Verlegeanweisung Holzterrassen 2024 – Achsmaß nach Brettstärke (Quelle GD Holz), Fuge 6–10 mm, zwei Schrauben je Auflager (gelesen 09.10.2026). Updated: .
How it is calculated
A 16 × 12 ft deck with 5.5-inch boards, a 1/4-inch gap and 16 ft boards needs 26 rows = 416 linear feet, so 29 boards with 10% waste. Underneath you need 13 joists at 16 inches on center (156 ft) and 676 screws.
How the decking calculator works
- Rows: deck width plus one gap, divided by board width plus gap, rounded up.
- Linear feet: rows × deck length. Add waste and divide by the board length to get the number of boards.
- Joists: they run across the boards. Deck length ÷ joist spacing, rounded up, plus one joist – each as long as the deck width.
- Screws: the 2021 International Residential Code (R507.7) requires wood decking to be attached to each supporting member with at least two 8d threaded nails or two No. 8 wood screws, so the calculator counts rows × joists × 2.
Joist spacing and gaps
For wood decking the maximum joist spacing comes from IRC Table R507.7: 1¼-inch wood decking laid perpendicular may span 16 inches on center when each board runs over three or more joists, but only 12 inches over just two joists or when laid diagonally; 2-inch wood decking allows 24 inches (18 inches diagonal over two joists). 16 inches on center is the common choice. Composite and PVC boards follow the manufacturer’s instructions. A European guide citing the German timber trade association GD Holz gives 400 mm for 20/21 mm boards, 500 mm for 26 mm and 700 mm for 40 mm, with a 6–10 mm gap.
Examples
- 16 × 12 ft, 5.5 in boards, 1/4 in gap, 16 ft boards, 16 in o.c.: 26 rows, 416 ft, 29 boards, 13 joists, 676 screws.
- 20 × 10 ft, 5.5 in boards, 3/16 in gap, 12 ft boards, 16 in o.c.: 22 rows, 440 ft, 41 boards, 16 joists (160 ft), 704 screws.
- 5 × 4 m, 145 mm boards, 7 mm gap, 4 m boards, 50 cm: 27 rows, 135 m, 38 boards, 11 joists, 594 screws.
For the base under a ground-level deck see the gravel calculator and the concrete calculator for footings.
Limits
This is an estimate for rectangular decks. Butt joints need doubled joists or blocking, and rim joists, beams, posts, ledger boards and stairs are not included. Joist size and span must follow the code tables or a designer. Picture-frame borders, angles and curves increase waste.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5.5-inch boards and a 1/4-inch gap: 26 rows × 12 ft = 312 linear feet. With 12 ft boards that is 26 boards, or 29 with 10% waste.
How do I calculate linear feet of decking?
Divide the deck width (plus one gap) by the board width plus gap, round up to get the rows, and multiply by the deck length.
How far apart should deck joists be?
16 inches on center is standard for 5/4 wood decking; the 2021 IRC Table R507.7 sets the maximum by board thickness, angle and the number of joists a board spans (1¼-inch wood: 16 in over three or more joists, 12 in over two). Composite boards often need 16 inches or less, diagonal boards closer spacing.
How many screws do I need for decking?
Two per board at every joist. On a 16 × 12 ft deck with 13 joists and 26 rows that is 676 screws – about 3.5 per square foot.
How big should the gap between deck boards be?
Commonly 1/8 to 1/4 inch for wood, depending on moisture; a European guide citing GD Holz gives 6 to 10 mm. Composite boards follow the manufacturer’s gap chart.
How much waste should I allow for decking?
Usually 5 to 10% for a simple rectangle, more for diagonal patterns, picture-frame borders or curves.
Sources and legal basis
- Terragarden/Habisreutinger, Verlegeanweisung Holzterrassen 2024 – Achsmaß nach Brettstärke (Quelle GD Holz), Fuge 6–10 mm, zwei Schrauben je Auflager (gelesen 09.10.2026)
- Penn State PHRC, “Decked Out” handouts 2025 – 2021 IRC R507.7 decking fasteners and joist spacing (read 09.10.2026)
As of: · Editorial policy
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