Deck Calculator
Boards, joists, and fasteners for a new deck.
Deck Calculator
Boards = ceil(deck width in / (board width + gap) × (1 + waste)); Joists = ceil(length in / spacing) + 1
Calculated instantly
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What This Calculator Does
The Deck Calculator estimates decking boards, board linear feet, joists, joist linear feet, and screws from the deck size, board dimensions, and joist spacing. It gives a shopping list rather than a single area figure.
How It Works
The number of board rows is the deck width in inches divided by the board width plus the gap. That is scaled by the waste allowance and rounded up. Joists run the other way: the deck length in inches divided by joist spacing, plus one for the end joist. Screws use a typical 2.5 per square foot.
Example
A 16 × 12 ft deck is 192 sq ft. With 5.5 inch boards, a 1/8 inch gap, and 10% waste, it needs 29 boards and 464 linear feet. Joists at 16 inches on centre need 13 pieces and 156 linear feet; screws come to about 480.
Formula
Boards = ceil((deck width in / (board width + gap)) × (1 + waste/100))Joists = ceil((deck length in / joist spacing)) + 1Screws ≈ Deck area × 2.5A planning estimate for two screws per board per joist crossing.
Reading the Results
Deck boards
Board count assuming boards run the long direction. Add extra if you plan a picture-frame border or diagonal layout.
Joists
Joist count including the rim joist at the far end. Ledger attachment and post spacing still need structural design.
Screws
An approximate fastener count. Hidden fastener systems use a different quantity per square foot.
Sources and References
International Residential Code (IRC) R507 — exterior decks
Joist spans, ledger attachment, and guard requirements
The IRC sets structural limits; this tool only estimates quantities.
American Wood Council — Prescriptive Residential Wood Deck Construction Guide
Joist spacing and fastener schedules
The guide gives prescriptive spans for common lumber sizes and deck loads.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How many deck boards do I need?
Divide the deck width in inches by the board width plus gap, then add 10% waste and round up. A 12 ft wide deck with 5.5 inch boards and a 1/8 inch gap needs about 29 boards.
What joist spacing should I use?
Sixteen inches on centre is common for wood decking; composite boards often require 12 inches. The span also depends on joist size, species, and load, so check the span table or a designer.
Should deck boards run parallel to the house?
Usually parallel to the house is the default because it looks calmer and is simpler to lay. Perpendicular or diagonal layouts change the board count and waste, so recalculate with the dimensions swapped.
How many screws per square foot of decking?
About 2.5 screws per square foot for a standard two-screw fixing at each joist. Hidden clips use roughly one clip per board per joist plus screws.