Deck Calculator
Plan the decking boards, joists and deck screws needed for a rectangular deck.
How to use the Deck Calculator
- Enter the deck length (along the house) and pick its unit.
- Enter the deck depth (out from the house) and pick its unit.
- Choose the joist spacing.
- Enter the deck board width (actual) and pick its unit – 5½ in for 5/4×6 and 2×6 boards.
- Enter the gap between boards and pick its unit.
- Choose the deck board length.
- Enter the waste allowance.
- Enter the price per deck board.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Deck Calculator
Most wood decks use joists 16 in on center; composite and PVC decking usually requires 16 in (12 in for diagonal layouts), so check the manufacturer's span rating. Leave about ⅛ in between wood boards for drainage, or follow the composite maker's gapping chart.
This estimate covers the decking surface, joists and screws. You will also need a ledger, rim and band joists, beams, posts, footings, joist hangers and flashing, sized according to your local code and span tables. If the deck is longer than your boards, stagger the butt joints and add blocking or a double joist under each seam.
Formula
Rows of decking = ⌈Deck depth ÷ (Board width + Gap)⌉
Decking linear feet = Rows × Deck length × (1 + waste %)
Boards = Linear feet ÷ Board length
Joists = ⌈Deck length ÷ Spacing⌉ + 1
The layout assumes joists run out from the house and decking runs parallel to the house, the most common arrangement. Screws assume two fasteners at every board-to-joist crossing.
Example calculation
Using the default values:
- Deck length (along the house) = 16 ft
- Deck depth (out from the house) = 12 ft
- Joist spacing = 16 in on center
- Deck board width (actual) = 5.5 in
- Gap between boards = 1/8 in
- Deck board length = 16 ft
- Waste allowance = 10%
the calculator returns:
- Deck boards: 29 boards
- Deck area: 192 ft²
- Rows of decking: 26 rows
- Joists: 13 × 12′ 0″ long
- Deck screws (2 per joist crossing): 676 screws
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5½ in boards and ⅛ in gaps running the 16 ft way, you need 26 rows of 16 ft boards, or about 29 boards with 10% waste.
What joist spacing should I use?
16 in on center is standard. Use 12 in for diagonal decking or thinner composite boards, and 24 in only where the decking is rated for it.
How many screws do I need per square foot of deck?
Plan on roughly 350 screws per 100 sq ft of deck with 16 in joist spacing and 5½ in boards.
Last updated: · Category: Construction calculators