Free Deck Calculator: Boards, Joists and Fasteners
Enter the deck size, the joist spacing and your deck boards. You’ll get the boards, the framing (joists, rim, beams, posts, ledger and hangers), the fasteners and the railing parts. These are the same rules LaraKey’s takeoff calculator uses.
Deck materials
Estimates only. Measure on site and check with your supplier before you order.
How to calculate deck materials
Board rows = Deck depth (in) ÷ (Board width + 3/16 in gap), rounded up
Boards = Rows × (Deck length ÷ Board length, rounded up) + waste
Joists = Deck length (in) ÷ Joist spacing + 1, + 5%
Posts = one every 8 ft along each beam, plus one at the end (1 beam if attached, 2 if freestanding)
Deck screws = Rows × Joists × 2 ÷ 350 per box + 5%. Hidden clips = Rows × Joists ÷ 90 per box + 5%.
Railing: a post every 6 ft, 3 balusters per foot, top and bottom rails.
Examples
A 16 × 12 ft attached deck
12 ft = 144 in ÷ 5.6875 in = 26 rows, 1 board per row on 16 ft boards → 26 + 10% = 29 boards. Joists at 16 in: 13, + 5% = 14 × 12 ft. 3 posts, 13 hangers, 3 boxes of deck screws.
A 20 × 14 ft freestanding deck
Two beams with 4 posts each = 8 posts. On 12 ft boards, 30 rows × 2 per row = 60 + 10% = 66 boards.
Deck Calculator: common questions
How many deck boards do I need?
Divide the deck depth by the board width plus the gap to get the rows, multiply by the boards per row, and add about 10% for waste.
What joist spacing should I use?
16 in on center is standard for wood decking. Many composite boards need 12 in, especially on diagonal layouts. Check the board maker’s instructions.
How far apart should deck posts be?
Often 6 to 8 ft along the beam, depending on beam size, height and local code. The calculator uses 8 ft.
How many deck screws do I need?
Two screws where each board crosses each joist. A 16 × 12 deck with 26 rows and 13 joists needs about 676 screws.
Should I use hidden fasteners?
Hidden clips give a clean surface and are common with composite boards. They cost more and take longer to install.
What is a ledger board?
The board bolted to the house that carries one end of the joists on an attached deck. It needs proper flashing and structural screws or bolts.
Does this include footings and concrete?
No. Use the Concrete Calculator with the post-hole option for footings.
Are these numbers exact?
They’re estimates. Check your local code for spans, post spacing and railing height.
Talk to a person who knows remodeling.
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