Deck Calculator
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- Imperial and metric
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Results
29 decking boards, 13 joists and 3 footings for 192 ft²
How to use the deck calculator
- Enter the deck length out from the house (the direction the joists run) and the width along the house (the direction the boards run).
- Choose the joist spacing from your plans. Composite and PVC boards list a maximum joist spacing in their installation instructions, and diagonal layouts usually need closer joists.
- Enter the actual board width, the gap between boards, the board length you plan to buy and a waste allowance.
- Enter the beam plies, the post spacing along the beam and the footing size from your plans or local code.
- Add prices if you want a cost. A decking price per square foot (or m²) can be used instead of a price per board.
How the deck calculation works
Boards = rows × ⌈deck width ÷ board length⌉ × (1 + waste)
Joists = ⌈deck width ÷ joist spacing⌉ + 1 · Joist hangers = joists
Ledger = front rim joist = deck width · Beam lumber = plies × deck width
Posts and footings = ⌈deck width ÷ post spacing⌉ + 1
Footing concrete = π × (tube diameter ÷ 2)² × depth × footings
Screws = 2 × rows × joists, plus 2 at each butt joint
The layout is the common attached deck: a ledger on the house, joists running out from it, a rim joist across their outer ends and one beam on posts near the outer edge. Decking runs parallel to the house. The joist count includes the two outer joists that form the sides of the frame, so there is always a joist at both edges. Stairs, railings, blocking, flashing and the post lumber itself are not included. A freestanding deck uses a second beam and row of footings instead of a ledger.
Worked example: a 12 × 16 ft deck
The deck extends 12 ft from the house and runs 16 ft along it. With 5.5 in boards and 3/16 in gaps, 144 inches of length takes 26 rows. Each row is one 16 ft board, so you need 26 boards, or 29 boards with 10% waste. At 16 in on center there are 13 joists, each 12 ft long, with 13 hangers at the ledger. The ledger and rim are 16 ft each, and a 2-ply beam takes 32 ft of lumber. With posts no more than 8 ft apart, the beam sits on 3 posts. Each 10 in footing tube filled 36 in deep holds 1.64 ft³, or 3 bags of 80 lb concrete, so 9 bags in all. Fastening the boards takes about 676 screws.
Boards, joists and footings by deck size
Length out from the house × width along it. 5.5 in boards with 3/16 in gaps and 10% waste, using the shortest 12, 16 or 20 ft board that spans the width; joists at 16 in on center; posts no more than 8 ft apart under one beam.
| Deck size | Area | Boards | Joists | Footings | Screws |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 25 × 12 ft | 9 | 3 | 396 |
| 10 × 12 ft | 120 ft² | 25 × 12 ft | 10 | 3 | 440 |
| 12 × 12 ft | 144 ft² | 29 × 12 ft | 10 | 3 | 520 |
| 12 × 16 ft | 192 ft² | 29 × 16 ft | 13 | 3 | 676 |
| 14 × 16 ft | 224 ft² | 33 × 16 ft | 13 | 3 | 780 |
| 16 × 16 ft | 256 ft² | 38 × 16 ft | 13 | 3 | 884 |
| 16 × 20 ft | 320 ft² | 38 × 20 ft | 16 | 4 | 1,088 |
| 20 × 20 ft | 400 ft² | 48 × 20 ft | 16 | 4 | 1,376 |
Deck building tips
- Get the framing from your plans or code. Joist size and span, beam size, post size and footing size depend on loads, wood species and grade, soil and local rules. This site does not publish span tables; your building department or a plans examiner can tell you what applies.
- Treat the ledger with care. On an attached deck the ledger holds up one end of every joist. Fasten it as your code requires and flash it so water stays out of the house wall.
- Use rated hardware. Pressure-treated lumber can corrode plain steel, so use hangers, screws and nails rated for treated wood, typically hot-dip galvanized or stainless steel.
- Plan the board lengths. Boards at least as long as the deck is wide avoid joints. Where boards must join, ends need a joist under them, and many builders add a second joist or blocking at the joint and stagger joints between rows.
- Gap for the material. Wood boards shrink and swell with moisture, so the right gap depends on how dry they are when installed. For composite and PVC, use the gap in the maker's instructions.
- Call before you dig. In the US, call 811 a few days before digging footings to have buried utility lines marked.
For other footing shapes, a bell base or more tube sizes, use the Sonotube calculator or the post hole concrete calculator. Fencing the yard as well? Try the fence calculator.
Frequently asked questions
How many deck boards do I need for a 12×12 deck?
With 5.5 in wide boards and 3/16 in gaps, 12 ft (144 in) of deck takes 144 ÷ 5.6875 = 25.3, so 26 rows. With 12 ft boards that is one board per row, or 29 boards with 10% waste. Boards shorter than the deck width need butt joints over a joist, which adds boards and offcuts.
How many joists does a 12 ft wide deck need?
Divide the width by the joist spacing, round up and add one, so there is a joist at both edges. For 12 ft (144 in) at 16 in on center, 144 ÷ 16 = 9, plus 1 = 10 joists. At 12 in on center it is 13 joists, and at 24 in it is 7. Your decking instructions and local code set the maximum spacing.
How many screws do I need for a deck?
Two screws per board at every joist it crosses, plus two more at each butt joint. A 12 × 12 ft deck with 26 rows of boards over 10 joists needs 26 × 10 × 2 = 520 screws. Buy a few extra for drops and stripped heads. Hidden fastener systems list their own counts in the instructions.
How much concrete do I need for deck footings?
A 10 in form tube filled 36 in deep holds about 1.64 cubic feet, which is three 80 lb bags per footing. A 12 × 12 ft deck with one beam and posts no more than 8 ft apart has 3 footings, so about 9 bags. Footing size and depth must come from your plans or local code.
What size joists and beams does my deck need?
That depends on the span, joist spacing, wood species and grade, the loads and your local code, so this calculator does not size framing. In the US many jurisdictions base their deck rules on the International Residential Code, and your building department can tell you what applies. For a high deck, long spans or a hot tub, ask a structural engineer.