Deck Calculator
Deck boards, joists, rim joists and screws for a rectangular deck, with waste included.
Step 1 · Deck size
Length runs along the house; depth runs out from it.
Units
ftm
ftm
Step 2 · Boards
Boards run along the length; waste covers offcuts.
inmm
inmm
ftm
Step 3 · Framing
Joists run out from the house, spaced along the length with one at each end.
Framing sizes, beams and posts depend on span tables and local code. This tool counts boards, joists and screws only.
- Deck boards
- 29
- 16 ft boards · 26 rows
- Joists
- 13
- 12 ft long · 16″ o.c.
- Rim joists
- 2
- 16 ft each
- Screws
- 676
- 2 per board at each joist
192 sq ft deck · 26 rows × 16 ft × 1.10 waste = 458 lin ft ÷ 16 ft = 29 boards
Results link:
Boards run along the length, joists run across the depth. A nominal 6″ board is 5½″ wide; a 5/4×6 is about 5½″.
Materials list
16 × 12 ft deck · joists 16″ o.c. · 10% wasteQuantities rounded up to whole boards. Beams, posts, footings, ledger hardware, joist hangers and railings are not included.
Open or edit this calculation:
How the quantities are calculated
Rows of boards are counted across the deck’s depth, turned into linear feet along its length and divided by your board length after adding waste. Joists sit at your spacing along the length, with one at each end.
rows = ⌈(depth + gap) ÷ (board width + gap)⌉
boards = ⌈rows × length × (1 + waste %) ÷ board length⌉
joists = ⌈length ÷ joist spacing⌉ + 1
rim joists = 2 × length
screws = rows × joists × 2
boards = ⌈rows × length × (1 + waste %) ÷ board length⌉
joists = ⌈length ÷ joist spacing⌉ + 1
rim joists = 2 × length
screws = rows × joists × 2
Assumptions
| Item | Assumption |
|---|---|
| Shape | Rectangular deck, boards laid square to the joists. |
| Joists | One at each end plus one every spacing; each as long as the deck depth. |
| Rim joists | Two, each as long as the deck. One may be a ledger on an attached deck. |
| Screws | Face-screwed, 2 per board at each joist. |
| Not included | Beams, posts, footings, hangers, flashing, stairs, railings, fascia. |
Worked example
A 16 × 12 ft deck with 5½″ boards, ⅛″ gaps, 16 ft boards, joists 16″ on center and 10% waste:
rows = ⌈144.125 ÷ 5.625⌉ = 26
boards = ⌈26 × 16 × 1.10 ÷ 16⌉ = 29
joists = ⌈192 ÷ 16⌉ + 1 = 13 at 12 ft
screws = 26 × 13 × 2 = 676
boards = ⌈26 × 16 × 1.10 ÷ 16⌉ = 29
joists = ⌈192 ÷ 16⌉ + 1 = 13 at 12 ft
screws = 26 × 13 × 2 = 676
Frequently asked questions
What joist spacing should I use?
16″ on center is the usual spacing for 5/4 wood and most composite decking laid square to the joists. Diagonal boards and some composites need 12″. 24″ spacing is generally limited to 2× wood decking. Follow IRC Table R507.7 and the decking manufacturer’s instructions.
How many deck boards do I need?
Count the rows across the deck depth (board width plus gap), multiply by the deck length to get linear feet, add waste for cuts, and divide by the board length you are buying. The calculator does this and rounds up to whole boards.
How much gap should I leave between deck boards?
Many installers leave about 1/8″ between wood boards. Composite and PVC makers specify their own gaps, which can depend on temperature at installation, so use the figure from the product instructions.
How many screws do I need for a deck?
Face-screwed boards normally get two screws at every joist they cross. Multiply rows of boards by the number of joists and by two. Hidden-fastener systems use clips instead; check the clip count per square foot on the package.
Does this calculator size beams, posts or footings?
No. Joist size, beam size, post spacing and footings depend on spans, loads and local code (IRC Section R507). Use the span tables or a designer for framing, and this tool for the board, joist and screw counts.
Sources
- International Code Council, International Residential Code, Section R507 (Decks), including Table R507.7 (decking joist spacing).
- Decking manufacturers’ installation instructions for gaps, spacing and fasteners.
Testing
Formulas are shown on this page, based on the cited sources, and covered by automated tests. Methodology