How to use the deck calculator
- Enter the deck length in the direction the boards run, and the width.
- Enter the actual board width (a nominal 6 in board is about 5½ in) and the gap between boards.
- Enter the board length you will buy.
- Choose the joist spacing from your plans.
- Keep about 10% waste for end cuts and defects.
Use the counts as a shopping list to check against your deck plan. Decoratoradvice is not a substitute for a design that meets your local code.
Deck formulas
Rows of boards = deck width ÷ (board width + gap), rounded up
Linear feet = rows × deck length
Boards = linear feet × (1 + waste) ÷ board length, rounded up
Joists = deck length ÷ joist spacing, rounded up, + 1
Screws = rows × joists × 2
Two screws per board at every joist follows the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6), which calls for 2 #8 screws (or 2 8d threaded nails) per joist and boards spaced about ⅛ in apart.
Worked example
A 16 × 12 ft deck, boards running the 16 ft length, 5½ in boards, ⅛ in gaps, 16 ft boards, joists 16 in on centre:
Rows: 144 in ÷ 5.625 in = 25.6, so 26
Linear feet: 26 × 16 = 416 ft
Boards: 416 × 1.10 ÷ 16 = 28.6, so 29 boards
Joists: 192 in ÷ 16 in = 12, + 1 = 13 joists, each 12 ft
Screws: 26 × 13 × 2 = 676
Framing and footings
This calculator counts decking and joists. Joist size, beam size, post spacing and footing size depend on spans and loads. DCA 6 has prescriptive tables for these, and requires footings to bear on undisturbed soil at least 12 in below grade or below the frost line, whichever is deeper. Footing concrete can be worked out with the concrete calculator.
Tips
- Each board should bear on at least three joists, as DCA 6 requires, so avoid short offcuts on long runs.
- Stagger board end joints so they do not line up across the deck.
- For stairs down to the yard, use the stair calculator.
- Hardwood and some composite boards have their own fastening rules; follow the manufacturer.
Common mistakes
- Using the nominal board width. A 6 in board is about 5½ in wide.
- Forgetting the gap. Over 26 rows, ⅛ in gaps add over 3 in.
- Counting joists without the end joist. 12 spaces need 13 joists.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5½ in boards and ⅛ in gaps: 26 rows of 12 ft, or 312 linear ft. With 10% waste, 343 ft, so 29 boards of 12 ft.
How much gap should I leave between deck boards?
DCA 6 calls for about ⅛ in for wood decking. Composite and hardwood products set their own gaps.
How many screws per deck board?
Two at each joist, per DCA 6. A board crossing 13 joists takes 26 screws.
What joist spacing should I use?
It comes from your plans and the decking's span rating, usually 12, 16 or 24 in on centre. Composite and PVC decking set their own maximum joist spacing; check the product.
How do I price the lumber?
Most decking and joists are priced per piece. For hardwood priced by volume, use the board foot calculator.
Can I add a fence or railing?
Yes. The fence calculator counts posts and pickets for a yard fence; deck railings must meet local code for height and strength.
Key takeaways
- Rows = width ÷ (board width + gap); boards = rows × length × (1 + waste) ÷ board length.
- Joists = length ÷ spacing + 1.
- Two screws per board at each joist, with about ⅛ in gaps, per DCA 6.
- Size joists, beams and footings from DCA 6 tables or your plans.