How to use the deck calculator
- Choose attached or freestanding. An attached deck hangs off a ledger on the house and needs one beam. A freestanding deck needs a beam on both sides.
- Enter the length and width. Length runs in the same direction as the deck boards; width runs along the joists (out from the house on an attached deck).
- Pick the board and joist spacing. The calculator counts board rows from the board width plus gap, then how many boards each row uses.
- Add your board price for a decking cost. The framing list tells you how many joists, rim boards and posts to buy.
How we calculate deck materials
Boards = Rows × (Length ÷ Board length) × (1 + Waste)
Joists = Length ÷ Joist spacing + 1, each as long as the width
Rim boards = 2 × Length
Screws = Rows × Joists × 2, plus 5%
Posts per beam = Length ÷ 8 ft + 1
Boards shorter than the deck are staggered, so offcuts from one row are reused in another; that's why the board count uses a fraction of a board per row rather than rounding each row up. Posts assume a beam supported every 8 ft, which is typical for a doubled 2×10 beam; your beam size and local code may allow more or less.
Maximum joist spacing for decking (IRC R507.7)
| Decking | Boards perpendicular | Boards diagonal (45°) |
|---|---|---|
| 5/4 (1¼″) wood | 16″ | 12″ |
| 2× (1½″) wood | 24″ | 16″ |
| Composite or PVC | Per manufacturer, usually 16″ (12″ diagonal) | |
Worked example
Boards run along the 16 ft side. With 5½″ boards and 3/16″ gaps, each row covers 5.69″, so 144″ ÷ 5.69″ = 25.3, or 26 rows. One 16 ft board per row plus 10% waste = 29 boards.
Joists at 16″ centres along 16 ft: 192 ÷ 16 + 1 = 13 joists, 12 ft long, plus two 16 ft rim boards. Screws: 26 × 13 × 2 = 676, plus 5% = 710. One beam with posts every 8 ft = 3 posts.
Frequently asked questions
How many deck boards do I need for a 12×12 deck?
With 5½″ boards and a 3/16″ gap you need 26 rows. Using 12 ft boards, that's 26 boards, or 29 with 10% waste.
How much gap should I leave between deck boards?
For kiln-dried wood and most composites, about 3/16″ (5 mm). Wet pressure-treated boards shrink as they dry, so they're often laid tight or with a ⅛″ gap. Always follow the composite manufacturer's gapping chart, which varies with temperature.
How many screws per deck board?
Two screws at every joist the board crosses. A 16 ft board over joists at 16″ centres crosses 13 joists, so it needs 26 screws.
What size joists do I need?
That depends on the joist span, spacing and wood species. Check the deck joist span table in your local code (IRC Table R507.6) before you buy.