Deck material calculator
Tell us the deck you want. You get every board, fastener and footing it needs — and the reasoning behind each number, so you can check our working.
- Your deck is 20 ft long but the boards are 16 ft, so every row needs a butt seam. Seams must land on a joist, and most builders double the joist under a seam line.
- Ledger attachment and flashing is the most common point of deck failure. The fastener schedule and flashing detail must come from AWC DCA6 or your local building department, not from a calculator.
- Span limits are not yet verified against AWC DCA6 and are withheld. Size your joists and beams with your local building department.
- Footings must reach below your local frost line. 42 in is a placeholder, not a recommendation — frost depth ranges from zero in the far south to 100+ in in the far north.
- Post spacing is assumed at 6 ft because the beam span table is not yet verified. Confirm beam size and post spacing before ordering.
- The stair needs 45.0 in (3.8 ft) of clear run at the bottom. Check it fits before you build.
Decking
| Item | Qty | Unit |
|---|---|---|
| 5/4x6 decking, 16 ft lengths 34 rows across 16 ft of width. Each board covers 5.75 in (5.50 in face + 0.25 in gap), so 16 ft of width needs 34 rows. 34 rows x 20 ft = 680 linear ft, plus 10% cutting waste for a straight layout = 748 linear ft, bought as 16 ft boards. | 47 | each |
Framing
| Item | Qty | Unit |
|---|---|---|
| 2x8 joists, 16 ft Joists run the 16 ft width, spaced 16 in on centre along the 20 ft length: floor(240 in / 16 in) + 1 = 16. The +1 is the joist that closes the far end. | 16 | each |
| 2x8 rim joist, 20 ft One rim joist closes the outboard edge, running the full length of the deck. | 1 | each |
| 2x8 ledger board, 20 ft The ledger carries the house-side end of every joist and runs the full length. | 1 | each |
| 2x8 blocking, cut to fit between joists A 16 ft joist span gets 2 row(s) of blocking (one per 8 ft of span). Each row needs a piece in each of the 15 bays: 2 x 15 = 30. | 30 | each |
| Structural posts One post per footing: 4. | 4 | each |
Hardware
| Item | Qty | Unit |
|---|---|---|
| Joist hangers One hanger per joist where it meets the ledger: 16 joists = 16 hangers. | 16 | each |
Foundation
| Item | Qty | Unit |
|---|---|---|
| 12 in diameter footings, 42 in deep One footing per post. Posts assumed at 6 ft on centre along the 20 ft beam: floor(20 / 6) + 1 = 4. | 4 | each |
| Concrete mix, 80 lb bags Each footing is a cylinder: pi x (0.50 ft radius)^2 x 3.50 ft deep = 2.75 cu ft. 4 footings = 11.00 cu ft total. One 80 lb bag yields 0.60 cu ft, so 11.00 / 0.60 = 19 bags. | 19 | bags |
Fasteners
| Item | Qty | Unit |
|---|---|---|
| Deck screws (face fastening) 2 screws at every board-to-joist crossing: 34 rows x 16 joists x 2 = 1088 screws, plus 5% spares = 1143. | 1,143 | each |
Railing
| Item | Qty | Unit |
|---|---|---|
| Guard posts Posts at no more than 6 ft apart on each open run (16 ft, 20 ft, 13 ft), plus one to close each run, less 2 shared corner posts = 11. | 11 | each |
| Top and bottom rails 49.0 ft of guard, with a top and a bottom rail: 49.0 x 2. | 98 | linear ft |
| 2x2 balusters A 4 in sphere must not pass through the guard (IRC R312.1.3). With 1.50 in balusters and a 3.75 in design gap, they sit 5.25 in on centre: 588 in of guard / 5.25 = 112. | 112 | each |
Stairs
| Item | Qty | Unit |
|---|---|---|
| 2x12 stringers Stringers no more than 16 in apart across a 36 in wide stair: ceil(36 / 16) + 1 = 4. | 4 | each |
| Tread stock (2 boards per tread) 4 treads x 2 boards x 3.0 ft wide = 24.0 linear ft. | 24 | linear ft |
How this calculator works
Actual dimensions, not nominal
A 2x8 is not eight inches wide. It is 7.25 in. A 5/4x6 deck board is 5.5 in wide, not six. Every quantity here is built from actual surfaced dimensions, because estimating on the label is the most common way a materials list comes out short.
The gap between boards is coverage too
Deck boards are installed with a gap for drainage and movement, typically 1/4 in. That gap is real coverage: a 5.5 in board with a 1/4 in gap covers 5.75 in of deck. Estimating on board width alone over-counts boards on every single deck.
Waste depends on the layout
A straight run wastes little, roughly 10%. A 45 degree layout wastes both ends of every board, closer to 15%. A picture-frame border adds mitred perimeter boards on top of that, around 18%. A single flat percentage cannot be right for all three.
Counts come from geometry
Joists are floor(span / spacing) + 1, because the last joist closes the far end. Fasteners come from the number of places a board actually crosses a joist — rows times joists — not from a screws-per-100-square-feet rule of thumb that ignores both board width and joist spacing.
What we will not guess
Joist and beam span limits are structural-safety data. Until our span tables are verified line by line against the published AWC DCA6 tables, this site withholds them and says so, rather than printing a number that looks authoritative. Footing depth must reach below your local frost line, which varies from nothing in the far south to over eight feet in the far north. Confirm both with your local building department — see our methodology.
Other calculators
- Decking board calculator How many boards, and how many linear feet. Counting the gap between them, because that gap is coverage.
- Deck framing calculator Joists, blocking, rim, ledger and hangers — counted from the frame geometry rather than a rule of thumb.
- Deck stair calculator Risers, treads, stringers and total run — with every riser held inside the 7.75 in code limit.
- Deck railing calculator Posts, rails and balusters, spaced so a 4 in sphere cannot pass — with margin, so site tolerance cannot push you over.