Deck Material Calculator
A deck is more than its surface boards. This deck material calculator builds a complete planning takeoff from your deck’s size: decking boards, joists, beams, posts, concrete footings, and the bags of concrete to fill them. The model assumes a simple ledger-attached rectangular deck — the most common DIY build — so treat the result as a shopping list to refine with your lumber yard, not a structural design.
Run parallel to the house (ledger side).
Distance the deck extends from the house.
Ground to top of decking — sets post length.
16″ oc is standard for wood decking.
Width of each concrete footing.
Sets how far apart posts sit under the beam.
Decking boards needed
35
- Joists
- 10
- Beam boards (doubled 2×)Each full deck length; ledger carries the house side
- 2
- Posts
- 3
- Post lumberCut to deck height
- 9ft
- Concrete footings
- 3
- Concrete (80-lb bags)≈0.6 ft³ per bag (rule of thumb)
- 12
How it was calculated
- Decking rows = ⌈(12 × 12) ÷ 5.75⌉ = 26; boards = ⌈26 × 16 ÷ 12⌉ = 35
- Joists = ⌊(12 × 12) ÷ 16⌋ + 1 = 10 at 16″ on center
- Beams = 2 boards (doubled beam, one beam line, each full deck length)
- Posts = ⌈16 ÷ 8⌉ + 1 = 3 (beam span 8 ft, capped at 8 ft)
- Post lumber = 3 × 3 ft = 9.0 linear ft
- Each footing = π × 0.50² × 3 = 2.36 ft³ → ⌈2.36 ÷ 0.6⌉ = 4 bags (80 lb, rule of thumb)
- Concrete = 3 footings × 4 = 12 bags
How it works
Decking and joists follow the board takeoff: rows = ⌈width in inches ÷ (5.5″ + ¼″ gap)⌉, boards = ⌈rows × length ÷ 12 ft⌉, joists = ⌊width ÷ spacing⌋ + 1.
Beams: one doubled beam line along the outer edge (2 full-length boards) — the ledger board bolted to the house carries the other side. Posts: ⌈length ÷ post spacing⌉ + 1, where post spacing is your beam span capped at 8 ft.
Footings match posts one-for-one. Each footing is modeled as a 3-ft-deep cylinder: π × (diameter ÷ 2)² × 3 ft³, filled with 80-lb concrete bags at about 0.6 ft³ per bag (rule of thumb), rounded up per footing.
Worked examples
16×12 ft deck, 3 ft high, 16″ joists, 12″ footings
Rows: ⌈144 ÷ 5.75⌉ = 26; boards: ⌈26 × 16 ÷ 12⌉ = 35. Joists: ⌊144 ÷ 16⌋ + 1 = 10. Beams: 2. Posts: ⌈16 ÷ 8⌉ + 1 = 3, with 3 × 3 = 9 linear ft of post lumber. Each footing: π × 0.5² × 3 = 2.36 ft³ → ⌈2.36 ÷ 0.6⌉ = 4 bags; 3 footings × 4 = 12 bags total.
20×10 ft deck, 4 ft high, 12″ joists, 12″ footings
Rows: ⌈120 ÷ 5.75⌉ = 21; boards: ⌈21 × 20 ÷ 12⌉ = 35. Joists: ⌊120 ÷ 12⌋ + 1 = 11. Beams: 2. Posts: ⌈20 ÷ 8⌉ + 1 = 4, with 4 × 4 = 16 linear ft of post lumber. Footings: 4, at 4 bags each = 16 bags of concrete.
How to measure
- Measure the deck footprint as the finished outside dimensions, not the ledger length.
- Measure deck height from the ground (or footing top) to the top of the decking — posts must reach it.
- Confirm your soil and frost line; the 3-ft footing depth used here is a rule of thumb, not your local code.
- Check the longest lumber your yard stocks — beams and joists over 16 ft may need splices or special orders.
- Sketch stairs, railings, and any cantilever before finalizing; they add lumber this takeoff does not count.
Assumptions & limitations
Planning estimate
- Simplified planning takeoff: assumes a rectangular ledger-attached deck with one beam line, 12-ft 5.5″ decking boards, and doubled 2× beam lumber — flag any deviation with your lumber yard.
- Post spacing from beam span (capped at 8 ft), 3-ft footing depth, and 80-lb bag yield of ≈0.6 ft³ are rules of thumb, not code values.
- Ledger board, flashing, joist hangers, railing, and stairs are not included.
- This is a planning estimate only — not a substitute for code compliance or professional design. Verify beam sizes, spans, and footing depths against IRC span tables and your local building code.
Frequently asked questions
Roughly: 35 decking boards (12-ft 5/4×6), 10–11 joists, 2 beam boards, 3–4 posts, plus concrete footings — about 12–16 bags of 80-lb concrete for 12″ × 3-ft footings. The calculator above sizes every line item to your exact dimensions.
A 12-inch-diameter, 3-ft-deep footing holds about 2.36 ft³ of concrete. At roughly 0.6 ft³ per 80-lb bag (rule of thumb), that is 4 bags per footing — always round up. Bigger diameters climb fast: an 18″ footing needs about 9 bags.
Below your local frost line — that is the code requirement, and it varies from about 12 inches in warm climates to 48 inches or more up north. The calculator uses 3 ft as a planning rule of thumb only. Call your building department for the real number.
Most residential decks use a beam — usually doubled 2×8 or larger — to carry the joists, with posts under the beam every 6–8 ft. The beam lets you pour fewer, deeper footings. Your local span tables dictate the actual beam size for your joist span.
A ledger-attached deck bolts to the house framing and needs only one beam line, which is what this calculator models. Freestanding decks need beams on both sides and more footings. If your house has brick veneer or you cannot flash a ledger properly, freestanding is the safer choice.