Deck Calculator — Boards, Joists, Beams & Footings
Decking, framing, footings and fasteners for a deck, with a cut list and code checks.
Units and currency
Deck size
Joists
Beams, posts and footings
Decking
Ledger fasteners
Spacing from IRC Table R507.9.1.3(1) for your joist span (interpolated; up to 18 ft). Stagger them top to bottom, 2 in from the ledger ends, and flash the ledger (R703.4).
Checks against the code and DCA 6
Cut list
| Member | Pieces | Cut to | Buy |
|---|
Shopping list
| Item | Quantity | Price | Cost |
|---|---|---|---|
| Total of the prices entered | — | ||
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Deck Calculator — Boards, Joists, Beams & Footings
Get the material list for a rectangular deck — attached to the house with a ledger, or freestanding. Enter the size, the joist spacing and overhang, the beams and posts and your decking boards, and the calculator counts the joists, rim joists and ledger, beam plies, posts and footings, joist hangers and ties, blocking and fasteners, and plans the decking: rows, joints on joists, every piece on at least three joists, and the fewest boards with offcuts reused.
It does not size the structure: the joist span, beam span and footing size are your choices, shown so you can check them in the IRC R507 tables or the American Wood Council's DCA 6 guide — the joist spacing for your decking (IRC Table R507.7), overhangs and ledger fasteners (Table R507.9.1.3(1)) are checked for you. This is an estimate, not a professional design: have plans approved where your building authority requires it.
How to use it
- Choose US or Metric units. Enter the deck's width along the house and depth out from it, and whether it is attached or freestanding.
- Set the joists: size label, spacing (12, 16 or 24 in), how far they run past the outer beam, and any beams under their middle to shorten the span.
- Set the beams and posts: plies, dropped or flush, the largest post spacing along a beam and the beam overhang, the post height and the footing size and depth — check them against IRC Table R507.3.1 or DCA 6.
- Describe the decking: wood or composite, board width and gap, across the joists or diagonal, overhangs and the board length you can buy. Read the checks, the cut list and the shopping list, type prices, and copy or download it.
Examples
13 joists 11′ 9″ long, span about 10′ 6″; 3 posts on one beam (7 ft spans, 1 ft overhangs); 13 joist hangers, 13 joist ties, 12 blocks; 39 rim fasteners and 26 ply nails. Ledger: 12 lag screws at about 17¼ in (Table R507.9.1.3(1), 40 psf).
26 rows of 16′ 2″: 26 boards of 20 ft and 676 screws (2 at each of the 13 joists in a row). A 16 ft board is 2 in short of a row, so in 16 ft boards it is 52 pieces cut from 30 boards, every joint on a joist and no two neighbouring rows joined on the same joist — and 728 screws, two more at each joint.
52 pieces (joints staggered over three joists) cut from 48 boards of 12 ft
1.18 ft³ each = 3.53 ft³ → 6 bags of 80 lb concrete mix (0.60 ft³ per bag)
13 joists of 3.51 m (span about 3.12 m); rows of 4.85 m need two pieces each: 30 boards + 5% = 32
How it is calculated
- Joists = width ÷ spacing, rounded up, + 1, each the depth minus the ledger (or house-side rim) and the outer rim. The joist span is the clear distance from the ledger (or house-side beam) to the outer beam, minus the overhang. A flush beam is the edge of the deck: the joists hang on its face in hangers, so they end there — no overhang and no rim joist on that side.
- Posts per beam = (beam length − both overhangs) ÷ the largest post spacing, rounded up, + 1; the beam span is the distance between posts.
- Decking rows = (depth + front overhang) ÷ (board width + gap), rounded up; diagonal rows are cut to the longest chord in each board's band. Rows longer than a board are cut with every joint on a joist (for diagonal rows approximately: see the limitations), every piece on at least three joists and the first joint of each row one joist further back than in the row before (over a cycle of three joists), so neighbouring rows never share a joint when the board length leaves a choice; the pieces are then cut from whole boards, longest first, reusing offcuts.
- Fasteners: two screws or nails at every joist a board crosses (IRC R507.7), one hidden clip per crossing; three nails or screws at each joist end on a rim (R507.6.2); two rows of 10d nails at 16 in along multi-ply beams (R507.5).
- Footing concrete = π/4 × diameter² (or side²) × concrete depth, per post.
What the checks use
- Joist spacing for the decking — IRC 2021 Table R507.7: 1¼-in wood decking 16 in across the joists (12 in where a board rests on only two joists) and 12 in (8 in) diagonally; 2-in wood 24 in across and 24 in (18 in) diagonally; plastic composite as its label says (R507.2.2).
- Overhangs — joists past the beam at most 1/4 of the joist span and the Table R507.6 value (DCA 6); beams past their end posts at most 1/4 of the beam span (R507.5).
- Ledger fasteners — on-centre spacing from Table R507.9.1.3(1) for the joist span and load (interpolated, up to 18 ft), 2 in from the ends of the ledger and staggered top to bottom; the ledger is at least a 2×8 and must be flashed (R507.9.1.1, R703.4).
- Footings — at least 12 in below undisturbed ground and frost-protected where required (R507.3.2, R507.3.3); their minimum size comes from Table R507.3.1 by tributary area and soil.
- Decking support — each piece on at least three joists and boards about 1/8 in apart (AWC DCA 6).
Spans are yours to check
Allowable joist and beam spans depend on the species, grade, size and load, so the calculator shows the spans of your layout and leaves the choice to the tables: IRC Table R507.6 (deck joists, with the maximum cantilever for each back span), Tables R507.5(1)–(4) (deck beams), Table R507.4 (post heights) and Table R507.3.1 (footings), or the prescriptive tables of DCA 6. If a span is too long, use deeper joists, closer spacing or a beam under the middle.
Limitations
- Rectangular decks only — no stairs, landings, guards or multi-level framing (see the stair calculator for stairs).
- It does not design or check structural sizes: spans, post heights and footing sizes must be checked in the IRC R507 tables, DCA 6 or by an engineer, and local codes and frost depths apply.
- The checks are those of the US International Residential Code (IRC) and the American Wood Council's DCA 6; building rules elsewhere can differ, so outside the US follow your own country's regulations.
- Diagonal decking needs blocking under the short corner boards. Its cut plan estimates the boards to buy: joints are spaced along each row in steps of the joist spacing × √2, but a row that starts between two joists will not have them exactly over one — mark each joint over a joist as you lay — and offcuts are not reused across the 45° ends.
- Hardware is counted per piece; choose hangers, ties and post bases rated for your lumber and loads.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How many deck boards do I need?
Divide the deck depth plus the front overhang by the board width plus the gap and round up to get the rows. If a row is no longer than your boards, buy one board per row; otherwise the calculator cuts each row into pieces that join on joists and works out how many whole boards those pieces need.
What joist spacing does my decking need?
IRC Table R507.7: for 5/4 (1¼-in) wood boards at most 16 in on centre across the joists, or 12 in for diagonal decking; for 2-in wood boards 24 in. Composite boards carry the maximum span on their label.
How many screws per deck board?
Two at every joist the board crosses (IRC R507.7 asks for at least two 8d threaded nails or two No. 8 wood screws per support), and two more at a butt joint, where both board ends sit on one joist. A 16 ft board across joists 16 in apart crosses 13 joists: 26 screws.
How far apart should ledger lag screws be?
IRC Table R507.9.1.3(1) sets it by joist span and load: for a 40 psf live load and ½ in lag screws, 18 in on centre for a 10 ft span, 15 in for 12 ft and 11 in for 16 ft. Through-bolts can be further apart. The calculator interpolates for your span and counts them along the ledger.
How much concrete for deck footings?
Volume per footing = π/4 × diameter² × depth for round footings (side² × depth for square), times the number of posts. An 18 in round footing 8 in deep is 1.18 ft³ — about two 80 lb bags of concrete mix at 0.60 ft³ each. The size itself comes from IRC Table R507.3.1 or DCA 6.