Deck Calculator

This deck calculator is for building a wooden or composite deck, not for card-game decks. A 16 × 12 ft deck attached to the house and 3 ft off the ground takes 13 joists of 2×8 at 16 in on center, one double 2×10 beam on 3 posts, three 18 in round footings (13 bags of 80 lb concrete) and 29 boards of 16 ft. Every span is checked against the prescriptive deck tables of the International Residential Code, Section R507.

How the deck calculator works

The frame is sized from the outside in. The joists run from the house out to the rim, so the joist span is the deck width minus any overhang past the beam. The IRC joist table (R507.6) gives the smallest joist that spans it at your spacing. The beam table (R507.5) then says how far the beam may span between posts for that joist span and overhang, which sets the number of posts. Each post carries a slice of deck, its tributary area, and the footing table (R507.3.1) turns that area and your soil into a footing size.

Joist span = width − overhang · Posts = length ÷ allowable beam span, rounded up, + 1 · Load per post = (joist span ÷ 2 + overhang) × post spacing

The boards are laid by the same engine as the deck board calculator: rows across the joists, split into stock lengths with butt joints only over joists, the fewest joints first and the least waste second.

Worked example: a 16 × 12 ft deck

Take the default: 16 ft along the house, 12 ft out, 3 ft high, attached with a ledger, pressure-treated Southern pine, joists 16 in on center and 12 in of overhang.

• Joist span: 12 ft − 12 in = 11 ft. A 2×8 spans 11 ft 10 in at 16 in, so 2×8 passes. Its maximum cantilever at an 11 ft back span is 28½ in (interpolated between 2 ft 6 in at 10 ft and 2 ft 3 in at 12 ft), so the 12 in overhang passes too.
• Joists: 16 ft ÷ 16 in = 12 bays, 13 joists.
• Beam: for 11 ft joists with a 1 ft cantilever the table column headed "12 & 1" governs. A double 2×10 spans 8 ft there, so 16 ft needs 2 spans and 3 posts.
• Load per post: (11 ÷ 2 + 1) × 8 = 52 sq ft. On 1,500 psf soil that is an 18 in round or 16 in square footing, 6 in thick.
• Concrete: an 18 in pad 6 in thick plus a 10 in tube up to grade from a 36 in hole is 2.25 cu ft. Three footings with 10% extra need 13 bags of 80 lb mix at 0.60 cu ft a bag (Quikrete's data sheet).
• Boards: 144 in plus a ⅛ in gap, divided by 5⅝ in per row, is 26 rows; each row is one 16 ft board. With 10% spare, 29 boards and 676 screws (2 per board at each of 13 joists).
• Ledger: at an 11 ft joist span, ½ in lag screws go 16½ in apart (Table R507.9.1.3(1)): 13 lags along 16 ft.

Joist spans the calculator uses

IRC Table R507.6, 40 psf live load, No. 2 grade, wet service:

The full table with cantilevers, the beam table and the snow-load rows are on the deck joist span calculator.

Attached or freestanding

An attached deck hangs its joists on a ledger bolted to the house band joist; the IRC wants that ledger at least a 2×8 and fastened to the table spacing, never to brick or stone veneer. A freestanding deck replaces the ledger with a second beam and posts next to the house. For the example that second beam carries only half the joist span and no overhang, so it may span 9 ft 4 in between posts; the deck ends up on 6 posts instead of 3.

Why the defaults are what they are

• 6×6 posts. The IRC allows 4×4 posts up to the heights in Table R507.4, but the American Wood Council's DCA 6 guide calls for 6×6 or larger, and a Southern pine or hem-fir 6×6 may stand 14 ft tall at every tributary area in the 40 psf table.
• 36 in frost depth. A placeholder: deck footings go at least 12 in below undisturbed ground (R507.3.2) and below your local frost line (R507.3.3). Ask the building department and enter their number.
• 1,500 psf soil. The lowest presumptive value in IRC Table R401.4.1(1), for clay and silt. Sand or gravel allow smaller footings.
• ⅛ in gap. DCA 6: "Space decking boards approximately 1/8" apart."

Stairs, railing and finish

A deck more than 30 in above grade needs a guard at least 36 in high on every open side (R312.1). Stairs follow the 7¾ in maximum riser: 36 in of height is 5 risers of 7³⁄₁₆ in. Plan the stringers with the stair calculator and, if the stairs land on a pad, the concrete steps calculator.

Calculators for each part of the job

• Boards by length, diagonal decks and picture frames: deck board calculator
• Every joist and beam span in the code tables: deck joist span calculator
• Footing size, hole depth and concrete per hole: deck footing calculator, with tube volumes in the Sonotube concrete calculator and bagged or ready-mix totals in the concrete calculator
• Stain for floor, railing and steps: deck stain calculator
• Price for your state, DIY or hired: deck cost calculator