Deck calculator

How to figure out materials for a deck

A deck is three layers: footings and posts, a beam and joists, then the boards you walk on. Each layer is sized from the one above it, so you work from the top down. Here is the math the calculator runs for its default deck: 16 ft along the house, 12 ft out, 3 ft off the ground, 5/4 treated boards on southern pine framing at 16 inches on center.

1. 1

   Count the rows of deck boards

   A 5/4x6 board is 5½ in wide. Add a ⅛ in gap and each row covers 5.625 in. The deck is 144 in deep, so 144 ÷ 5.625 = 25.6, which rounds up to 26 rows. The rows run 16 ft, so 16 ft boards fit with no joints. Add 5 percent for warped or split boards and buy 28.

2. 2

   Pick the joist size from the span table

   The joists run 12 ft from the house to the beam. In the DCA 6 table, a southern pine 2x8 at 16 in reaches only 11 ft 10 in, so the calculator steps up to 2x10, good for 14 ft 0 in. A 16 ft run at 16 in on center needs 16 × 12 ÷ 16 = 12 spaces, so 13 joists, plus a rim joist and a ledger.

3. 3

   Size the beam and space the posts

   With a 12 ft joist span, a two-ply 2x12 southern pine beam can span 8 ft 4 in between posts. Three posts 8 ft apart carry the 16 ft beam. The calculator tries every common beam and picks the cheapest mix of beam lumber and post-and-footing sets.

4. 4

   Size the footings

   DCA 6 Table 4 gives a 24 in round footing, 10 in thick, for an 8 ft beam span and joists up to 14 ft on average soil. Add a 10 in form tube from the footing up to just above grade. At 36 in deep that is about 3.9 cubic feet of concrete per post. With 10 percent extra, that is 8 bags of 80 lb mix per footing, 24 for the deck.

5. 5

   Add railing, stairs and hardware

   A 3 ft high deck has 36 in of rise, so the stairs need 5 risers of 7.2 in and 4 treads. Railing runs around the 40 ft of open edge, minus the 4 ft stair opening. Then come hangers, hurricane ties, ledger lags, joist tape and screws. All of it lands in the shopping list.

Joist span table

Maximum joist span for No. 2 lumber, 40 psf live load and 10 psf dead load, from the American Wood Council's DCA 6 guide (Table 2). Spans are measured between supports and don't include a cantilever.

SP is southern pine, the common treated lumber east of the Rockies. DF/HF/SPF covers Douglas fir-larch, hem-fir and spruce-pine-fir, which is what most western yards stock. Redwood and western cedar span a little less again; the calculator handles all three.

Joist spacing by decking

The boards set the joist spacing before the span table does. A 5/4 board flexes noticeably on 24 in centers. Composite is softer still, and it creeps over time if the joists are too far apart.

Composite makers publish their own limits, and some lighter lines call for 12 in even when laid straight. Read the install guide for the exact board before you order joists.

What a deck costs

At October 2026 prices, the default 16 x 12 deck with railing and stairs comes to about $2,600 in treated lumber, hardware and concrete, or about $13.50 per square foot. Switch to composite and the boards alone go from under $600 to about $1,300, and composite railing kits cost several times more than a site-built wood rail. HomeGuide's 2026 survey puts a finished treated-pine deck at $25 to $50 per square foot installed, composite at $40 to $80, and deck stairs at $1,400 to $2,600 on top. Labor runs $11 to $22 per square foot.

Doing it yourself saves the labor but not the paperwork. Most towns want a permit with a drawing for any attached deck, plus a footing inspection before you pour. A pro is worth it when the deck is high, on a slope, attached to brick or a cantilevered floor, or anywhere the ledger can't bolt to a solid rim joist.

Tips from the job site

• The ledger is where decks fail. Bolt it with ½ in lag screws or through-bolts in a staggered pattern, never nails, and flash it so water can't reach the house rim joist. DCA 6 bans lead anchors outright.
• Use fasteners rated for treated lumber. Modern treated wood eats plain zinc. Buy hot-dip galvanized (G185 or better) or stainless hangers, screws and bolts.
• Crown every joist. Sight down each board and install it with the hump up. A row of crowned and sagging joists shows through the decking forever.
• Gap wet treated boards tight. Treated lumber often arrives soaking wet and shrinks as it dries. Butted boards open up to about ⅛ in on their own. Composite needs the gap the maker specifies, often 3/16 in.
• Tape the tops of joists. Butyl joist tape keeps water from sitting in screw holes and adds years to the frame, which matters more under composite boards that outlast wood.
• Call 811 before you dig. It's free, and gas and power lines often run right where a deck footing wants to go.

Sources and assumptions

Structural sizes are prescriptive, not engineered: a single-level deck attached to the house, 40 psf live load, 10 psf dead load, No. 2 lumber, soil good for 1,500 psf, and joists bearing on a beam at the outer edge with no cantilever. Prices are US retail prices checked in October 2026 and every board price can be edited.

• Joist spans, beam spans, footing sizes and ledger fastener spacing come from the American Wood Council's DCA 6 Prescriptive Residential Wood Deck Construction Guide (2015 IRC edition, Tables 2, 3A, 4 and 5). The calculator always uses 6x6 posts, which that guide requires.
• Decking spans, guards and stairs follow the 2021 International Residential Code, Section R507 and R311.7/R312: a guard above 30 in, risers of 7¾ in or less, and 36 in minimum stair width. Your town may have amended the code.
• Lumber and hardware prices are October 2026 list prices at Menards: 5/4x6x12 treated decking $15.49, 2x6x12 $19.48, 2x8x12 $23.60, 2x10x12 $33.83, 2x12x12 $42.95, 6x6x8 $44.99, 6x6 post base $24.99, joist hangers $2.68 to $2.98. Composite boards ran $21 to $40 per 12 ft board there; the default of $36 is a mid-grade line.
• Concrete uses 0.60 cubic feet per 80 lb bag at $6.48, the same figures as our concrete calculator, with 10 percent extra.
• Installed cost ranges come from HomeGuide's cost to build a deck (February 2026): $25 to $50 per sq ft for treated pine, $40 to $80 for composite, $1,400 to $2,600 for stairs.
• Screw count assumes two screws per board at every joist and about 350 deck screws per 5 lb box. Hidden clips assume one clip per board per joist.

This is an estimate for planning and budgeting. It isn't a stamped design, and your building department has the final word. Decks taller than about 8 ft, freestanding decks, hot tubs, and anything on poor soil need more than a prescriptive table.

Common questions