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Deck calculator

Deck boards, joists, beam posts and screws for a deck attached to your house, with joist and beam sizes from the American Wood Council's deck guide.

Start from:

Your deck adds up to

29 deck boards, 16 ft

Deck area
192 sq ft
Decking
26 rows, 416 linear ft
Joists
13 × 12 ft 0 in
Joist size, minimum
2x10 at 16 in.
Posts for a 3-2x10 beam
3, up to 8 ft 9 in apart
Ledger and rim joist
16 ft each
Deck screws
676

The joist span from the ledger to the beam is 12 ft 0 in; a 2x10 at this spacing is allowed up to 14 ft 0 in. Boards include your extra and assume joints land on joists. Screws are 2 per board at every joist. Sizes are from prescriptive tables for typical decks: follow your local code and have your plans approved by your building department.

How to calculate deck materials

The calculator assumes a deck attached to the house with a ledger board, joists running out from the house to a single beam on posts, and deck boards running along the house across the joists. The joist and beam sizes come from the span tables in the AWC DCA 6, Prescriptive Residential Wood Deck Construction Guide (2015 IRC).

  1. Deck boards: rows = (deck depth + gap) ÷ (board width + gap), rounded up; linear feet = rows × deck length; boards = linear feet × (1 + extra %) ÷ board length, rounded up
  2. Joists = deck length ÷ joist spacing, rounded up, plus one for the far end
  3. Joist size: the smallest joist whose allowable span in the guide's Table 2 is at least the joist span (deck depth less the overhang)
  4. Posts = deck length ÷ the beam's allowable span from Table 3A, rounded up, plus one
  5. Screws = 2 × rows × joists (two #8 screws per board at each joist)

Worked example

A 16 × 12 ft deck in southern pine with joists 16 in. apart has a 12 ft 0 in joist span. A 2x8 reaches only 11 ft 10 in at that spacing, so the minimum is a 2x10 (up to 14 ft 0 in). It takes 192 ÷ 16 + 1 = 13 joists. A 3-2x10 beam carrying that span can go 8 ft 9 in between posts, so 16 ft needs 3 posts. With 5.5 in. boards and 1/8 in. gaps, 144 in. of depth takes 26 rows, or 416 linear ft; with 10% extra that's 29 boards of 16 ft and 676 screws.

Deck joist size by span

Minimum joist size for southern pine, from DCA 6 Table 2 (40 psf live load, 10 psf dead load, No. 2 grade, wet service). The next table lists the spans for every species. A dash means nothing in the table reaches.

Joist span12 in. o.c.16 in. o.c.24 in. o.c.
6 ft2x62x62x6
8 ft2x62x62x8
10 ft2x82x82x10
12 ft2x82x102x12
14 ft2x102x10–
16 ft2x102x12–

Maximum joist spans

SpeciesJoist12 in. o.c.16 in. o.c.24 in. o.c.Overhang at 16 in.
Southern pine2x69 ft 11 in9 ft 0 in7 ft 7 in1 ft 1 in
Southern pine2x813 ft 1 in11 ft 10 in9 ft 8 in2 ft 0 in
Southern pine2x1016 ft 2 in14 ft 0 in11 ft 5 in3 ft 5 in
Southern pine2x1218 ft 0 in16 ft 6 in13 ft 6 in4 ft 2 in
Douglas fir-larch, hem-fir, spruce-pine-fir2x69 ft 6 in8 ft 4 in6 ft 10 in1 ft 0 in
Douglas fir-larch, hem-fir, spruce-pine-fir2x812 ft 6 in11 ft 1 in9 ft 1 in1 ft 10 in
Douglas fir-larch, hem-fir, spruce-pine-fir2x1015 ft 8 in13 ft 7 in11 ft 1 in3 ft 2 in
Douglas fir-larch, hem-fir, spruce-pine-fir2x1218 ft 0 in15 ft 9 in12 ft 10 in3 ft 11 in
Redwood, western cedars, ponderosa pine, red pine2x68 ft 10 in8 ft 0 in6 ft 10 in0 ft 10 in
Redwood, western cedars, ponderosa pine, red pine2x811 ft 8 in10 ft 7 in8 ft 8 in1 ft 7 in
Redwood, western cedars, ponderosa pine, red pine2x1014 ft 11 in13 ft 0 in10 ft 7 in2 ft 7 in
Redwood, western cedars, ponderosa pine, red pine2x1217 ft 5 in15 ft 1 in12 ft 4 in3 ft 9 in

Beam spans (post spacing)

Maximum distance between posts for built-up beams, from DCA 6 Table 3A, by the joist span the beam carries. The guide also lists solid and glued-laminated beams.

Southern pine

BeamJoists to 6 ftJoists to 8 ftJoists to 10 ftJoists to 12 ftJoists to 14 ftJoists to 16 ftJoists to 18 ft
2-2x66 ft 8 in5 ft 8 in5 ft 1 in4 ft 7 in4 ft 3 in4 ft 0 in3 ft 9 in
2-2x88 ft 6 in7 ft 4 in6 ft 6 in5 ft 11 in5 ft 6 in5 ft 1 in4 ft 9 in
2-2x1010 ft 1 in8 ft 9 in7 ft 9 in7 ft 1 in6 ft 6 in6 ft 1 in5 ft 9 in
2-2x1211 ft 11 in10 ft 4 in9 ft 2 in8 ft 4 in7 ft 9 in7 ft 3 in6 ft 9 in
3-2x67 ft 11 in7 ft 2 in6 ft 5 in5 ft 10 in5 ft 5 in5 ft 0 in4 ft 9 in
3-2x810 ft 7 in9 ft 3 in8 ft 3 in7 ft 6 in6 ft 11 in6 ft 5 in6 ft 1 in
3-2x1012 ft 9 in11 ft 0 in9 ft 9 in8 ft 9 in8 ft 3 in7 ft 8 in7 ft 3 in
3-2x1215 ft 0 in13 ft 0 in11 ft 7 in10 ft 6 in9 ft 9 in9 ft 1 in8 ft 7 in

Douglas fir-larch, hem-fir, spruce-pine-fir; Redwood, western cedars, ponderosa pine, red pine

BeamJoists to 6 ftJoists to 8 ftJoists to 10 ftJoists to 12 ftJoists to 14 ftJoists to 16 ftJoists to 18 ft
2-2x65 ft 2 in4 ft 5 in3 ft 11 in3 ft 7 in3 ft 3 in2 ft 10 in2 ft 6 in
2-2x86 ft 7 in5 ft 8 in5 ft 1 in4 ft 7 in4 ft 3 in3 ft 10 in3 ft 5 in
2-2x108 ft 1 in7 ft 0 in6 ft 3 in5 ft 8 in5 ft 3 in4 ft 10 in4 ft 5 in
2-2x129 ft 5 in8 ft 2 in7 ft 3 in6 ft 7 in6 ft 1 in5 ft 8 in5 ft 4 in
3-2x67 ft 1 in6 ft 5 in5 ft 9 in5 ft 3 in4 ft 10 in4 ft 6 in4 ft 3 in
3-2x89 ft 5 in8 ft 3 in7 ft 4 in6 ft 8 in6 ft 2 in5 ft 9 in5 ft 5 in
3-2x1011 ft 9 in10 ft 2 in9 ft 1 in8 ft 3 in7 ft 7 in7 ft 1 in6 ft 8 in
3-2x1213 ft 8 in11 ft 10 in10 ft 6 in9 ft 7 in8 ft 10 in8 ft 3 in7 ft 10 in

Deck questions

How many deck boards do I need?

Divide the deck's depth by the board width plus the gap to get the rows, multiply by the deck length for linear feet, and divide by the board length. A 16 × 12 ft deck in 5.5 in. boards takes 26 rows and about 29 boards of 16 ft with 10% extra. Choose a board length that divides the deck neatly, or plan where the butt joints fall over joists: the guide says each piece of decking must bear on at least 3 joists.

What size joists do I need for a deck?

It depends on the span from the ledger to the beam, the spacing and the wood. In southern pine at 16 in. on center, the guide allows 2x6 joists up to 9 ft 0 in, 2x8 up to 11 ft 10 in, 2x10 up to 14 ft 0 in and 2x12 up to 16 ft 6 in. Joists can run past the beam by the lesser of the table's overhang and a quarter of the span.

How far apart should deck posts be?

As far as the beam can span, which shrinks as the joists it carries get longer. Table 3A of the guide lists it by beam size and joist span; the calculator reads it for your beam and deck depth and counts posts with one at each end. The guide also sizes footings and limits post height, which this calculator doesn't cover, so read those sections before you dig.

How much space should be between deck boards?

The guide says to space wood decking about 1/8 in. apart. Composite and PVC boards have their own gap rules, so use the maker's instructions and enter that gap above.

How wide are deck boards, and what screws do they need?

A 2x6 deck board is 5.5 in. wide actual and a 2x4 is 3.5 in. 5/4 decking and composite boards vary by maker, so measure one and enter its width. The guide calls for two 8d threaded nails or two #8 screws at each joist, which is what the screw count above uses. Use fasteners rated for the treated lumber or the decking you choose.

How much does it cost to build a deck?

Enter a price per deck board above for the decking. Framing lumber, footings, hardware, railings, stairs, permits and labor all add to it and vary widely by region and design, so get quotes for the whole job. For the steps down from the deck, use the stair calculator and press "Deck".