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Deck Board Calculator

Rows across the depth, boards along the run, and the gap between them counted.

A material estimate from a prescriptive guide, not a design. Board widths vary by product, so check yours before you order, and your local building department decides what is built.

16' · 192 in · 16 ft · 4.8768 m

12' · 144 in · 12 ft · 3.6576 m

4' · 48 in · 4 ft · 1.2192 m

192 ft²

10% covers decking cut to length and culled, the figure Decks.com adds for a straight lay (15% for a diagonal one). It is added once, to the deck boards — the joists, beam, posts and footings are counted off the frame and read off the DCA 6 tables.

This page recalculates as you type. There is no Calculate button.

Answer

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Deck boards

29 boards

192 ft² of deck · 13 2×8 joists · 3 footings

Deck area 192 ft²
Joists 13 joists
Beam 2-2×12 nominal
Posts 3 posts
Footings 3 at 20" footings
Footing concrete 4.36 ft³
Decking fasteners 338 clips
More numbers
Joist size 2×8 nominal
Ledger bolts 12 screws
Joists on center 16"
Joist overhang past the beam 24"
Guard to build 40 ft
Materials $3,600.60
DrawingThe project drawn to scale. Deck 1: 16' × 12'.16'12'Deck 1

Buying noteThe concrete is the footing pads only. A pier or a tube form from the pad up to the ground is extra, and so is anything the frost line adds below it.
Buying noteDCA 6 gives a maximum spacing for guard posts on a stair, not for ones on the deck surface. The 6 ft used here is that stair figure; your building department decides.
Buying noteA corner post over 2 ft tall needs diagonal bracing parallel to the beam (DCA 6 p. 10). The bracing is not in the list — it is two short pieces of the same framing lumber per corner.

Breakdown

Deck Measurements Qty Area Share
Deck 1 16' × 12' × 4' Add ×1 192 ft²

What to buy

Buying noteThe concrete is the footing pads only. A pier or a tube form from the pad up to the ground is extra, and so is anything the frost line adds below it.
Buying noteDCA 6 gives a maximum spacing for guard posts on a stair, not for ones on the deck surface. The 6 ft used here is that stair figure; your building department decides.
Buying noteA corner post over 2 ft tall needs diagonal bracing parallel to the beam (DCA 6 p. 10). The bracing is not in the list — it is two short pieces of the same framing lumber per corner.

Ways to buy the footing concrete

The footings only. The lumber and the connectors are priced by their own rows below.

7 × 80 lb + 1 × 40 lb Bags 8 Left over 0.14 ft³ Weight 600 lb Cost $43.74
7 × 80 lb + 1 × 40 lb Bags 8 Left over 0.14 ft³ Weight 600 lb Cost $43.74
8 × 80 lb Bags 8 Left over 0.44 ft³ Weight 640 lb Cost $45.44 No reference price. Type what your store charges.
10 × 60 lb Bags 10 Left over 0.14 ft³ Weight 600 lb Cost $49.80 No reference price. Type what your store charges.
15 × 40 lb Bags 15 Left over 0.14 ft³ Weight 600 lb Cost $59.70 No reference price. Type what your store charges.

Shopping list

29 29 × 16 ft $2,088.00 Reference price $4.50. Type your store's.
4 buckets $216.00 Reference price $54.00. Type your store's.
13 13 × 12 ft $265.20 Reference price $1.70. Type your store's.
1 1 × 16 ft $27.20 Reference price $1.70. Type your store's.
1 1 × 16 ft $27.20 Reference price $1.70. Type your store's.
12 screws $22.80 Reference price $1.90. Type your store's.
16 ft $33.60 Reference price $2.10. Type your store's.
2 2 × 16 ft $54.40 Reference price $1.70. Type your store's.
3 3 × 8 ft $118.56 Reference price $4.94. Type your store's.
3 post bases $72.00 Reference price $24.00. Type your store's.
13 hangers $31.20 Reference price $2.40. Type your store's.
13 ties $14.30 Reference price $1.10. Type your store's.
8 7 × 80 lb + 1 × 40 lb $43.74
8 posts $256.00 Reference price $32.00. Type your store's.
4 4 × 20 ft $128.00 Reference price $1.60. Type your store's.
88 balusters $202.40 Reference price $2.30. Type your store's.
Ticked items $3,600.60

Untick what you already have. Lumber is priced by the foot beside its row; the concrete's bags are priced in the table above.

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Standards

We hold the criteria and the clause, not the wording of the standard.

Meets DCA 6-15 · Table 2

Joists span 10', inside the 11' 10" the guide allows a 2×8 at this spacing.

Meets DCA 6-15 · Table 2

The joists pass the beam by 2', inside the 2' allowed.

Meets DCA 6-15 · Table 3A

A 2-2x12 beam spans 8' between posts, inside the 9' 2" allowed.

Meets DCA 6-15 · Table 4

The posts stand 2' 4 9/16", inside the 14' the table allows.

Meets DCA 6-15 · Table 5

The ledger bolts go in at 18" on center.

Not checked IRC 2021 · R507.7

Composite decking has no row in this clause: from IRC 2021 the span is whatever the board's own label says. The joists here are at 16", which is what Trex and TimberTech publish — read the figure printed on your boxes before you frame.

Meets IRC 2021 · R312.1

At 48" the deck is over 30", and the list carries a guard.

How this was worked out

Every number below is yours. Each constant links to the row where you can change it.

Deck 1 16' × 12' 192 ft²
Joist spacing Composite deck boards laid Square to the joists allows joists up to 16" apart 16"

Change:Composite joists, square lay

Rows of decking 144" deep ÷ (5 1/2" board + 1/4" gap) 26

Change:Deck board widthGap the hidden fastener sets

Deck boards 26 rows × 16 ft run, cut from 16 ft boards 26
After waste 26 boards + 10% 29

Change:Decking waste

Joists 16 ft along the house, none more than 16" apart 13
Joist size from the table spans 10 ft at 16", so 2×8 11.83 ft
Beam from the table 16 ft on 3 posts is 8 ft a span 2-2×12
Posts 3 under each of 1 beams 3
Footing from the table beam 8 ft, joists 10 ft 20" round, 8" thick

Change:Footing below groundYour frost depth

Footing concrete π × (20" ÷ 2)² × 8" × 3 footings 4.363 ft³
Hidden fasteners 26 rows × 13 joists 338
Ledger fasteners 16 ft of ledger, none more than 18" apart 12
Guard 40 ft of guard on 8 posts 88

Change:Most between guard postsWidest opening in a guard

Bags 4.363 ft³ as 7 × 80 lb + 1 × 40 lb 8

Change:Yield, 80 lb bagYield, 60 lb bagYield, 40 lb bag

Cheapest combination 29 × 16 ft × $4.50/ft + 4 × $54.00 + 13 × 12 ft × $1.70/ft + 1 × 16 ft × $1.70/ft + 1 × 16 ft × $1.70/ft + 12 × $1.90 + 16 × $2.10/ft + 2 × 16 ft × $1.70/ft + 3 × 8 ft × $4.94/ft + 3 × $24.00 + 13 × $2.40 + 13 × $1.10 + 7 × $5.68 + 1 × $3.98 + 8 × $32.00 + 4 × 20 ft × $1.60/ft + 88 × $2.30 $3,600.60

Assumptions

Every one of these is editable. Change one and the answer changes with it.

Assumption Value Confidence
DCA 6 T2 joist span, Southern PineDCA 6-15 Table 2, allowable span L_J: how far a joist of this size may span at this spacing, face of support to face of support. Assumes 40 psf live load, 10 psf dead load, No. 2 grade, wet service and L/360 deflection. Species: Southern Pine. DCA 6-15 From a data sheet or an agency document
DCA 6 T2 overhang, Southern PineDCA 6-15 Table 2, allowable overhang L_O: how far a joist of this size may pass the beam. The overhang used is the lesser of this and a quarter of the actual span. The column does not always fall as the spacing widens - the guide shades those cells as deflection controlled - and the figures are carried as printed. DCA 6-15 From a data sheet or an agency document
DCA 6 T3A beam, 2-ply, Southern PineDCA 6-15 Table 3A, beam span L_B: how far a beam of this make-up may span between posts, by the span of the joists it carries. The column is read as "joist spans less than or equal to", so a joist span is rounded up to the next column before the table is read. Assumes 40 psf live, 10 psf dead, No. 2 grade and wet service. DCA 6-15 From a data sheet or an agency document
DCA 6 T3A beam, 3-ply, Southern PineDCA 6-15 Table 3A, beam span L_B: how far a beam of this make-up may span between posts, by the span of the joists it carries. The column is read as "joist spans less than or equal to", so a joist span is rounded up to the next column before the table is read. Assumes 40 psf live, 10 psf dead, No. 2 grade and wet service. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 footing diameterDCA 6-15 Table 4. Assumes 1,500 psf soil bearing and 150 pcf concrete; the guide allows the value to be multiplied by 0.9 for a corner post, which this page does not do. Both axes are read as "less than or equal to", so a span is rounded up to the next column before the table is read. The diameter of a round footing under one post. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 footing thicknessDCA 6-15 Table 4. Assumes 1,500 psf soil bearing and 150 pcf concrete; the guide allows the value to be multiplied by 0.9 for a corner post, which this page does not do. Both axes are read as "less than or equal to", so a span is rounded up to the next column before the table is read. The thickness of the footing pad. Assumes 2,500 psi concrete. DCA 6-15 From a data sheet or an agency document
DCA 6 T4 post height, Southern PineDCA 6-15 Table 4, post heights for a 6x6: how tall a post may stand, measured from grade or the top of the foundation, whichever is higher, to the underside of the beam. An 8x8 may stand anywhere in the table up to 14 ft. Species: Southern Pine. A blank cell is the guide's "NP" - not permitted - and the page says so rather than fill it in. DCA 6-15 From a data sheet or an agency document
DCA 6 T5 lag spacingDCA 6-15 Table 5: the on-center spacing of ledger fasteners, by what the house's band or rim joist is and how far the joists span. Deck live load 40 psf, dead load 10 psf; 15/32 in maximum sheathing; fasteners staggered. The 1 in and 1 1/8 in rows are LVL rim joists, the 1 1/2 in row solid-sawn lumber. Half-inch lag screws, tips fully past the inside face of the band. DCA 6-15 From a data sheet or an agency document
Deck board widthThe width a nominal 6 in deck board really is: 5 1/2 in. Trex publishes its boards as 0.94 in x 5.5 in, and PS 20 dresses a nominal 6 in face to 5 1/2 in, so wood and composite are the same width. Decks.com counts rows at the nominal 6 in and so leaves three rows out of a 16 ft depth. in From a data sheet or an agency document
Gap the hidden fastener setsTrex: the hidden fastener sets a 6.4 mm gap between boards, which is 0.252 in. Wider than the 1/8 in a wood deck is laid at, so a composite deck takes fewer rows over the same depth. in From a data sheet or an agency document
Composite board thicknessTrex publishes the actual dimensions of its grooved and square-edge boards as 0.94 in x 5.5 in. The thickness only moves the post height, which is read off a table in whole feet, but a frame worked out to the inch should not guess it. in From a data sheet or an agency document
Composite joists, square layComposite decking laid perpendicular to the joists: joists 16 in on center for a residential deck. Trex install guide 2025, p. 20, and TimberTech Composite Installation Guide REV 09/23, p. 7, give the same figure. in From a data sheet or an agency document
A guard is needed aboveAWC DCA 6-15, p. 19: a deck more than 30 in above grade has to have a guard [IRC R312.1]. in From a data sheet or an agency document
Widest opening in a guardAWC DCA 6-15, Figure 24 and its commentary on p. 41: no opening in a guard may let a 4 in diameter sphere pass. The commentary extends it to the openings under a fixed bench used in place of a guard, which Figure 24 itself leaves out. in From a data sheet or an agency document
Most between guard postsMost between guard posts. AWC DCA 6-15, Figure 30, gives 6 ft for the guard on a deck STAIR. The guide gives no maximum for a guard on the deck surface, so this figure is carried across to it and the page says so beside the count; your building department decides, and some ask for 4 ft. in From a data sheet or an agency document
Screws a board a joistAWC DCA 6-15, p. 3: attach decking to each joist with 2-8d threaded nails or 2-#8 screws. Omni uses a flat 3.5 screws a square foot, which only matches at 16 in on center. screws From a data sheet or an agency document
Hangers a joistAWC DCA 6-15, p. 9 and Figure 7: one joist hanger at the ledger for each joist. At the beam the joists bear over it and take a hurricane tie instead (Figure 6, Option 2), so they are not hung twice. hangers From a data sheet or an agency document
Ties a joist at the beamAWC DCA 6-15, p. 9, Figure 6 Option 2: one mechanical fastener or hurricane clip per joist at the beam, with at least 100 lb capacity in uplift and in lateral load. ties From a data sheet or an agency document
Smallest ledger and guard joistAWC DCA 6-15, Table 2 footnote 6 and p. 19: the ledger is a minimum 2x8 nominal, and so is any joist or rim joist a guard post is attached to. in nominal From a data sheet or an agency document
Post face, 6×6 dressedAWC DCA 6-15, p. 10: every deck post is 6x6 nominal or larger. 5 1/2 in is what a nominal 6 in face dresses to under PS 20. in From a data sheet or an agency document
Brace a corner post overAWC DCA 6-15, p. 10: diagonal bracing parallel to the beam at every corner post more than 2 ft 0 in tall. The guide prohibits it on center posts. in From a data sheet or an agency document
Footing below groundAWC DCA 6-15, p. 11 [R403]: footings bear on undisturbed soil at least 12 in below the undisturbed ground surface, or below the frost line, whichever is deeper. in From a data sheet or an agency document
Your frost depthHow far down the ground freezes where you are building. AWC DCA 6-15, p. 11 [R403]: a footing bears 12 in below the undisturbed surface or below the frost line, whichever is deeper, and the guide says to ask the authority having jurisdiction for the depth - no document publishes it for your town. Zero is no frost allowance, which is what the guide's own 12 in base case assumes; type your depth and every hole follows it. in From a data sheet or an agency document
The wide allowance fromNIST PS 20-20, Table 3: the dressed allowance goes from ½ in to ¾ in at 8 in nominal width. in From a data sheet or an agency document
Dressed, 8 in and widerNIST PS 20-20, Table 3: 8 in nominal and wider is dressed dry ¾ in under it - 8 to 7¼, 10 to 9¼, 12 to 11¼. in From a data sheet or an agency document
Dressed, 2 in up to 8 inNIST PS 20-20, Table 3: from 2 in nominal and under 8 in wide, lumber is dressed dry ½ in under it - 2 to 1½, 4 to 3½, 6 to 5½. in From a data sheet or an agency document
Yield, 80 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document
Yield, 60 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document
Yield, 40 lb bagQuikrete Concrete Mix 1101 data sheet. ft³/bag From a data sheet or an agency document

How many deck boards for a 20 by 12 foot deck

Count the rows, not the area. Twelve feet of depth is 144 inches; a 5/4 × 6 board is 5½ inches wide and sits about ⅛ inch off its neighbor, so a row covers 5⅝ inches. 144 ÷ 5.625 = 25.6, so the deck takes 26 rows, each one a 20 foot board. With 10% for cuts and culls that is 29 boards.

Change the run and the count of boards in a row changes with it. A 24 foot deck is two boards a row, and the page buys 12 foot boards for it rather than 20 foot ones, because two 12s meet at 24 feet with nothing left over and two 16s leave 8 feet on the floor. Sizing a deck to a board length is the oldest piece of advice in decking; this is it done as arithmetic.

Two ways the popular calculators get this wrong

Decks.com counts rows at the nominal 6 inches a board is called rather than the 5½ it measures. Over a 16 foot depth that is 192 ÷ 6 = 32 rows where 192 ÷ 5.5 = 34.9 means 35: three rows short, and the last foot and a half of the deck has no boards for it.

Inch Calculator's article divides the area by the area of one board: "200 sq ft ÷ 7.34 sq ft = 24.25 boards … 25 boards". The division is wrong — 200 ÷ 7.33 is 27.3, so 28 — and the method is wrong too, because it drops the gap between boards and it drops the butt joints where a row needs more than one board. Inch's own tool returns 28, so the article and the tool disagree with each other.

What else the boards decide

The joists under them cannot stand further apart than the decking spans: IRC R507.7 gives 16 inches for 5/4 laid square to the joists and 12 inches on the diagonal, and 24 inches for a full 2 inch board. Switch the decking or the angle at the top of the form and the frame below follows. Fasteners follow too — two screws a board at every joist it crosses.