Deck Calculator

A 12 × 16 ft deck takes 29 deck boards (16 ft long, 10% extra), 13 joists at 16 in on center, 4 posts and about 676 deck screws. Enter your own size to get the full list for a wood, composite or PVC deck: boards for a straight, diagonal or herringbone layout, joists, beams, posts and footings checked against the American Wood Council's span tables, screws or clips, railing and stairs. Add local prices if you want a total.

By Ellyana, creator of CalcClearly · Last updated 2026-10-27

Deck Details

Size, boards, framing, railing and stairs

Units

Deck type

Length along the house (ft)

Width out from the house (ft)

Height above ground (ft)

Joists run across the width, from the house to the outer beam. Boards run along the length.

Decking material

Laying pattern

Board width (in)

Gap (in)

Board length

Extra boards (%)

Use the actual width: a 2x6 or 5/4x6 board measures 5½ in (14 cm). DCA 6 spaces wood boards about ⅛ in apart; TimberTech asks for ⅛ to 3/16 in on composite. Extra boards start at 10% for straight and about 15% for diagonal (TimberTech).

Framing lumber

Joist spacing (on center)

Joist size

Beam (built up from 2x lumber)

Spans and post spacing come from the American Wood Council's DCA 6 tables.

Board fastening

Footing concrete bag

Railing

Railing section length (ft)

Stairs

Stair width (in)

Use the section length of the railing you plan to buy. Stairs follow the IRC (risers 7¾ in max, treads 10 in min) and DCA 6 (36 in wide min, stringers 18 in apart max, at least 3).

Prices (optional)

Per deck board

Per joist

Beam lumber per ft

Per post

Per concrete bag

Per screw / clip

Per railing section

Per stair stringer

Currency

Your deck material list will appear here

Enter the deck size and click calculate for boards, joists, beams, posts, footings, fasteners, railing and stairs.

Quick Answer

Deck boards = deck area ÷ (board width + gap) × board length, plus 10% for straight boards or about 15% for diagonal. Joists = length ÷ joist spacing, rounded up, + 1. A 12 × 16 ft deck with 16 ft boards needs 29 boards, 13 joists (2x10 at 16 in), 4 posts on 21 in footings and 676 screws.

How It Works: Formula & Variables

Boards = ⌈area ÷ ((width + gap) × length) × (1 + waste)⌉

Waste: Straight 10%, diagonal 15% (TimberTech). Board makers give no figure for herringbone, so it uses the diagonal 15% as a minimum.
Joists: ⌈deck length ÷ spacing⌉ + 1, each as long as the deck is wide
Beams: ⌈deck width ÷ allowed joist span⌉ beam lines for an attached deck, one more for a freestanding deck
Posts per beam: ⌈deck length ÷ allowed beam span⌉ + 1
Footings: One per post, sized from DCA 6 Table 4 by beam span and joist span
Fasteners: ⌈deck width × sin(board angle) ÷ (board width + gap)⌉ × joists × 2 screws (or 1 clip)
Stairs: risers = ⌈height ÷ 7.75 in⌉; treads = risers − 1, at least 10 in deep

Maximum joist spans used (DCA 6 Table 2)

Species	Joist	12 in o.c.	16 in o.c.	24 in o.c.
Southern pine	2x6	9 ft 11 in	9 ft	7 ft 7 in
Southern pine	2x8	13 ft 1 in	11 ft 10 in	9 ft 8 in
Southern pine	2x10	16 ft 2 in	14 ft	11 ft 5 in
Southern pine	2x12	18 ft	16 ft 6 in	13 ft 6 in
Douglas fir-larch, hem-fir, SPF	2x6	9 ft 6 in	8 ft 4 in	6 ft 10 in
Douglas fir-larch, hem-fir, SPF	2x8	12 ft 6 in	11 ft 1 in	9 ft 1 in
Douglas fir-larch, hem-fir, SPF	2x10	15 ft 8 in	13 ft 7 in	11 ft 1 in
Douglas fir-larch, hem-fir, SPF	2x12	18 ft	15 ft 9 in	12 ft 10 in
Redwood, western cedar, ponderosa / red pine	2x6	8 ft 10 in	8 ft	6 ft 10 in
Redwood, western cedar, ponderosa / red pine	2x8	11 ft 8 in	10 ft 7 in	8 ft 8 in
Redwood, western cedar, ponderosa / red pine	2x10	14 ft 11 in	13 ft	10 ft 7 in
Redwood, western cedar, ponderosa / red pine	2x12	17 ft 5 in	15 ft 1 in	12 ft 4 in

40 psf live load, 10 psf dead load, No. 2 grade, wet service. Joists are limited to 18 ft for footing design.

Sources: American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide (Tables 2, 3A and 4; decking, guard and stair requirements), TimberTech composite decking installation guide (joist spacing, gaps, 10% scrap, screws and clips, stair stringers), TimberTech on diagonal decking, TimberTech on herringbone decks, Trex, how to space and install deck joists, International Residential Code R311.7.5 and R312.1 (stair risers and treads, guards), and Quikrete Concrete Mix data sheet (bag yields).

Worked Examples

Example 1: 12 × 16 ft wood deck

A low deck on the back of a house: 16 ft along the wall, 12 ft out. 5½ in boards, 16 ft long, ⅛ in gap, laid straight. Southern pine 2x10 joists at 16 in, a doubled 2x10 beam, 80 lb bags for the footings.

Area: 16 × 12 = 192 sq ft
One board covers 5.625 in × 16 ft = 7.50 sq ft, so 192 ÷ 7.50 = 25.6 boards, plus 10% = 29 boards
Joists: 192 in ÷ 16 = 12 bays, + 1 = 13 joists, each 12 ft
A 2x10 at 16 in spans 14 ft, so one 12 ft span from the ledger to 1 beam is enough
That beam carrying 12 ft joists spans 7 ft 1 in: 16 ft ÷ 7.1 rounds up to 3 spans, so 4 posts
Footings: 21 in round, 8 in thick, 1.60 cu ft each, 11 bags of 80 lb
Screws: 26 rows × 13 joists × 2 = 676 screws

Example 2: 14 × 20 ft diagonal composite deck with railing and stairs

20 ft along the house, 14 ft out, 3 ft above the yard. Composite boards at 45°, hidden clips, joists at 12 in, a tripled 2x10 beam, 6 ft railing sections on the three open sides, and a 36 in stair down the middle of the outer side.

Boards: 280 sq ft ÷ 7.50 = 37.3, plus 15% = 43 boards
Joists: 240 ÷ 12 = 20, + 1 = 21 joists (a 2x10 at 12 in spans 16 ft 2 in)
Posts: the beam spans 8 ft 3 in with 14 ft joists, so 4 posts on 24 in footings (18 bags of 80 lb)
Clips: at 45° each joist carries 22 boards, × 21 joists = 462 clips
Railing: 45 ft once the stair opening is left out, 10 sections and 12 posts
Stairs: 36 ÷ 7.75 = 4.6, so 5 risers of 7.2 in, 4 treads, 3 stringers at DCA 6's 18 in maximum (composite treads may need them closer; check the board's stair chart); with 5 risers it needs a handrail

Example 3: when the joists are too small

The same 12 × 16 ft deck, framed with 2x8 joists to save money.

A southern pine 2x8 at 16 in spans 11 ft 10 in, two inches short of 12 ft.
So the calculator adds a beam in the middle: 2 spans of 6 ft and 2 beams.
That takes the deck from 4 posts to 6 posts, and two more footings to dig. Moving up to 2x10 joists is usually the cheaper fix.

Key Concepts

Length and width are not interchangeable. Joists run from the house out to the beam, so the width out from the house is the joist span. A 12 × 16 ft deck framed with 16 ft joists is a different structure from one framed with 12 ft joists.

Pattern changes the framing, not only the waste. Diagonal and herringbone boards need joists at 12 in, which on a 16 ft deck means 17 joists instead of 13. Herringbone also needs doubled joists where board ends meet.

Actual sizes. A 2x6 or 5/4x6 deck board measures 5½ in wide. Composite boards are close to that; check the label. Entering the nominal 6 in for a board that is really 5½ in makes the count about 8% low.

Gaps. DCA 6 spaces wood boards about ⅛ in apart. TimberTech asks for ⅛ to 3/16 in side to side on composite. Hidden clips set the gap for you.

What DCA 6 covers. A single-level deck attached to the house, no more than 40 psf snow load. Footings sit at least 12 in below undisturbed ground or below the frost line, whichever is deeper. For anything bigger or more complex, ask a designer or your building department.

Common Mistakes

Counting boards by length only. Rows × length works for a straight deck, but not for diagonal or herringbone. Work from area and add the waste for the pattern.

Keeping 16 in joists under diagonal composite. Composite boards at an angle sag between joists set for straight boards. Plan 12 in spacing before you buy the framing.

Stretching a joist past its span. A 2x8 that reaches 12 ft is not a 2x8 that can carry 12 ft. Check the span table, not the lumber rack.

Forgetting the stair stringers and the guard. Above 30 in the deck needs a guard, and four or more risers need a handrail. Both add posts and lumber.

Digging before the utilities are marked. Call 811 a few business days ahead, and ask your building department about permits before you order anything.

Frequently Asked Questions

29 boards, if they are 16 ft long and 5½ in wide with a ⅛ in gap. The deck is 192 sq ft and each board covers 7.50 sq ft, so 25.6 boards cover it exactly. Add 10% for cuts and you buy 29. Laid diagonally, with 15% extra, it comes to 30.

For most decks, 16 in on center. TimberTech caps composite decking at 16 in for straight boards and 12 in for any diagonal or angled pattern, and Trex says the same about diagonal boards. With wood framing, the joist size and spacing together set how far a joist can span: in the DCA 6 tables, a southern pine 2x10 spans 14 ft at 16 in on center and 16 ft 2 in at 12 in.

DCA 6 Table 2, for southern pine at 16 in on center: 2x6 spans 9 ft, 2x8 spans 11 ft 10 in, 2x10 spans 14 ft and 2x12 spans 16 ft 6 in. Douglas fir, hem-fir and SPF span a little less (2x10: 13 ft 7 in). The table assumes 40 psf live load, No. 2 lumber and wet service. A joist can overhang the beam by a quarter of its span at most.

It depends on the beam. A beam can only span so far between posts, and that distance shrinks as the joists it carries get longer. For the 12x16 deck with a doubled 2x10 southern pine beam, DCA 6 allows 7 ft 1 in between posts, so 16 ft needs 4 posts. Each sits on a footing: here 21 in across and 8 in thick. Footings go at least 12 in down, and below the frost line where that is deeper.

Count where boards cross joists. DCA 6 and TimberTech both use two screws at each joist, and hidden clips use one. On the 12x16 deck, 26 rows of boards cross 13 joists, so 338 crossings and 676 screws. Buy a box or two more than that for the ones you drop.

Divide the deck height by 7¾ in, the tallest riser the IRC allows, and round up. That is the number of risers; there is one tread fewer. A deck 36 in high needs 5 risers of 7.2 in and 4 treads. DCA 6 asks for stairs at least 36 in wide with 2x12 stringers no more than 18 in apart, and at least three of them. Four or more risers need a handrail.

Under the IRC, a deck more than 30 in above the ground needs a guard. Lower decks often get one anyway. The calculator counts railing sections and posts from the section length of the railing you plan to buy, and leaves a gap where the stair comes down.

Wood boards usually cost less to buy; composite and PVC cost more but need no staining. The framing underneath is the same pressure-treated lumber either way, though composite needs joists at 16 in or closer. Put your local prices into the calculator for both and compare the totals, because board prices vary a lot by region and brand.

Last reviewed 2026-10-27. For educational purposes only — not professional advice.