Deck Calculators

Boards, joists, beams, footings, railings and the ledger — one job, with DCA 6 behind it.

These pages size a deck's frame against DCA 6-15 and the IRC. They are an estimate for planning; a licensed trade or engineer signs off the work, and your local authority decides which edition applies.

An isometric deck frame with the decking boards shaded

Which of these 10 pages is yours

Not sure which one? Start with the Deck Calculator — it handles the general case, and the links under its answer carry your measurements to the next page.

Page	The question it answers	The number it prints largest
Deck	Your deck, end to end — the boards, the frame under them, the footings under that, and the guard around it.	Prints: Deck boards
Deck boards	Rows across the depth, boards along the run, and the gap between them counted.	Prints: Deck boards
Composite decking	Boards, clips, and the joist spacing the board you chose asks for.	Prints: Deck boards
Joist span	The shallowest joist that reaches, with the clause it comes from.	Prints: Joist size
Beam span	The lightest beam that spans, on the fewest posts that carry it.	Prints: Beam
Deck footings	The pad under each post, sized off the beam and joist spans, in the bags a store sells.	Prints: Footings
Deck railing	The guard a deck over 30 inches has to have, counted post by post.	Prints: Guard to build
Deck screws	Two at every crossing, counted from the joists you actually have.	Prints: Decking fasteners
Deck ledger	The bolts that hold the deck to the house, at the spacing the table gives.	Prints: Ledger bolts
Deck cost	Every row priced, every price a box. Untick what you already have.	Prints: Materials

How a deck is measured on site

A deck is measured three ways at once, and the three numbers are not interchangeable. The length runs along the house wall, because that is what the ledger bolts to and what the beam has to span. The width is how far the deck comes out from the wall, which is what the joists span. The height is from the finished deck surface down to the ground at the outside corner — not to the sill, and not to the bottom of the footing. Those three, in that order, are what every page here asks for.

Boards are the one place where the name and the measurement disagree. A deck board called 6 inches is 5½ inches wide, and it is laid about ⅛ inch off its neighbor, so a row covers 5⅝ inches. Twelve feet of depth is 144 inches, which is 25.6 rows — so the deck takes 26, and 25 would leave three and a half inches of joist showing. Counting rows by dividing the area by the board's nominal size is how a short order happens.

Joist spacing is not a preference on these pages. It is decided by the decking: 16 inches on center for a straight lay, 12 inches at 45 degrees, because a board crossing the frame at an angle bridges a longer gap. That is the intersection of what Trex and TimberTech both allow. The two name their angles differently, and where they disagree the page says so as a warning rather than picking one and calling it a constant.

Where the frame numbers come from

Every lumber size on these pages is read off a table in the American Wood Council's Prescriptive Residential Wood Deck Construction Guide, DCA 6-15. Joist spans and overhangs are Table 2, beam spans Table 3A, footing sizes and post heights Table 4, and the ledger bolt pattern Table 5. All four were pulled out of the published PDF by script and checked against the numbers worked by hand, so a transcription slip in one of them cannot sit here unnoticed. Guards come from IRC R312.1 and the decking span from R507.7.

The tables assume an ordinary residential deck: 40 psf live load, 10 psf dead, No. 2 grade, wet service. They are given per species group, and the pages do not average them — Southern Pine, the Douglas Fir group and the cedars and redwoods each have their own spans, and the species is a choice you can change.

The beam is pushed as far in as the guide allows rather than set under the outside edge. A shorter joist span buys a shallower joist, a shorter beam span and a smaller footing, so the overhang is the lesser of the table's figure and a quarter of the span, and it grows with the deck until the table stops it.

A composite board's span is a label, not a code table

Ask a composite deck page whether a 16 inch joist spacing passes and the honest answer is that no code table says. The 2021 and 2024 IRC dropped composite from R507.7 — the allowable span of a composite board is whatever its manufacturer's ASTM D7032 report says it is, printed on the product. So on these pages a composite span check never returns a pass. It returns the clause, says the span is the product's to state, and leaves the green tick to the label on the box.

What these pages do not do

They do not design reinforcement, and they do not size anything for a hot tub, a planter, a roof above the deck or snow beyond the 40 psf the DCA 6 tables assume. Those are loads the prescriptive guide does not cover, and the right answer to them is a designer rather than a table lookup.

They do not work out the lateral load connection at the ledger. The IRC asks for one and names a device; choosing and placing it is a detail on a drawing, not a quantity.

They do not lay out boards against the real outline of your deck. The cut list on the decking pages solves one direction — it reuses the offcut from one row at the start of the next, which is why 24 feet taken from 16 foot boards costs three boards and not four — but it does not know where your stairs land or which corner is out of square.

And they do not decide whether your build needs a permit, an inspection or a different edition of the code than the one the panel reads. That is the local authority's call, and nothing on this site changes it.

What this cluster assumes

45 assumptions: 45 from a data sheet or an agency document

Every constant these pages show under Assumptions, on the project each one opens with and on every choice its form offers, with its tier and the document it was read from. Each one is editable on the page that uses it; the waste each page starts at is in its own box, beside the form.

Assumption	Value	Confidence
Yield, 40 lb bagQuikrete Concrete Mix 1101 data sheet.	0.3 ft³/bag	From a data sheet or an agency document · source
Yield, 60 lb bagQuikrete Concrete Mix 1101 data sheet.	0.45 ft³/bag	From a data sheet or an agency document · source
Yield, 80 lb bagQuikrete Concrete Mix 1101 data sheet.	0.6 ft³/bag	From a data sheet or an agency document · source
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.	5 1/2 in	From a data sheet or an agency document · source
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.	24 in	From a data sheet or an agency document · source
Composite joists, 45° layComposite decking at 45 degrees to the joists: Trex takes 4 in off its 16 in, and TimberTech prints 12 in for 45 degrees and caps every diagonal lay at 12 in. Both give 12.	12 in	From a data sheet or an agency document · source
Composite joists, 60° layComposite decking at 60 degrees to the joists: Trex takes 2 in off its 16 in (install guide 2025, p. 20), giving 14 in; TimberTech prints 14 in for the angle it calls 30 degrees (REV 09/23, p. 7). The two agree on the number. What they do not agree on is the name of the angle, and that is said on the page as a warning rather than stored here.	14 in	From a data sheet or an agency document · source
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.	16 in	From a data sheet or an agency document · source
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.	0.94 in	From a data sheet or an agency document · source
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.	12 in	From a data sheet or an agency document · source
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.	0 in	From a data sheet or an agency document · source
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.	0.252 in	From a data sheet or an agency document · source
Gap between screwed boardsAWC DCA 6-15, p. 3: space decking boards approximately 1/8 in apart.	1/8 in	From a data sheet or an agency document · source
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.	4 in	From a data sheet or an agency document · source
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].	30 in	From a data sheet or an agency document · source
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.	1 hangers	From a data sheet or an agency document · source
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.	8 in nominal	From a data sheet or an agency document · source
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.	5 1/2 in	From a data sheet or an agency document · source
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.	72 in	From a data sheet or an agency document · source
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.	2 screws	From a data sheet or an agency document · source
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.	1 ties	From a data sheet or an agency document · source
Dressed, boards 1 to 1½ inNIST PS 20-20, Table 3 (p. 16): a board from 1 in and under 2 in nominal is dressed dry ¼ in under it - 1 to ¾, 1¼ to 1, 1½ to 1¼.	1/4 in	From a data sheet or an agency document · source
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.	1/2 in	From a data sheet or an agency document · source
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.	3/4 in	From a data sheet or an agency document · source
The wide allowance fromNIST PS 20-20, Table 3: the dressed allowance goes from ½ in to ¾ in at 8 in nominal width.	8 in	From a data sheet or an agency document · source
DCA 6 T3A beam, 2-ply, other speciesDCA 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. The row is printed "3x6 or 2-2x6": a solid 3x and a two-ply 2x read the same line. Species: every group but Southern Pine.	DCA 6-15	From a data sheet or an agency document · source
DCA 6 T3A beam, 3-ply, other speciesDCA 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. Species: every group but Southern Pine.	DCA 6-15	From a data sheet or an agency document · source
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 · source
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 · source
R507.7 decking, diagonal layIRC R507.7, maximum joist spacing for wood decking. A single span is decking bearing on two joists; continuous is three or more. DCA 6 requires every segment of decking to bear on at least three supports, so the continuous column is the one this page reads. Diagonal covers every angle up to 45 degrees off perpendicular.	IRC 2021 and IRC 2024, R507.7	From a data sheet or an agency document · source
R507.7 decking, square layIRC R507.7, maximum joist spacing for wood decking. A single span is decking bearing on two joists; continuous is three or more. DCA 6 requires every segment of decking to bear on at least three supports, so the continuous column is the one this page reads.	IRC 2021 and IRC 2024, R507.7	From a data sheet or an agency document · source
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 · source
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 · source
DCA 6 T2 overhang, Douglas Fir groupDCA 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 · source
DCA 6 T2 overhang, cedar groupDCA 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 · source
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 · source
DCA 6 T2 joist span, Douglas Fir groupDCA 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: Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir (incising assumed).	DCA 6-15	From a data sheet or an agency document · source
DCA 6 T2 joist span, cedar groupDCA 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: Redwood, Western Cedars, Ponderosa Pine and Red Pine.	DCA 6-15	From a data sheet or an agency document · source
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 · source
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 · source
DCA 6 T4 post height, Douglas Fir-LarchDCA 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: Douglas Fir-Larch. 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 · source
DCA 6 T4 post height, Hem-Fir and cedarsDCA 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: Hem-Fir and Western Cedars. 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 · source
DCA 6 T4 post height, SPF groupDCA 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: Ponderosa Pine, Red Pine and Spruce-Pine-Fir. 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 · source
DCA 6 T4 post height, redwoodDCA 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: Redwood. 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 · source
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 · source

Code editions these pages read

DCA 6-15 — Table 2, Table 3A, Table 4, Table 5
IRC 2021 — R312.1, R507.7

The clauses above are the ones the default edition states. Every page here lets you switch edition, and the clause numbers and limits change with it: IRC 2021 · IRC 2024

Estimates for planning. A licensed trade or engineer signs off the work, and your local authority has the last word on which edition applies.

Every Decking calculator 10

Read at the source

Every constant on this site carries the tier it sits in and the document it came from. These are the documents, read in the text and not quoted from another calculator.

IRC 2021
IRC 2024
DCA 6-15
PS 20-20