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Deck Footing Calculator

The pad under each post, sized off the beam and joist spans, in the bags a store sells.

The pads only; a pier above them and anything the frost line adds below are extra. Soil bearing has to be verified in the field, and your building department decides.

20' · 240 in · 20 ft · 6.096 m

12' · 144 in · 12 ft · 3.6576 m

3' · 36 in · 3 ft · 0.9144 m

240 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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Footings

3 at 23" footings

240 ft² of deck · 16 2×8 joists · 3 footings

Deck boards 29 boards
Deck area 240 ft²
Joists 16 joists
Beam 3-2×12 nominal
Posts 3 posts
Footing concrete 6.49 ft³
Decking fasteners 832 screws
More numbers
Joist size 2×8 nominal
Ledger bolts 15 screws
Joists on center 16"
Joist overhang past the beam 24"
Guard to build 44 ft
Materials $2,428.95
DrawingThe project drawn to scale. Deck 1: 20' × 12'.20'12'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.

Breakdown

Deck Measurements Qty Area Share
Deck 1 20' × 12' × 3' Add ×1 240 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.

Ways to buy the footing concrete

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

11 × 80 lb Bags 11 Left over 0.11 ft³ Weight 880 lb Cost $62.48
11 × 80 lb Bags 11 Left over 0.11 ft³ Weight 880 lb Cost $62.48
11 × 80 lb Bags 11 Left over 0.11 ft³ Weight 880 lb Cost $62.48 No reference price. Type what your store charges.
15 × 60 lb Bags 15 Left over 0.26 ft³ Weight 900 lb Cost $74.70 No reference price. Type what your store charges.
22 × 40 lb Bags 22 Left over 0.11 ft³ Weight 880 lb Cost $87.56 No reference price. Type what your store charges.

Shopping list

29 29 × 20 ft $783.00 Reference price $1.35. Type your store's.
3 boxes $98.91 Reference price $32.97. Type your store's.
16 16 × 12 ft $326.40 Reference price $1.70. Type your store's.
1 1 × 20 ft $34.00 Reference price $1.70. Type your store's.
1 1 × 20 ft $34.00 Reference price $1.70. Type your store's.
15 screws $28.50 Reference price $1.90. Type your store's.
20 ft $42.00 Reference price $2.10. Type your store's.
3 3 × 20 ft $102.00 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.
16 hangers $38.40 Reference price $2.40. Type your store's.
16 ties $17.60 Reference price $1.10. Type your store's.
11 11 × 80 lb $62.48
9 posts $288.00 Reference price $32.00. Type your store's.
5 5 × 20 ft $160.00 Reference price $1.60. Type your store's.
97 balusters $223.10 Reference price $2.30. Type your store's.
Ticked items $2,428.95

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 3-2x12 beam spans 10' between posts, inside the 11' 7" allowed.

Meets DCA 6-15 · Table 4

The posts stand 1' 4 1/2", inside the 14' the table allows.

Meets DCA 6-15 · Table 5

The ledger bolts go in at 18" on center.

Meets IRC 2021 · R507.7

Joists at 16" are inside the 16" this decking spans.

Meets IRC 2021 · R312.1

At 36" 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 20' × 12' 240 ft²
Joist spacing 5/4 × 6 treated decking laid Square to the joists allows joists up to 16" apart 16"
Rows of decking 144" deep ÷ (5 1/2" board + 1/8" gap) 26

Change:Deck board widthGap between screwed boards

Deck boards 26 rows × 20 ft run, cut from 20 ft boards 26
After waste 26 boards + 10% 29

Change:Decking waste

Joists 20 ft along the house, none more than 16" apart 16
Joist size from the table spans 10 ft at 16", so 2×8 11.83 ft
Beam from the table 20 ft on 3 posts is 10 ft a span 3-2×12
Posts 3 under each of 1 beams 3
Footing from the table beam 10 ft, joists 10 ft 23" round, 9" thick

Change:Footing below groundYour frost depth

Footing concrete π × (23" ÷ 2)² × 9" × 3 footings 6.492 ft³
Deck screws 26 rows × 16 joists × 2 832

Change:Screws a board a joist

Ledger fasteners 20 ft of ledger, none more than 18" apart 15
Guard 44 ft of guard on 9 posts 97

Change:Most between guard postsWidest opening in a guard

Bags 6.492 ft³ as 11 × 80 lb 11

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

Cheapest combination 29 × 20 ft × $1.35/ft + 3 × $32.97 + 16 × 12 ft × $1.70/ft + 1 × 20 ft × $1.70/ft + 1 × 20 ft × $1.70/ft + 15 × $1.90 + 20 × $2.10/ft + 3 × 20 ft × $1.70/ft + 3 × 8 ft × $4.94/ft + 3 × $24.00 + 16 × $2.40 + 16 × $1.10 + 11 × $5.68 + 9 × $32.00 + 5 × 20 ft × $1.60/ft + 97 × $2.30 $2,428.95

Assumptions

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

Assumption Value Confidence
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
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 between screwed boardsAWC DCA 6-15, p. 3: space decking boards approximately 1/8 in apart. 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, 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. 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
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

Deck footings: how wide, how thick, how many bags

A 16 by 12 foot deck on three posts, its beam spanning 8 feet and its joists 10, takes a 20 inch round footing 8 inches thick under each post. π × 10² × 8 is 2,513 cubic inches, 1.45 cubic feet, and three of them are 4.36 cubic feet — eight bags, seven of 80 pounds and one of 40.

Footing size is DCA 6-15 Table 4, read against the beam span and the joist span, and both axes are "less than or equal to", so a span between two columns is read in the wider one. The table assumes soil that bears 1,500 psf and concrete at 150 pcf. Widen the post spacing and the footings grow fast: the same deck on two posts would want a far bigger pad than on three.

How deep the hole goes

The guide puts every footing on undisturbed soil at least 12 inches below the undisturbed ground surface, or below the frost line, whichever is deeper. Only your building department knows the local frost depth; type it into Your frost depth under Assumptions and the hole follows it. A footing closer than 5 feet to an exterior house foundation wall has to bear at the same elevation as the house footing.

Do not dig over a septic field, a leach field, a utility line or an enclosed meter, and call 811 first.

The concrete here is the pads only

What this counts is the footing pad the table sizes. A pier or a tube form carrying the post from the pad up to the ground is extra concrete, and it is not in the bag count — the volume depends on a form diameter nothing on this page asks for yet. The same goes for whatever the frost line adds below the pad. Deeper holes mean more concrete than the eight bags above.

Post anchors have to be galvanised or stainless, and the pad thickness has to suit the anchor you buy. Posts are 6×6 nominal at the smallest, centered on the footing, with cut ends and notches field-treated. These are estimates from a prescriptive guide: a licensed contractor or engineer, and your local authority, decide what is built.