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Deck footing calculator

Enter the deck size, height and frost depth. We size every footing from DCA 6 Table 4 and count the bags of concrete to fill them.

This deck needs 3 footings. Each is a 20 in round (or 18 in square) pad, 8 in thick, with the bottom 36 in below grade. Concrete for all of them: about 10.3 cu ft, which is 18 bags of 80 lb mix and 3 concrete tube forms (12 in wide, 4 ft long).

Footing sizes from DCA 6 Table 4 for soil that bears 1,500 psf and 2,500 psi concrete. The row is the beam span between posts (7'-0"), the column is your joist span (9'-10 1/2"). Footings within 5 ft of the house foundation wall bear at the house footing depth.

Footing size by beam span and joist span, DCA 6 Table 4 (round diameter / thickness, inches)
Beam span up toJoists 10 ftJoists 14 ftJoists 18 ft
6 ft18 / 721 / 824 / 10
8 ft20 / 824 / 1027 / 11
10 ft23 / 927 / 1131 / 13
12 ft25 / 1030 / 1334 / 15
14 ft27 / 1132 / 1437 / 16
16 ft29 / 1235 / 1540 / 18
18 ft31 / 1337 / 1642 / 19

Depth rule: the bottom of each footing sits at least 12 in below grade or below the frost line, whichever is deeper. Soil that bears less than 1,500 psf needs a soils investigation.

Design this deck in 3D, free

Carries over: size, height, attachment, species, spacing and frost depth. Not carried: that you confirmed the frost depth with your building department, so the designer will ask again. Stairs start off, as in this calculator.

Planning figures from the AWC DCA 6-2015 guide, not an engineer's design or a permit. Your building department has the final word, and local rules may be stricter.

How the footings are sized

DCA 6 Table 4 gives the footing for each post from two numbers: the distance between posts along the beam, and the length of the joists the beam carries. Wider spacing and longer joists put more weight on each post, so the footing grows. The calculator finds the beam and joists for your deck, reads the cell, and counts the posts.

Round or square

The table gives a round diameter and a square side, and both carry the same load. Round footings are poured in a form tube or a dug hole, square ones in a built form. The calculator counts concrete for the round footing.

Soil and concrete assumptions

Table 4 is written for soil that bears 1,500 psf and concrete rated 2,500 psi. Soil that bears less needs a soils investigation, and the calculator does not shrink footings for firmer soil.

Footings near the house

A footing within 5 ft of the house foundation wall bears at the house footing elevation, so ask for that depth before you dig.

Stair posts

A 4x4 post that shortens a stair stringer sits on a 12 in round footing 6 in thick. The stair stringer calculator tells you when you need one.

Where the numbers come from

Every limit on this page is read from the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), 2015 edition, a guide written for the 2015 International Residential Code. The deck designer, the builder's plan and these tools all read the same tables. The span tables assume 40 psf live load, 10 psf dead load, No. 2 grade lumber in wet service, and footings on soil that bears at least 1,500 psf.

A number that is our own choice and not in the guide, such as the 24 in joist overhang we draw by default, is never presented as a code requirement. Where a value comes from somewhere other than DCA 6, the page says so.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

It depends on the beam span and the joist span. In DCA 6 Table 4, on soil that bears 1,500 psf, the smallest footing is 18 in round and 7 in thick, and the largest in the table is 42 in round and 19 in thick. A 16 by 12 ft deck in Southern Pine with joists at 16 in on center needs 3 footings of 20 in round by 8 in thick. Enter your own deck above to get yours.

DCA 6 puts the bottom of the footing on undisturbed soil at least 12 in below grade or below the frost line, whichever is deeper. The frost line depends on where you live, and your building department publishes it. The calculator starts at 36 in, which is our own placeholder and not a code figure, so enter your local depth.

In cold climates, yes, so the ground cannot heave them. Where the frost line is shallow, the 12 in minimum applies. Ask your building department which depth it enforces.

For our default 16 by 12 ft deck, each footing takes 6 bags of 80 lb mix: a pad 20 in round and 8 in thick, plus a 12 in tube up to 2 in above grade. Bag counts assume 0.6 cu ft per bag, which is the mix maker's figure. The calculator recounts them for your deck.