Concrete foundations set out on a construction site

Reference

Deck footing sizes and spacing

A footing has two sizes and people usually only think about one. The tube sets how much concrete you pour; the pad at the bottom is what actually stops the deck sinking, and it is sized by load and soil, not by the post above it. Here is both, plus the frost rule that decides how deep you dig.

Form tube sizes and concrete per foot

A form tube is a cylinder, so the concrete it takes is fixed by its diameter and how deep you go. These figures are per foot of depth — multiply by your dig and by the number of piers.

Tube	Per foot of depth	80 lb bags per foot	Suits
6 in	0.20 ft³	0.33	Light posts, mailbox, small pergola
8 in	0.35 ft³	0.58	The usual minimum under a 4×4 deck post
10 in	0.55 ft³	0.91	6×6 posts, standard residential deck
12 in	0.79 ft³	1.31	6×6 posts carrying a longer beam span
16 in	1.40 ft³	2.33	Heavy beams, roof loads over the deck
18 in	1.77 ft³	2.95	Structural piers, poor soil

A 10 in tube at a 42 in frost depth is 3.5 ft of pour, so 1.9 cubic feet, or about four 80 lb bags per pier. Over twelve piers that is a mixer job, not a wheelbarrow job.

Deck boards being fixed down over the frame — The part you see is held up by the part you do not

Pad size by load, not by post

The pad at the bottom spreads the load into the soil. Size it from the tributary area — the patch of deck each post carries, which is half the joist span by half the beam span on each side — at a residential design load of 50 lb per square foot, and divide by what the soil can take.

load = tributary area × 50 lb/ft²
pad area = load ÷ soil bearing
diameter = √(4 × pad area ÷ π)

Tributary area	Load	Pad on 1 500 psf soil	Pad on 2 000 psf soil
24 ft²	1 200 lb	0.80 ft² — 12 in round	0.60 ft² — 11 in round
36 ft²	1 800 lb	1.20 ft² — 15 in round	0.90 ft² — 13 in round
48 ft²	2 400 lb	1.60 ft² — 17 in round	1.20 ft² — 15 in round
64 ft²	3 200 lb	2.13 ft² — 20 in round	1.60 ft² — 17 in round
80 ft²	4 000 lb	2.67 ft² — 22 in round	2.00 ft² — 19 in round

Notice how quickly the pad outgrows the tube. A 10 in tube on a 36 ft² tributary wants a 15 in pad under it — which is why a bell footing, or a square pad poured under the tube, is standard rather than exotic. Soil bearing of 1 500 psf is the usual presumptive value for ordinary soil; sand and gravel take more, soft clay takes far less.

Which tube for a 4×4 or a 6×6 post

Post	Actual size	Minimum tube
4×4	3.5 in	8 in
6×6	5.5 in	10 in
8×8	7.25 in	12 in

The post does not sit in the concrete on a deck — it sits on a galvanised base anchored to the top of the pier, clear of standing water. The tube only has to be wide enough to take that base and its bolt.

Frost depth decides the dig

A footing that stops above the frost line gets lifted by the ground freezing under it, and it does not go back down evenly. The bottom of the footing has to sit below the local frost depth — a figure your building department publishes, and the only number on this page you cannot work out yourself.

Region	Typical frost depth	10 in tube takes
Gulf coast, southern Florida	0–12 in	under 1 bag
Mid-Atlantic, Pacific Northwest	18–24 in	2 bags
Midwest, New England	36–48 in	3–4 bags
Northern plains, interior Alaska	60–100 in	5–8 bags

Size the footings for your deck →

Setting posts in concrete instead of on piers? That is the post hole calculator. For the bag yields behind every figure here, see concrete bag yields and mix ratios, and for the steel in a larger pier, rebar sizes and weights.

The rest of the deck

These tables set how far the frame can reach. Every deck tool on the site works inside them.

Tools for the job

What makes this job easier

Tools and materials that suit this kind of job. These are affiliate links: if you buy through one, Renovimo may earn a commission at no extra cost to you. It never changes the figures on this page.