What Size Footing for a 6x6 Post?
Concrete to buy
0.45 cubic yards
Four round 12 in piers, 3 ft below grade with 6 in standing above, ten percent waste.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Pier shapeRound, sonotube
- Diameter or side12 in
- Depth below grade3 ft
- Height above grade6 in
- How many piers4 count
- Waste allowance10 %
Concrete to buy0.45 cubic yards
| Height of each pier | 3.5 ftbelow grade plus what stands above it |
|---|---|
| Section area | 0.785 sq ftround |
| Concrete per pier | 2.75 cu ft |
| Concrete for all piers | 11 cu ft |
| With waste | 12.1 cu ft |
| 60 lb bags | 270.45 cu ft each |
| 80 lb bags | 210.60 cu ft each |
| Sonotube at 12 ft | 23 piers cut from one tube |
| Vertical rebar | 39 ft3 bars per pier |
| Bearing at 2,000 psf | 1,571 lbper pier, presumptive soil |
The material list this produces
| Concrete, cu yd | 0 |
|---|---|
| Or 80 lb bags | 21 |
| Sonotube, 12 ft lengths | 2 |
| Rebar, ft | 39 |
| Rebar tie wire, lb | 1 |
| Post bases or anchor bolts | 4 |
Footing diameter compared
Diameter swept with the depth and the count held. The right hand column is what each footing carries on presumptive 2,000 psf soil, and it is the column the size is actually chosen from. The rebar column steps rather than climbing, because the engine puts two bars in a pier under 10 in, three from 10 to 16 and four above that.
| Footing diameter | Concrete to buy | Vertical rebar | Bearing at 2,000 psf |
|---|---|---|---|
| 8 in round | 0.2 cubic yards | 26 ft | 699 lb |
| 10 in round | 0.31 cubic yards | 39 ft | 1,091 lb |
| 12 in round | 0.45 cubic yards | 39 ft | 1,571 lb |
| 16 in round | 0.8 cubic yards | 39 ft | 2,793 lb |
| 18 in round | 1.01 cubic yards | 52 ft | 3,535 lb |
| 24 in round | 1.79 cubic yards | 52 ft | 6,284 lb |
What else matters here
A 12 in round footing 3 ft below grade is 2.75 cu ft of concrete, and on the presumptive 2,000 psf soil most residential codes allow without a soils report it carries about 1,571 lb. Four of them comes to 0.45 cu yd, 21 eighty pound bags and 2 sonotubes.
That bearing figure is the answer to the question, and the honest version of the answer is that it depends on the load. A 6x6 holding up a porch roof and a 6x6 holding up the corner of a loaded deck are the same post carrying very different numbers, and what the footing has to be is set by the load above it and the soil below it, not by the post. Nothing about a 6x6 requires a 12 in footing. It is simply the size most often used under one.
What the table below is for is seeing the shape of the trade. Going from 8 in to 12 in takes the bearing from 699 lb to 1,571, more than double, for 12 more bags across four footings. Going from 12 in to 24 in takes it to 6,284 lb for 81 bags against 21. Diameter is squared in that calculation and depth is not, which is why widening a footing buys bearing so much faster than deepening one does.
Depth is still the thing that has to be right. A footing that stops above the frost line lifts every winter and takes the post, and whatever the post is holding, with it. Depth is set by the published frost depth where you build, and then diameter is chosen for the load.
What this calculator assumes
- Depth below grade is measured to the bottom of the pier. A footing that stops above the frost line heaves, and the frost depth where you are building is published by your building department.
- Concrete yields are the printed ones: a 60 lb bag makes 0.45 cubic feet and an 80 lb bag makes 0.60.
- Sonotube is nested out of 12 ft lengths rather than divided into them, so a 4.5 ft pier gets two out of a tube and a 6.5 ft pier gets one. The tube line is what you actually buy.
- Bearing is shown at 2,000 pounds per square foot, which is the presumptive value most residential codes allow without a soils report. Soft or filled ground is less and a report can allow more.
- Vertical steel is two bars in a pier under 10 inches, three from 10 to 16, and four above that, each run the full height with three inches of cover at the bottom.
- Anything over about a cubic yard in total is usually cheaper and far faster from a ready mix truck with a short load fee than it is from bags.
Questions people ask
What size footing is needed for a 6x6 post?
It depends on the load and the soil, not on the post. A 12 in round footing is the size most commonly used and it carries about 1,571 lb on presumptive 2,000 psf soil. A 16 in carries 2,793 and a 24 in carries 6,284.
How much concrete for a 6x6 post footing?
A 12 in round footing 3 ft below grade with 6 in above is 2.75 cu ft, which is about 5 eighty pound bags. Four of them is 0.45 cu yd and 21 bags.
Is it better to go deeper or wider?
Wider, for bearing. The diameter is squared in the section area and the depth is not, so going from 8 in to 12 in more than doubles what the footing carries. Depth is set by the frost line and is not a lever you get to pull.
The concrete footing calculator takes any dimensions you like and hands back the same list.