What size concrete tube for a post?
8 in for a 4x4, 10 to 12 in for a 6x6.
But that is a starting point, not the answer. The tube carries the load into the soil, so what actually sizes it is how much weight sits on the post and how soft the ground is.
Concrete per tube, by diameter
| Tube | Per foot | 4 ft deep | 80 lb bags (4 ft) | Typical use |
|---|---|---|---|---|
| 6 in | 0.20 cu ft | 0.79 cu ft | 2 | 4x4 post, light rail |
| 8 in | 0.35 cu ft | 1.40 cu ft | 3 | 4x4 post, light rail |
| 10 in | 0.55 cu ft | 2.18 cu ft | 4 | 4x4 or 6x6 deck post |
| 12 in | 0.79 cu ft | 3.14 cu ft | 6 | 4x4 or 6x6 deck post |
| 14 in | 1.07 cu ft | 4.28 cu ft | 8 | heavy load, poor soil |
| 16 in | 1.40 cu ft | 5.59 cu ft | 11 | heavy load, poor soil |
| 18 in | 1.77 cu ft | 7.07 cu ft | 13 | heavy load, poor soil |
Volume goes up with the square of the diameter, which is the thing that surprises people at the till. A 12 in tube holds 2.25 times the concrete of an 8 in one, not one and a half times. Stepping a whole deck's footings up one size is a much bigger order than it sounds.
Why the post does not size the footing
A post transfers load to the footing; the footing spreads it over enough soil that the ground can carry it. Those are separate problems. A 6x6 carrying a porch roof in soft clay may need a 16 in base even though the post is 5.5 in across, while the same 6x6 holding a handrail on rock needs almost nothing.
The number that matters is the soil's bearing capacity, and the honest version is that most residential ground is assumed to carry around 1,500 lb per square foot unless somebody has tested it. A 12 in circle is 0.79 sq ft, so it carries roughly 1,180 lb before the soil is the limiting factor — which is why 12 in is the common deck answer and why a heavy roof load pushes past it.
Common questions
- Do I need a tube at all?
- Not for a fence post — a dug hole is fine and the rough sides actually grip better. A tube is for a footing you want to be a clean cylinder of known diameter: deck piers, pergola legs, anything inspected.
- Can I fill the tube and set the post in it?
- You can, but the better detail is a post base anchor set into the wet concrete, holding the post an inch clear of the surface. Wood set into concrete sits in a cup that collects water, and that is where posts rot first.
- How deep does the tube go?
- Below the frost line, same as any footing. The table above prices 4 ft because that covers most of the frost belt; multiply the per-foot column for anything else.
- What does a 12 in sonotube hold?
- 0.79 cu ft per foot of depth. At 4 ft that is 3.14 cu ft, or 6 bags of 80 lb.
- Should the bottom be wider than the tube?
- A flared or belled base spreads load over more soil without more tube, and in frost country it also stops the footing being lifted. Some inspectors expect one; plastic footing forms that clip to the tube base exist for exactly this.