How much concrete a round footing takes
A sonotube footing is a cylinder, so the volume is the area of the circle
times the depth. The thing worth noticing is how strongly the diameter
dominates.
volume = pi × (diameter ÷ 2 ÷ 12)² × depth in feet
8 in × 4 ft = 1.40 ft³
10 in × 4 ft = 2.18 ft³ (+56%)
12 in × 4 ft = 3.14 ft³ (+125%)
Same depth, same hole, and more than twice the concrete between the
smallest and largest common tube. Volume goes with the square of the
radius, so guessing a tube size up "to be safe" is an expensive habit.
Depth is not ours to guess
Footings go below the frost line, and that depth is set by your local
building department. It ranges from essentially nothing in the far south
to over six feet in the northern states, so there is no national number to
print here and this calculator does not pretend otherwise. Get the figure
locally, then come back and enter it.
The rest of the deck
Once the footings are in, the
deck board calculator
works out the decking those posts will carry, counting the gap between
boards. If you are setting posts in dug holes rather than tubes, the
post hole calculator
subtracts the post from the hole, which a plain cylinder calculation
cannot do.
Concrete per footing, at 10% waste
One footing each. Multiply by your footing count, and remember a flared base form adds concrete this table does not include.
Common questions
How many bags of concrete for a sonotube footing?
A 10 inch tube four feet deep holds about 2.2 cubic feet, which is four bags of 80 lb with a ten percent allowance. Diameter matters far more than depth because the volume goes up with the square of the radius: going from a 10 to a 12 inch tube adds nearly half again as much concrete for the same depth.
How deep do deck footings need to be?
Below your local frost line, and that number varies enormously — from almost nothing on the Gulf Coast to over six feet in northern Minnesota. There is no national figure and this calculator will not invent one. Your building department publishes the depth for your area, and it is usually the first thing they check on an inspection.
What size sonotube should I use?
That is a structural question set by the load each post carries and by your soil bearing capacity, not by preference. Eight, ten and twelve inch tubes cover most residential decks, with bigger tubes under posts carrying beams or a roof. Your deck plan or your inspector will specify it.
Can I pour footings without a tube?
You can pour straight into a clean hole in stable soil, and plenty of decks are built that way. The tube gives you a consistent diameter and a clean above-grade finish, and it stops the sides collapsing in loose or wet ground. It also means the volume is predictable, which is what makes a calculation like this reliable.
Should I use a footing form at the base?
A flared base spreads the load over more soil, and in many jurisdictions it is required. It also adds concrete this calculator does not count — the tube volume is the cylinder only, so add the base form separately from its own stated volume.
How much extra concrete should I mix?
Ten percent, which is the default here. Tubes get overfilled, some mix always stays in the barrow, and running short halfway up a footing leaves a cold joint exactly where you least want one. It is the cheapest insurance on the whole job.
Sources
- Concrete mix density
- 140 lb/ft3
- 80 lb bag yield
- 0.6 ft³
- 60 lb bag yield
- 0.45 ft³
- 50 lb bag yield
- 0.375 ft³
- 40 lb bag yield
- 0.3 ft³
- 90 lb bag yield
- 0.675 ft³
- Default waste allowance
- 10%
Bag yield is approximate and varies with aggregate and water content. Density 140 lb/ft3 is the ASTM C138 unit weight of mixed bagged concrete, about 3,780 lb/yd3, and is what every calculation here uses. Ready-mix delivered by truck is denser, about 150 lb/ft3 or 4,050 lb/yd3; that figure is stored separately as readyMixDensity and appears only where a page discusses ordering ready-mix. The two are not interchangeable. The 90 lb bag is regionally stocked rather than nationally, so it is flagged and listed last.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures