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.
The rest of the deck
Footings are the first job and nothing like the last. The
deck board calculator
works out the decking and the rows it lands in, the
deck stain calculator
covers finishing it, and the
board foot calculator
prices the framing the way a yard quotes it. Setting posts rather than
pouring piers? The
post hole calculator works
out the concrete around each one, and the
fence post depth calculator
settles how deep the hole goes.
Common questions
How deep is the frost line where I am?
Ask your building department, once, and write it down. This page will not guess it for you and no page should. The frost depth tables circulating online come off maps nobody publishes freely, and more importantly the physical frost depth is not what governs — your local code minimum is, and the two are different numbers. Enter the figure you were given and the calculator checks the pier against it and tells you if it is short. A footing that stops above the frost line lifts every winter, and it takes the deck with it.
How many tubes do I buy?
Fewer than you have footings, because tube is sold in long lengths you cut down. Six footings at three and a half feet come out of two twelve foot tubes with three feet left over, not six tubes. Tell the calculator what length your merchant sells and the ticket gives you the count, how many footings come out of each length, and what is left. If a pier is longer than the stock it says so rather than quoting a fraction of a tube — tube is cut, not joined.
I am in the UK, Canada or Australia. Does this work in metric?
Yes, and the shopping list changes with it rather than just the arithmetic. Switch to metric and the answer comes in cubic metres and the bags become 20 kg and 30 kg — the sizes those merchants actually stock. It used to convert the volume and leave the packaging American, which gave you the right amount of concrete in a bag nobody near you sells. Yields are the published ones: Blue Circle put a 20 kg bag at about 0.01 m3, and Sakrete Canada put a 30 kg bag at about 0.014 m3. Prices can be entered in GBP, EUR, CAD or AUD.
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
- 30 kg bag yield
- 0.49440533379999996 ft³
- 20 kg bag yield
- 0.35314666699999997 ft³
- Default waste allowance
- 10%
- QUIKRETE Concrete Mix No. 1101 data sheet — published yields of 0.30, 0.375, 0.45, 0.60 and 0.675 ft3 for the 40, 50, 60, 80 and 90 lb bags, the 90 lb noted as regional availability
- QUIKRETE Concrete Mix No. 1101 data sheet — unit weight approximately 140 lb/ft3 per ASTM C138
- Sakrete Concrete Mix product data sheet, King Packaged Materials / Sika Canada — a 30 kg (66 lb) bag yields approximately 0.014 m3 (1/2 ft3)
- Blue Circle Multi-Purpose Concrete technical information, Tarmac — one 20 kg bag typically produces around 0.01 m3 of fresh concrete, supplied throughout the United Kingdom in 20 kg bags
- NRMCA Concrete in Practice CIP 36, Structural Lightweight Concrete — "Structural lightweight concrete has an in-place density (unit weight) on the order of 90 to 115 lb/ft3 (1440 to 1840 kg/m3) compared to normalweight concrete with a density in the range of 140 to 150 lb/ft3 (2240 to 2400 kg/m3)."
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. Bag sizes are per market: the 40 to 90 lb bags are what North American merchants stock, and the 20 and 30 kg bags are what the UK, Canadian and Australian ones do. Switching the calculator to metric switches the shopping list with it, because a metric answer priced in pounds of product is not an answer anyone can buy. The 150 lb/ft3 ready-mix figure was carried for a long time as a point value with nothing behind it. NRMCA CIP 36 publishes it as the TOP of a range — normalweight concrete is 140 to 150 lb/ft3, and structural lightweight is 90 to 115 — so densityRanges now holds both and the weight calculator reports a span rather than a single number it cannot justify. Where one figure has to be picked, the top of the range is the safe end: over-stating a weight risks specifying too much truck, and under-stating it risks overloading one.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
