Sonotube Concrete Calculator
Find out how much concrete your deck, porch or post footings need. Enter the tube diameter, how deep the holes go and how many you are pouring.
How to measure Sonotube footings
Sonotube is a well-known brand of cardboard tube form, and the same method works for any round column form. You need three measurements for each footing:
- Tube diameter: the inside diameter of the form, usually printed on it. Common sizes include 8, 10, 12, 14 and 16 inches.
- Depth below grade: how far the concrete goes into the ground. Footings generally need to reach below your local frost line, which your building department can confirm.
- Height above grade: how far the tube sticks up out of the ground, often a couple of inches, so water drains away from the post base.
Enter the number of identical tubes. If your footings come in different sizes, run the calculator for each size and add the results.
The cylinder volume formula
A tube form is a cylinder, so its volume is the area of the circle times the height:
Volume = π × (diameter ÷ 2)² × (depth + height above grade)Cubic feet = cubic inches ÷ 1,728Bags = total volume ÷ bag yield, rounded upThe calculator uses typical premix yields of 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb and 0.60 cu ft for 80 lb, and the same yield per kilogram for 20, 25 and 30 kg bags in metric mode. It also shows the bags needed per tube before any extra, which is handy when you mix one tube at a time.
Worked example
A deck needs six footings in 10 inch tubes. The holes are 42 inches deep and each tube will stand 2 inches above the ground. You add 10% extra.
- Height of concrete per tube: 42 + 2 = 44 in.
- Volume per tube: π × 5² × 44 = 3,456 cu in, which is 3,456 ÷ 1,728 = 2.0 cu ft.
- Six tubes: 6 × 2.0 = 12.0 cu ft, plus 10% = 13.2 cu ft (0.49 yd³).
- 80 lb bags: 13.2 ÷ 0.6 = 22 bags. With 60 lb bags you would need 30, with 40 lb bags 44.
- Tube length to buy: 6 × 44 in = 264 in, or 22 ft of tube. Tubes are sold in fixed lengths, so plan your cuts to avoid waste.
A metric version with six 25 cm tubes, 105 cm deep plus 5 cm above ground, needs about 54 litres per tube, 0.36 m³ in total with 10% extra, or 31 bags of 25 kg.
Why add extra concrete?
Holes are rarely the exact shape of the form. The bottom of the hole is often wider than the tube, especially when it has been dug with a post-hole digger or auger, and some concrete is lost when filling and tamping. 10% is a sensible starting allowance. If your design includes a flared bottom or a square pad under the tube, that volume can be larger than the tube itself, so calculate it separately using the footing mode of the concrete calculator.
Estimating the cost
For a typical set of deck or porch footings, bagged concrete is the usual choice, because the total volume is small and the holes are spread out. Enter the price of your largest bag size to estimate the concrete cost. The default price is an example only. Also budget for the tube forms themselves, any post bases or anchors, rebar if your plans call for it, and gravel for the bottom of the holes if required. If you have many large tubes, it is worth comparing the bag cost with a small ready-mix delivery; enter the total cubic yards from this calculator when you ask for a quote.
Pouring tips
- Brace each tube so it stays plumb and does not float or shift as you fill it.
- Fill in layers and rod or tap the sides to release air pockets.
- Set anchor bolts or post bases while the concrete is still workable, checking alignment with a string line.
- Follow the curing guidance on the bag before loading the footing.
Footing size, spacing and depth affect how safely your structure carries its load, so always follow your approved plans and local code. This calculator estimates material quantities only.
Frequently asked questions
How many bags of concrete do I need for a Sonotube?
Work out the tube volume with π × radius² × height, then divide by the yield of your bag. A 10 in tube filled to 44 in holds about 2.0 cubic feet, which is about 3.3 bags of 80 lb premix (0.6 cu ft each), so you would buy 4 bags per tube before adding extra.
What size Sonotube do I need for deck footings?
Tube size depends on the load each footing carries and your soil, so it is set by your deck plans, local code or an engineer. Many residential decks use 8 to 12 inch tubes, often on a wider footing at the bottom of the hole.
How deep should Sonotubes go?
Footings generally need to extend below the local frost line, which varies widely by region. Check your local building department for the required depth before you dig.
Should I include a bell footing or spread footing?
This calculator covers the straight cylinder only. If your design flares into a bell or sits on a square pad, calculate that part separately with the footing mode of our concrete calculator and add the two volumes.
Can I use fast-setting concrete in Sonotubes?
Fast-setting mixes are common for fence posts but may be less convenient for tall forms, because they need to be placed quickly. Follow the manufacturer's directions for the product you choose.