Sonotube Calculator: How Many Bags of Concrete per Tube Form

Pick your sonotube diameter from the list (12 inch is pre-selected because it is the most common size for deck and porch footings), type the height of concrete in each tube in feet, and enter how many tubes you are pouring. The calculator instantly tells you how many 80 lb bags of concrete mix to buy and the total volume in cubic yards, already padded with a 10% waste allowance.

Need more control? Open More options to switch to metric, enter an odd custom diameter, change the bag size to 60, 50 or 40 lb, set your own waste percentage, or add a price per bag to estimate the cost of the pour. If you plan to order ready-mix instead of bags, use the cubic-yard figure when you call the supplier.

A sonotube (also called a concrete tube form or cardboard column form) is simply a cylinder, so the concrete it holds depends on just two measurements: the inside diameter and the height you fill it to. Because volume grows with the square of the diameter, going up one size makes a big difference. A 12 inch tube holds about 0.79 cubic feet per foot of height, while a 16 inch tube holds about 1.40 cubic feet per foot — nearly 80% more concrete for only 4 extra inches.

That is why a question like *"how many bags of concrete for a 12 inch sonotube 4 feet deep"* has a very different answer from the same question for an 8 inch tube. Remember to measure the full height of the concrete, including the part buried below grade and any section that sits above ground, plus any bell-shaped footing base you add at the bottom (estimate that separately with a general concrete calculator).

Pre-mixed bags are sold by weight but they fill a fixed volume once mixed: a standard 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45 cubic feet, a 50 lb bag about 0.375 cubic feet and a 40 lb bag about 0.30 cubic feet. Since you cannot buy part of a bag, the calculator always rounds bags up to the next whole number.

Volume of one tube (cu ft) = π × (diameter in inches ÷ 24)² × height in feet

Dividing the diameter by 24 converts inches to feet and halves it to get the radius in one step.

Total volume = volume per tube × number of tubes × (1 + waste % ÷ 100)

Cubic yards = total cubic feet ÷ 27

Bags = total cubic feet ÷ bag yield, rounded up (80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft).

In metric mode, height in meters is converted to feet (× 3.28084) and a custom diameter in centimeters is converted to inches (÷ 2.54) before applying the same formula; results are also shown in cubic meters.

Example 1 – how many 80 lb bags for 10 sonotubes (12 inch, 4 ft): One tube = π × 0.5² × 4 = 3.142 cu ft. Ten tubes = 31.42 cu ft. Adding 10% waste gives 34.56 cu ft, or about 1.28 cubic yards. 34.56 ÷ 0.60 = 57.6, so buy 58 bags of 80 lb mix (or 77 bags of 60 lb mix).

Example 2 – deck footings with 8 inch tubes, 3 ft deep, 4 tubes: One tube = π × 0.333² × 3 = 1.047 cu ft. Four tubes = 4.19 cu ft; with 10% waste = 4.61 cu ft. That is 8 bags of 80 lb concrete (4.61 ÷ 0.60 = 7.7) or 11 bags of 60 lb.

Example 3 – 6 fence or pergola piers, 10 inch tubes, 4 ft tall: One tube = π × 0.4167² × 4 = 2.182 cu ft. Six tubes = 13.09 cu ft; with waste = 14.40 cu ft, which works out to 24 bags of 80 lb mix.

- Buy one or two extra bags. Running out mid-pour leaves a cold joint; leftover sealed bags can usually be returned.
- Keep the 10% waste allowance for uneven hole bottoms, spillage, and concrete that soaks into loose soil around the base.
- Check local frost depth. Footings normally need to extend below your area's frost line, so your tube height may be deeper than you expect.
- Brace the tube plumb with stakes or scrap lumber before filling, and rod or tap the sides to release trapped air.
- Consider ready-mix for big jobs. Once you need more than about 1 cubic yard (45 bags of 80 lb), a delivered truck or trailer load can be faster and cheaper.
- Cut the tube to length with a handsaw, and leave a little above grade so water drains away from the post base.