What is a Concrete Cylinder?
A concrete cylinder is a circular cross-section form filled with concrete, used in various construction applications such as pier footings, post supports, bollards, and cylindrical structural elements. Cardboard tube forms (Sonotubes) are the most popular method for creating concrete cylinders on job sites, available in diameters ranging from 6 inches to 60 inches.
Concrete cylinders are also used in material testing. Standard test cylinders measure 6 inches in diameter by 12 inches tall, and they are cast during a concrete pour to verify the compressive strength of the mix at 7 days and 28 days after placement. This quality control step is mandatory on commercial and public construction projects.
Cylinder Volume Formula
Where d is the diameter and h is the height or depth. To convert cubic inches to cubic feet, divide by 1,728. To convert cubic feet to cubic yards, divide by 27. Concrete weighs approximately 150 pounds per cubic foot (normal weight) or 110 pounds per cubic foot (lightweight).
Common Cylinder Sizes
| Diameter | Depth 24" | Depth 36" | Depth 48" | Common Use |
|---|---|---|---|---|
| 8" | 0.058 yd³ | 0.087 yd³ | 0.116 yd³ | Light post footings |
| 10" | 0.091 yd³ | 0.136 yd³ | 0.182 yd³ | Fence post footings |
| 12" | 0.131 yd³ | 0.196 yd³ | 0.262 yd³ | Deck piers |
| 18" | 0.294 yd³ | 0.441 yd³ | 0.589 yd³ | Heavy piers |
| 24" | 0.523 yd³ | 0.785 yd³ | 1.047 yd³ | Bridge piers |
Concrete Strength Testing
Concrete test cylinders are cast according to ASTM C31 standards. The standard test cylinder is 6 inches in diameter by 12 inches tall, though 4" x 8" cylinders are becoming common. These specimens are cured under controlled conditions and then crushed in a compressive testing machine per ASTM C39 to determine the concrete's compressive strength (f'c). Most structural concrete has a design strength of 3,000 to 5,000 psi, while high-performance concrete can exceed 10,000 psi.
Worked Example
Calculate concrete for a cylinder 12 inches in diameter and 48 inches deep:
= 5,428.67 ÷ 1,728 = 3.14 ft³
= 3.14 ÷ 27 = 0.116 yd³
You would need approximately 6 bags of 80-lb concrete mix, and the finished cylinder would weigh about 471 pounds.
Frequently Asked Questions
How deep should concrete cylinder footings be?
Footing depth depends on the frost line in your area. In cold climates, footings must extend below the frost line, which can be 36 to 60 inches deep. In warmer climates, a minimum depth of 12 to 18 inches is typical. Always check local building codes for your specific frost depth requirement.
What diameter Sonotube do I need for a deck?
For a typical residential deck, 10 to 12-inch diameter tubes are standard. Larger decks or those with heavy loads (such as hot tubs) may require 16 or 18-inch tubes. The required diameter depends on the load-bearing capacity needed and the soil's bearing capacity at your site.
How many bags of concrete for a 12-inch Sonotube?
For a 12-inch diameter tube that is 4 feet deep, you need approximately 6 bags of 80-lb concrete mix (or 8 bags of 60-lb mix). Each 80-lb bag yields about 0.6 cubic feet of mixed concrete. Always round up and buy an extra bag for waste.
Should I use quick-set or regular concrete for tube forms?
Regular concrete mix is recommended for structural tube forms because it develops higher ultimate strength. Quick-set concrete is acceptable for non-structural applications like fence post footings where you need to continue work quickly. Quick-set typically reaches a walkable state in 20-40 minutes but may have slightly lower final strength.