Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Outer diameter 24 in = 2 ft; Inner diameter 18 in = 1.5 ft; Length or height 6 ft
- Outer radius R = 1 ft, inner radius r = 0.75 ft
- Volume = π × (1² − 0.75²) × 6 × 1 = 8.2467 ft³
- Net volume = 8.2467 ft³ ÷ 27 = 0.3054 yd³ (× 0.0283168 = 0.2335 m³)
- Order quantity = 0.3054 yd³ × (1 + 10%) = 0.336 yd³ = 9.0713 ft³
- 80 lb bags = 9.0713 ft³ ÷ 0.6 ft³ per bag = 15.12 → 16 bags (always round up)
- Weight = 9.0713 ft³ × 150 lb/ft³ = 1,361 lb
How to measure
Measure the outer diameter across the outside of the finished tube and the inner diameter across the opening. The wall thickness is half the difference between them. If you know the wall thickness instead, add twice the thickness to the inner diameter to get the outer diameter.
Length is the height or length of the pour. Rings stacked on top of each other add together, and the quantity box handles identical sections.
The hollow cylinder formula
Subtract the volume of the hole from the volume of the outer cylinder. In one step that is π × (R² − r²) × length, where R is the outer radius and r is the inner radius. Using this form avoids subtracting two large numbers and is the same formula the original calculator on the home page uses.
Wall thickness has a large effect. Doubling the wall thickness of a thin ring more than doubles the concrete, because the area grows with the square of the radius.
Formula: V = π × (R² − r²) × L × quantity, in feet.
Worked example: a 24 in ring, 6 ft long
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Outer diameter 24 in = 2 ft; Inner diameter 18 in = 1.5 ft; Length or height 6 ft
- Outer radius R = 1 ft, inner radius r = 0.75 ft
- Volume = π × (1² − 0.75²) × 6 × 1 = 8.2467 ft³
- Net volume = 8.2467 ft³ ÷ 27 = 0.3054 yd³ (× 0.0283168 = 0.2335 m³)
- Order quantity = 0.3054 yd³ × (1 + 10%) = 0.336 yd³ = 9.0713 ft³
- 80 lb bags = 9.0713 ft³ ÷ 0.6 ft³ per bag = 15.12 → 16 bags (always round up)
- Weight = 9.0713 ft³ × 150 lb/ft³ = 1,361 lb
| Quantity | Result |
|---|---|
| Net volume | 0.305 yd³ (8.25 ft³, 0.234 m³) |
| Order with 10% waste | 0.34 yd³ (0.26 m³) |
| 80 lb bags | 16 |
| 60 lb bags | 21 |
| Weight at 150 lb/ft³ | 1,361 lb |
That tube has a 3 in wall and needs 0.34 yd³, or 16 bags of 80 lb mix. Precast pipe suppliers sell by the piece, so this is mostly useful when you are casting your own.
Where hollow shapes show up
- Round planters and raised garden rings.
- Fire pit walls.
- Culvert sections and drainage rings cast in place.
- Well curbs and septic risers.
- A column with a void, such as a form around a service pipe.
Wall thickness table
Concrete in one linear foot of a 24 in outer diameter tube, for different wall thicknesses:
| Wall | Inner diameter | ft³ per foot | Feet per yd³ |
|---|---|---|---|
| 2 in | 20 in | 0.960 | 28.1 |
| 3 in | 18 in | 1.374 | 19.6 |
| 4 in | 16 in | 1.745 | 15.5 |
| 6 in | 12 in | 2.356 | 11.5 |
Tips for casting hollow shapes
An inner form (a cardboard tube, a collapsible core or a smaller form) must be held in the center while the concrete is placed, otherwise it floats or shifts. Brace it and fill evenly around the ring. Stripping an inner form early is easier than stripping it late, but cure the wall properly: see the curing guide.
Frequently asked questions
How do I calculate the concrete in a tube?
Use π × (R² − r²) × length with radii in feet. For a 24 in outer and 18 in inner tube 6 ft long: π × (1² − 0.75²) × 6 = 8.25 ft³, or 0.31 yd³.
What if I know the wall thickness instead of the inner diameter?
Inner diameter = outer diameter − 2 × wall thickness. A 24 in outer tube with a 3 in wall has an 18 in inner diameter.
Why does the calculator refuse some inputs?
The inner diameter must be smaller than the outer diameter. If it is equal or larger there is no wall.
Can I use this for a square tube?
No. Use two slab calculations: the outer box minus the inner box.
How much waste should I add?
Ten percent is a good start for cast-in-place hollow shapes, because of form flex and trapped air.
Do I need rebar in a ring?
Often yes. The design depends on the load and the diameter. Use the specification supplied with your drawing.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.