Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Diameter or side 12 in = 1 ft; Height 4 ft
- Radius = 1 ÷ 2 = 0.5 ft
- Volume = π × 0.5² × 4 × 1 = 3.1416 ft³
- Net volume = 3.1416 ft³ ÷ 27 = 0.1164 yd³ (× 0.0283168 = 0.089 m³)
- Order quantity = 0.1164 yd³ × (1 + 10%) = 0.128 yd³ = 3.4558 ft³
- 80 lb bags = 3.4558 ft³ ÷ 0.6 ft³ per bag = 5.76 → 6 bags (always round up)
- Weight = 3.4558 ft³ × 150 lb/ft³ = 518 lb
How to measure
For a round column measure the inside diameter of the form, not the outside of the cardboard tube. A tube with a 12 in label has a 12 in inside diameter, and the wall is paper-thin. For a square column measure the inside of the formwork on one side.
Height is the vertical pour height from the bottom of the footing or pier to the top of the column. If the column starts in a footing, calculate the footing separately.
Round and square column formulas
A round column is a cylinder. The volume is π × radius² × height, where the radius is half the diameter. A square column is a box: side × side × height. In both cases work in feet and divide by 27 for cubic yards.
The difference is bigger than it looks. A round column holds only 78.5% of the concrete of a square one whose side equals the diameter, because π ÷ 4 = 0.785. That is why swapping a 12 in square form for a 12 in tube cuts the concrete by about a fifth.
Round: V = π × (D ÷ 2)² × H. Square: V = S² × H. Both in feet, then ÷ 27 for yd³.
Worked example: four 12 in columns, 8 ft tall
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Diameter or side 12 in = 1 ft; Height 8 ft
- Radius = 1 ÷ 2 = 0.5 ft
- Volume = π × 0.5² × 8 × 4 = 25.1327 ft³
- Net volume = 25.1327 ft³ ÷ 27 = 0.9308 yd³ (× 0.0283168 = 0.7117 m³)
- Order quantity = 0.9308 yd³ × (1 + 10%) = 1.0239 yd³ = 27.646 ft³
- 80 lb bags = 27.646 ft³ ÷ 0.6 ft³ per bag = 46.08 → 47 bags (always round up)
- Weight = 27.646 ft³ × 150 lb/ft³ = 4,147 lb
| Quantity | Result |
|---|---|
| Net volume | 0.931 yd³ (25.13 ft³, 0.712 m³) |
| Order with 10% waste | 1.02 yd³ (0.78 m³) |
| 80 lb bags | 47 |
| 60 lb bags | 62 |
| Weight at 150 lb/ft³ | 4,147 lb |
Round columns need 1.02 yd³. The same four columns in a 12 in square form need 1.30 yd³, which is 12 more 80 lb bags.
Concrete per foot for common tube sizes
These are rounded figures for a round form with no waste, handy for a quick check on site.
| Diameter | ft³ per foot | Feet per yd³ |
|---|---|---|
| 8 in | 0.349 | 77.3 |
| 10 in | 0.545 | 49.5 |
| 12 in | 0.785 | 34.4 |
| 16 in | 1.396 | 19.3 |
| 18 in | 1.767 | 15.3 |
| 24 in | 3.142 | 8.6 |
For the full size range, including tube forms sold by brand names, use the Sonotube calculator.
Reinforcement and anchor bolts
Columns carry load, so they are reinforced. The vertical bars and ties take up almost no volume, so you do not subtract them. Their quantity and size come from the engineer. If you need an estimate of weight and bar count for a grid, the rebar calculator covers slabs; column steel is normally drawn on the plans.
If anchor bolts or a base plate will be set in wet concrete, have them ready and templated before the truck arrives. You cannot go back for them.
Mistakes that cost a second delivery
- Using the outside diameter of the tube. Cardboard forms are thin, but 24 in versus 24.5 in over a 10 ft column is an extra 2 ft³.
- Forgetting that a long column is a deep pour. Concrete may need to be placed in lifts and vibrated.
- Pouring from a truck chute straight into a narrow form: plan for a funnel or bucket, or a pump.
- Not allowing for fall-off at the bottom where the tube meets an uneven footing.
Frequently asked questions
How do I calculate concrete for a round column?
Halve the diameter to get the radius, square it, multiply by π and by the height, all in feet. For 12 in by 8 ft: 3.1416 × 0.5² × 8 = 6.28 ft³ per column, which is 0.23 yd³.
How many bags of concrete fill a 12 in Sonotube?
A 12 in tube holds 0.785 ft³ per foot of height. At 4 ft that is 3.14 ft³, about 5.2 bags of 80 lb mix at 0.60 ft³ per bag, so buy 6.
What is the difference between a column and a pier?
In pouring terms, none. Both are vertical members. A pier usually sits under a beam and goes down to the footing; a column carries load above grade.
Should I add extra for tube forms?
Ten percent is a good default. Forms can bulge and the bottom is rarely level.
How much does a concrete column weigh?
A 12 in round column 8 ft tall is about 942 lb at 150 lb/ft³ before reinforcement, so plan lifting accordingly if you cast it off site.
Can I use this for a round footing?
Yes. A round footing is a short, wide cylinder, so enter its diameter and thickness.
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.