How to use the sonotube calculator calculator
- 1
Find the inside diameter
Tubes are normally sold by their inside diameter, which becomes the diameter of the finished pier. If you measure across the tube yourself, measure the inside rather than the outside, since the cardboard wall is not filled with concrete.
- 2
Enter the height you will fill
Enter the height of concrete rather than the full length of the tube. Tubes are often cut to length or set with the top above the intended pour line, and only the filled part counts.
- 3
Set the number of tubes
Enter how many identical tubes you are pouring. The per-tube volume is multiplied by the count.
- 4
Add an allowance if the base is open
A tube set on undisturbed soil with an open bottom, or one with a bell or flared base, will take more than the cylinder figure. Add a percentage of your own or calculate the base separately.
- 5
Add a bag yield or your pricing
Enter the yield from the bag label for a bag count, or your quoted price per cubic yard or per bag for a cost estimate.
Formula
Cubic feet per tube = pi x (inside diameter(ft) / 2)^2 x filled height(ft)
Multiply by the number of tubes, then divide by 27 for cubic yards. This is the same cylinder formula used for post holes and round footings.
Worked example
Six form tubes, each 10 inches inside diameter, filled to 4 feet
- 1Convert the diameter: 10 in / 12 = 0.833 ft, so the radius is 0.417 ft.
- 2Find the area: pi x 0.417^2 = 0.545 square feet.
- 3Volume per tube: 0.545 x 4 ft = 2.18 cubic feet.
- 4Total for six: 2.18 x 6 = 13.09 cubic feet.
- 5Convert: 13.09 / 27 = 0.48 cubic yards.
Six tubes of this size take about 13.09 cubic feet, or 0.48 cubic yards.
Worked examples
Stepping up one tube size
The same six tubes at 12 inches inside diameter instead of 10, still 4 feet tall
- 1Radius: 12 in = 1 ft, so the radius is 0.5 ft.
- 2Area: pi x 0.5^2 = 0.785 square feet.
- 3Volume per tube: 0.785 x 4 = 3.14 cubic feet.
- 4Total for six: 3.14 x 6 = 18.85 cubic feet, or 0.70 cubic yards.
Going from 10 inch to 12 inch tubes raises the concrete by 44 percent, because volume scales with the square of the diameter rather than with the diameter itself.
Bag count for a small pier job
The 13.09 cubic feet from the first example, at a 0.6 cubic foot bag yield
- 1Divide by the yield entered: 13.09 / 0.6 = 21.82 bags.
- 2Round up: 22 bags.
- 3That is between 3 and 4 bags per tube.
The six tubes take 22 bags at that example yield. Confirm the yield on the bag you buy.
The round tube formula
A filled form tube is a cylinder, so its volume is the circular area of the tube multiplied by the height of concrete. The area is pi times the radius squared, and the radius is half the inside diameter.
Working in feet keeps the arithmetic clean: convert the diameter from inches by dividing by 12, halve it for the radius, square that, multiply by pi, then multiply by the filled height in feet. Divide by 27 at the end for cubic yards.
It is exactly the same calculation as a post hole or a round footing, which is why those pages share this engine. The difference is practical rather than mathematical: a tube gives you a known, consistent diameter, while a dug hole does not.
How to measure diameter and height
Use the inside diameter. Form tubes are normally specified and sold by inside diameter, because that is what determines the size of the finished pier, and the cardboard wall itself is not filled. Measuring across the outside adds two wall thicknesses to the figure.
For height, measure the concrete rather than the tube. Tubes are frequently cut on site, and it is common to set the top slightly above the finished level and trim later. If you are filling a 5 foot tube to 4 feet, the height is 4 feet.
If the tubes sit at different heights because the ground slopes, they are not identical and the quantity multiplier will not represent them correctly. In that case calculate the tallest and shortest separately, or enter each group with its own height and add the results.
It helps to record each tube against a mark rather than a plan dimension. Tubes are usually set to a common finished level, so the length below ground varies with the grade even when the tops all line up. Measuring from the top of each tube down to the bottom of its hole gives the height that will actually be filled, and on a sloping site that figure can differ by a foot or more between the highest and lowest pier.
Why diameter matters so much
Because area depends on the square of the radius, small changes in diameter produce large changes in volume. A 12 inch tube holds 44 percent more concrete per foot than a 10 inch tube, and an 8 inch tube holds only 44 percent of what a 12 inch tube holds.
This catches people out when a plan changes size late. Swapping to the next tube size up is a small decision at the supplier and a significant one on the concrete order, particularly across a dozen piers.
Height, by contrast, is linear and easy to reason about. Adding a foot to every tube adds the same volume per tube regardless of how tall they already are.
Common mistakes
The recurring errors on form tube estimates:
- Measuring the outside diameter of the tube rather than the inside.
- Using the full tube length instead of the height actually being filled.
- Entering the radius in the diameter field, which changes the answer by a factor of four.
- Treating tubes at different heights as identical and multiplying by a count.
- Forgetting a belled or flared base, which can add a substantial amount per pier.
- Assuming a bag yield rather than reading it off the bag.
- Ordering the exact figure when tubes are set on open ground, where concrete can escape at the base.
Form tube volume examples by diameter and height
Concrete needed to fill one round form tube at each size, with the last column showing how many tubes of that size one cubic yard would fill. These sizes demonstrate the arithmetic and are not pier design recommendations.
| Diameter x depth | Cubic feet each | Cubic yards each | Number that fit in 1 cubic yard |
|---|---|---|---|
| 6 in x 36 in | 0.589 | 0.0218 | 45 |
| 8 in x 36 in | 1.05 | 0.0388 | 25 |
| 8 in x 48 in | 1.4 | 0.0517 | 19 |
| 10 in x 48 in | 2.18 | 0.0808 | 12 |
| 12 in x 48 in | 3.14 | 0.116 | 8 |
| 12 in x 60 in | 3.93 | 0.145 | 6 |
| 16 in x 60 in | 6.98 | 0.259 | 3 |
| 18 in x 72 in | 10.6 | 0.393 | 2 |
Frequently asked questions
- How much concrete does a 12 inch form tube hold?
- A 12 inch inside diameter tube holds 0.785 cubic feet per foot of height. Filled to 4 feet that is 3.14 cubic feet, and filled to 5 feet it is 3.93 cubic feet. One cubic yard fills about 8 tubes at 4 feet each.
- How many bags of concrete for one form tube?
- Divide the tube volume by the yield printed on your bag. A 10 inch tube filled to 4 feet is 2.18 cubic feet, which is about 4 bags at a 0.6 cubic foot yield or 5 bags at a 0.45 yield.
- Do I use the inside or outside diameter?
- Inside. Form tubes are sold by inside diameter, which is also the diameter of the finished pier. The cardboard wall is not filled with concrete, so measuring the outside overstates the volume.
- Is Sonotube the same as a concrete form tube?
- Sonotube is a brand name for round concrete forming tubes and is widely used as a search term for them. YardCalc uses the term here for that reason only and is not affiliated with or endorsed by any manufacturer. The cylinder calculation is the same for any round form tube.
- What size tube do I need?
- This calculator does not size piers. Diameter, depth, embedment, reinforcement, and bearing depend on the loads, the soil, the frost depth, and local code, so take them from your drawings or a qualified designer.
- Does this include a flared base?
- No. The result covers the cylindrical part of the tube only. If your design has a belled or flared base, or a separate pad beneath the pier, calculate that volume separately and add it.
Form tube volume uses the exact cylinder formula with the inside diameter you enter. Sonotube is a brand name used here as a search term only; YardCalc is not affiliated with any manufacturer. No pier size, bag yield, or price is assumed. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.