A 12-inch-inside-diameter tube filled with concrete for 3 feet of height holds 2.356 cubic feet, before any quantity allowance. At the cited 0.60 cu ft mixed yield, that rounds up to 4 bags of 80-lb concrete mix.1 Use actual inside diameter and the full concrete fill height, including any portion above grade. This is a material estimate, not a footing-size or excavation-depth recommendation.
The cylinder formula works for a plain filled tube. A spread base, bell, cap, post occupying part of the volume or other noncylindrical detail needs separate treatment. Sonotube is a brand of concrete form; its Builder’s Tube sheet labels diameters nominal and says to check size when critical.2 Do not use the cardboard outside diameter as the concrete diameter.
Tube concrete formula
Radius (ft) = inside diameter (in) / 24.
Concrete volume (cu ft) = π × radius² × fill height (ft).
Total with allowance = volume × number of identical tubes × (1 + allowance percent / 100).
Bags = total volume / exact mixed yield per bag, rounded up.
Cubic yards = cubic feet / 27. Our tables use full π and only round the final bag count.
Fill height is concrete height, not necessarily the tube length or excavation depth. If the tube extends 6 inches above ground and is filled through 30 inches below ground, the concrete height is 36 inches. Exclude a gravel zone only if the checked assembly actually uses gravel there. Do not subtract bedding depth from a separate concrete base that still needs to be poured.
Mixed-concrete bag yields
| QUIKRETE Concrete Mix bag | Approximate mixed yield |
|---|---|
| 40 lb | 0.30 cu ft |
| 50 lb | 0.375 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
| 90 lb | 0.675 cu ft |
The cited Concrete Mix sheet gives these yields.1 The same bag weight from another mix is not guaranteed to produce the same volume. Read the exact product packaging and follow its mixing, placement and curing instructions. Dry bag weight is not itself a measure of finished concrete volume.
Tube chart: concrete volume and 80-lb bag count
Calculated for the listed inside diameters, no displacement, no base/cap and no allowance. Each cell is one tube’s net volume and whole bags at 0.60 cu ft per bag. Fill heights are quantity examples, not recommended footing depths. Sizes beyond the cited Builder’s Tube range are geometric examples, not claims that this product comes in those sizes.
| Inside diameter | Volume per 1 ft | 2 ft fill | 3 ft fill | 4 ft fill |
|---|---|---|---|---|
| 6 in | 0.196350 cu ft | 0.393 cu ft / 1 bags | 0.589 cu ft / 1 bags | 0.785 cu ft / 2 bags |
| 8 in | 0.349066 cu ft | 0.698 cu ft / 2 bags | 1.047 cu ft / 2 bags | 1.396 cu ft / 3 bags |
| 10 in | 0.545415 cu ft | 1.091 cu ft / 2 bags | 1.636 cu ft / 3 bags | 2.182 cu ft / 4 bags |
| 12 in | 0.785398 cu ft | 1.571 cu ft / 3 bags | 2.356 cu ft / 4 bags | 3.142 cu ft / 6 bags |
| 16 in | 1.396263 cu ft | 2.793 cu ft / 5 bags | 4.189 cu ft / 7 bags | 5.585 cu ft / 10 bags |
| 18 in | 1.767146 cu ft | 3.534 cu ft / 6 bags | 5.301 cu ft / 9 bags | 7.069 cu ft / 12 bags |
| 24 in | 3.141593 cu ft | 6.283 cu ft / 11 bags | 9.425 cu ft / 16 bags | 12.566 cu ft / 21 bags |
Manufacturer chart versus full-precision geometry
The Sonotube Builder’s Tube sheet lists rounded volume per foot for nominal 6-, 8-, 10- and 12-inch forms, including 0.80 cu ft per foot for the 12-inch form.2 A mathematically exact 12-inch inside cylinder is π × 0.5² = 0.785398 cu ft per foot. Those are close rounded planning values, not identical exact measurements. Use the actual internal dimensions and exact mix yield when a bag boundary matters.
Three checked examples
1. One 12-inch tube, 3 feet of fill
Radius = 12 / 24 = 0.5 ft. Volume = π × 0.5² × 3 = 2.356194 cu ft. At 0.60 cu ft per bag: 3.926991, rounded to 4 bags. At 0.45 yield: 6 bags; at 0.375 yield: 7 bags. No quantity allowance is included.
2. Six 12-inch tubes, 3 feet of fill each
Net volume: 6 × 2.356194 = 14.137167 cu ft, or 0.523599 cu yd. At 0.60 yield: 23.561945, rounded to 24 bags. With a separately chosen example 10% allowance: 15.550884 cu ft / 0.60 = 25.918139, rounded to 26 bags. The 10% is a scenario, not a universal allowance.
3. One 18-inch tube, 4 feet of fill
Radius = 0.75 ft. Volume = π × 0.75² × 4 = 7.068583 cu ft. At 0.60 yield: 11.780972, rounded to 12 bags. Compared with a 12-inch cylinder at the same height, the volume ratio is (18 / 12)² = 2.25. This is a size-comparison calculation, not a footing recommendation.
Add bases, caps and displacement separately
A cylindrical tube does not automatically include a spread footing under it, a bell-shaped base or a larger cap. Calculate each concrete shape separately, without double-counting an overlapping region. A post occupying part of the tube reduces only the zone it actually occupies. Reinforcement and anchor details should follow the design; this guide does not determine them.
For a cylinder supporting a load, diameter, foundation depth, frost protection, soil bearing, connection and required inspections need local/design checks before material calculation. Do not size a deck footing by which row uses fewer bags. A form manufacturer’s product limits, bracing and placement directions also need checking; the geometry table does not approve unsupported pour height.
Batch rounding and quantity allowance
Multiply unrounded volume by the tube count before rounding the total number of bags when material can be shared under the mix’s working-time limits. Rounding per tube may overstate the total. Conversely, planning many holes does not mean all concrete should be mixed at once. Stage the pour according to the exact product and form instructions.
No fixed “one extra bag” rule is given here. Measure the actual fill height and internal diameter, account for the complete checked assembly, then choose any quantity allowance for handling, overfill or irregular shapes. Diameter errors matter: volume grows with diameter squared, so a 24-inch cylinder has four times the volume of a 12-inch cylinder at the same height.
Metric formula
Cubic meters = π × (inside diameter in meters / 2)² × concrete height in meters. For a 30.48 cm inside diameter and 91.44 cm fill height, convert to 0.3048 m and 0.9144 m before multiplying. That equals the first example, about 0.066720 cubic meters. Use the product’s stated metric yield or consistent units when dividing for bags; do not divide cubic meters by a cubic-foot yield.
FAQ
How many bags for a 12-inch Sonotube?
At exactly 12 inches inside diameter and 3 feet of concrete height, 4 bags at 0.60 cu ft yield before allowance. The inside diameter, fill height and bag product determine the count; the tube name alone does not.
Can I round to one bag per foot?
Not reliably. A 10-inch exact cylinder is about 0.545415 cu ft per foot, but a final whole-bag count depends on total height, yield and allowance. Use the unrounded total volume rather than a rounded bags-per-foot shortcut.
Should I subtract gravel at the bottom?
Only if gravel occupies part of the selected fill height under the checked design. If there is a separate concrete base, calculate it rather than removing it. Do not invent a bedding layer from the quantity formula.
Does the slab calculator handle these tubes?
The concrete calculator is a rectangular slab quantity model, not a round-tube input form. Use the cylinder formula here for tubes. The post calculator subtracts listed post displacement and is not the same as a completely filled pier.
Before excavation
Contact 811 and follow your state’s locate, response, waiting and safe-digging rules; private lines may need separate locating. Get the structural dimensions, outlet/utility conflicts where relevant, frost requirements and inspection needs checked before digging. Follow exact concrete/form instructions for placement, curing and loading.
Related guides
Post concrete bags | Slab bag guide | Rectangular concrete calculator
Sources
- QUIKRETE: Concrete Mix sheet (approximate mixed yields and product instructions)
- Sonotube: Builder’s Tube concrete forms sheet (rounded per-foot quantities, nominal diameters and product limits; cited sheet, not all form variants)