Tube Form Calculator
Round tube forms are the usual way to pour cylindrical columns, deck piers and tidy fence-post holes. Set the diameter, the height or depth, and how many you are pouring; the calculator shows the volume of one tube and of the whole run. Every result is an estimate for planning — the tube size, the depth and the count come from your project and local practice.
Step by step
How to use this calculator
- Set the number. Enter how many tubes or holes you are pouring, from one post hole to a whole row of deck piers.
- Enter the diameter in inches — that is the inside diameter of the form, or the width of the hole you dug.
- Enter the vertical dimension. For a column or pier it is the height of concrete; for a post hole it is the depth. It is the same field, labeled for whichever job you are doing.
- Read both volumes. You get one tube and the total for the whole count. To add wastage and other shapes in one project, use the main concrete calculator.
Formulas
- One tube: volume in ft³ = π × (diameter in ÷ 24)² × height (ft). A 12-inch, 4-foot tube holds π × 0.5² × 4 ≈ 3.14 ft³.
- Total: per-tube volume × count. Cubic yards = ft³ ÷ 27.
- Metric: volume in m³ = π × (diameter m ÷ 2)² × height (m). One cubic meter holds about 1.31 yd³.
Worked example
Worked Example
One column, 12 in diameter × 4 ft tall
π × (12 ÷ 24)² × 4 = 3.14 ft³, which is 0.116 cubic yards. One 80 lb bag yields roughly 0.6 ft³, so a single tube like this takes about six bags.
Six deck piers, 12 in × 4 ft
Six tubes at 0.116 yd³ each come to 0.70 yd³ in total. With a 10% allowance that is about 0.77 yd³ — small enough to mix from bags, big enough to plan the morning around.
Eight fence post holes, 12 in across × 2 ft deep
π × 0.5² × 2 × 8 = 12.6 ft³, or 0.47 yd³ for the whole line of posts. Digging usually takes longer than pouring at this size.
Fence posts, piers and post holes
For fence posts, a hole roughly three times the width of the post is a common starting example, and the depth usually depends on two things: how tall the fence is and how deep the ground freezes where you live. Many people also add a shovel of gravel in the bottom of the hole so the post end sits on drainage instead of soil. Local practice varies, so it is worth asking around before digging a long line of holes.
For deck piers, the tube form keeps the concrete neat from the hole up to above grade. The calculator assumes a straight cylinder: if your plans call for a wider footing pad at the bottom, run the pad through the footing shape on the slab calculator and add the two volumes together.
Whichever job you are doing, brace the forms so they stay plumb while you pour, and check the count before the truck or the bags arrive — an extra hole discovered afterwards always seems to be the deepest one.
Knowledge base
Frequently Asked Questions
How deep should a fence post hole be?
A common starting example is a third of the above-ground post height, but the depth also depends on frost in cold regions and on wind exposure. Local practice is the best guide; this calculator only turns the depth you choose into a volume.
What diameter tube or hole do I need?
For fence posts, a hole about three times the post width is a frequent example. For columns and deck piers, the diameter comes from your plans or the hardware you are attaching. Measure the inside of the form, not the outside.
Can I pour concrete directly into a dug hole?
Yes — below grade the soil is the form. Above grade, a tube form holds the concrete to a tidy column instead of letting it slump out. That is the two-part job this calculator describes with one height or depth field.
How many bags does one post hole take?
A 12-inch hole, 2 ft deep, holds about 1.6 ft³ — roughly three 80 lb bags at a typical 0.6 ft³ yield. The concrete bag calculator counts bags for any hole size, with an editable allowance.
Does the count change the per-tube volume?
No. The per-tube volume only depends on diameter and height or depth; the count simply multiplies it into a total. Both numbers are shown so you can check them against each other.
What about flared or bell-shaped footing bottoms?
This calculator assumes a straight cylinder, which is what a standard tube form gives you. If your project uses a flared bottom, estimate the straight part here and the flare as a separate shape, then add the volumes.
This calculator is an estimating tool for planning only. Quantities depend on site conditions, wastage and suppliers, and are not a quotation. Thickness, footing depth, reinforcement and mix strength must come from your plans, an engineer or your local building authority.
More Calculators
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Compare the cost of bags against a ready-mix truck with your own prices.