What a column or pier volume actually is
A concrete column is a prism: one cross-section extruded through a height. Every column volume on a job site is therefore the same two-step calculation - find the area of the section, multiply by the height - and the only thing that changes between a fibre tube, a drilled caisson and a formed pilaster is how you get the area.
For a round form the area is πr², where r is half the inside diameter. Fibre tube forms are sold by inside diameter, which is convenient: a tube labelled 10 in gives you a 10 in concrete column, not a 10 in outside dimension with a wall thickness to subtract. For a rectangular column the area is width times depth, measured face to face on the inside of the forms.
What makes piers error-prone is not the geometry but the units. The diameter is in inches, the height is in feet, the bag yield is in cubic feet, and the ready-mix truck is ordered in cubic yards. Four different units in one calculation, and the most common mistake - squaring the diameter in inches and then dividing by 12 rather than dividing first - is off by a factor of twelve, which is large enough to be obvious and small enough to survive a distracted check.
If you are pouring a flat pad rather than a column, use the concrete slab calculator; for a spread footing under a wall, use the concrete footing calculator.
The formula, variable by variable
Start with the section. For a round column, halve the diameter and convert to feet in the same move: r = (d ÷ 12) ÷ 2. Square that, multiply by π, and you have square feet. A 10 in tube gives r = 0.41667 ft, r² = 0.173611 ft², and an area of 0.545415 ft².
Multiply the area by the height in feet and you have the volume of one column in cubic feet. Multiply by the number of columns for the total. Divide by 27 to reach cubic yards, because a cubic yard is 3 × 3 × 3 feet. Multiply cubic feet by 0.0283168 for cubic metres.
Bag counts come from the yield printed on the bag rather than from the dry weight. An 80 lb bag of standard concrete mix yields about 0.60 ft³ of mixed concrete and a 60 lb bag about 0.45 ft³. Always read the yield off the bag you are actually buying, since high-early, crack-resistant and fibre-reinforced products differ.
Waste is applied last, to the total. On piers the losses are real: the tube tops get over-filled and screeded off, a shovel of mix goes on the ground at every hole, and an auger-drilled hole is always slightly larger than the bit. Five to ten percent covers it for tubes. For an unformed drilled pier in loose soil, allow considerably more - the hole belled at the bottom is concrete you cannot see and cannot recover.
Worked example: eight 10 in piers, 4 ft deep
You are setting a deck on eight piers. Each is a 10 in fibre tube, 4 ft from the bottom of the hole to the top of the form, and you want 8 percent waste.
- Radius in feet. 10 ÷ 12 = 0.833333 ft diameter; half of that is r = 0.416667 ft.
- Area. π × 0.416667² = π × 0.173611 = 0.545415 ft².
- Volume per pier. 0.545415 × 4 = 2.181662 ft³.
- Total neat volume. 2.181662 × 8 = 17.453293 ft³.
- Cubic yards. 17.453293 ÷ 27 = 0.646418 yd³.
- With 8 percent waste. 0.646418 × 1.08 = 0.698131 yd³, so order 0.75 yd³ if your supplier sells in quarter-yard steps.
- Or in bags. 17.453293 × 1.08 = 18.849556 ft³; at 0.60 ft³ per 80 lb bag that is 31.4, so 32 bags. At 0.45 ft³ per 60 lb bag it is 41.9, so 42 bags.
Thirty-two 80 lb bags is 2,560 lb of material to carry, mix and place by hand. That is the point at which most crews call for a short load instead, and it is worth pricing both - a short-load surcharge can easily exceed the bag saving on a job this size.
Reading the result and choosing bags or ready-mix
The number that decides your day is the total in cubic yards. Below about a quarter of a yard, bags win on every count: no truck, no minimum, no waiting. Between a quarter yard and a yard it is genuinely a coin flip and depends on how far you are carrying material and whether you have a mixer. Above a yard, mixing by bag becomes a serious labour item, and above two yards it is rarely the right call for a crew that has a ready-mix supplier within reach. Treat those figures as a working rule of thumb rather than a rule, and price the short-load fee before you decide.
The per-column volume matters for a different reason: it tells you how much material each hole swallows. A 6 in tube 3 ft deep holds 0.59 ft³ of concrete, which is less than a single 80 lb bag. If the design load needs more bearing than that section provides, the answer is a wider base at the bottom, not a taller pier - bearing comes from the base area, which is what the deck footing size calculator sizes.
Depth is usually set by frost, not by load. Local codes fix a minimum footing depth below finished grade for the frost line, and the height you enter here should run from the bottom of that hole to the top of the form, including the part you will never see.
Concrete per foot of column height
| Section | ft³ per ft of height | yd³ per ft of height | 80 lb bags per ft |
|---|---|---|---|
| 6 in round | 0.196350 | 0.007272 | 0.33 |
| 8 in round | 0.349066 | 0.012928 | 0.58 |
| 10 in round | 0.545415 | 0.020200 | 0.91 |
| 12 in round | 0.785398 | 0.029089 | 1.31 |
| 14 in round | 1.069014 | 0.039593 | 1.78 |
| 16 in round | 1.396263 | 0.051713 | 2.33 |
| 18 in round | 1.767146 | 0.065450 | 2.95 |
| 24 in round | 3.141593 | 0.116355 | 5.24 |
| 12 × 12 in square | 1.000000 | 0.037037 | 1.67 |
| 16 × 16 in square | 1.777778 | 0.065844 | 2.96 |
Bag counts are neat volume divided by a 0.60 ft³ yield, with no waste added, and are shown to two decimals so they can be summed before rounding up.
Mistakes that put the order wrong
- Squaring inches and dividing by 12 afterwards. Convert the diameter to feet first, then square. Doing it the other way overstates the area twelvefold.
- Using the radius where the formula wants the diameter, or the reverse. A 10 in tube has a 5 in radius. Halving twice quarters the volume, and the number still looks plausible.
- Measuring the height to grade instead of to the bottom of the hole. The buried length is most of the pour on a frost-depth pier.
- Ordering neat volume. A truck that arrives one wheelbarrow short leaves you with a cold joint in the last pier. Waste is cheaper than a second placement.
- Assuming bag weight scales with yield. Two 60 lb bags do not equal one 80 lb bag and a bit; check the yield in cubic feet printed on the bag.
- Ignoring the bell at the bottom of a drilled hole. An unformed pier takes whatever the auger removed, which is always more than the nominal cylinder.
Volume is not the same as capacity
This calculator answers how much concrete fills the form. It says nothing about whether the pier is strong enough or the soil beneath it can carry the load. Column capacity depends on concrete strength, reinforcement and slenderness, and is governed by ACI 318; bearing depends on the soil and is governed by the presumptive values in IRC Table R401.4.1 or by a geotechnical report. Size the base for bearing and the section for load before you order anything.
Where this sits among the other takeoff tools
Piers are one line on a concrete takeoff. If the same pour includes a slab, a grade beam or a footing, calculate each separately and add them before you place the order, because each has its own waste behaviour - a slab loses concrete to subgrade irregularity, a pier loses it at the top of the tube.
For bagged work, the concrete bag calculator converts any volume into bag counts across the common product sizes. If you are batching on site rather than buying pre-blended bags, the concrete mix ratio calculator splits a volume into cement, sand and aggregate. And where the piers support a deck, the deck footing size calculator and the deck board calculator handle the two ends of the same structure.
One practical note on ordering: ready-mix suppliers sell in quarter-yard or half-yard increments and charge a short-load fee below a threshold that is usually a few cubic yards. Ask what the threshold is before you decide between a truck and a pallet of bags, because on a pier job the fee is often the largest single number in the comparison.