Concrete for round pier forms — bags per pier and for the job, with belled bases and footing pads handled properly.
Sonotube Calculator
Inputs
Stock sizes are 6, 8, 10, 12, 16 and up. Deck piers are usually 10 or 12.
Below frost line to a few inches above grade — the whole height of concrete, not just the buried part.
Results
Concrete needed
36 × 80 lb bags
V = π × (diameter ÷ 24)² × depth, per pier
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/sonotube-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
A pier is a cylinder, so the volume is π r² h and the arithmetic is genuinely simple. What makes people order wrong is everything around it: the diameter is given in inches while the depth is in feet, the answer usually lands in an awkward fraction of a cubic yard, and most piers are not plain cylinders at all — they sit on a pad or flare into a bell at the bottom, and that base is frequently a third of the concrete.
So the calculator works in the units the job is actually specified in, gives you the bag count per pier as well as the total, and handles the pad and the bell properly. The per-pier figure is the one that matters when you are carrying bags across a yard: knowing six piers need 36 bags is less useful than knowing each one takes six.
V = π × (diameter ÷ 24)² × depthV = width² × thickness ÷ 1728, in ft³V = πh/3 × (R² + Rr + r²)ft³ ÷ 0.60ft³ ÷ 0.45Because that is how the job is specified — the same units a post hole is dug to. Tubes are sold by inch diameter, frost depth is quoted in feet, and converting between them by hand is where the mistakes happen — dividing a 12 inch diameter by 12 gives a 1 ft diameter, but the formula wants the radius, so the real divisor is 24. Halving that number by mistake quarters the volume, and it is the single most common error in pier estimating.
A belled base flares from the tube diameter at the top to a wider diameter at the bottom. Calculating it as a cylinder of the bell diameter is badly wrong, because the flare is a cone, not a barrel: on a bell twice the tube width the cylinder figure is about 70% too high. The correct volume is the frustum formula: πh/3 × (R² + Rr + r²). Worth noting that some jurisdictions do not permit belled piers at all — they are hard to inspect and hard to form — so check before digging one.
You cannot pour four-fifths of a bag into one pier and carry the rest to the next: each pier is a separate pour, mixed and placed before it starts to set. A 10 inch pier 4 ft deep is 2.18 ft³, which is 3.6 bags — so 4 bags per pier, and six piers is 24 bags rather than the 22 the raw total suggests. The calculator gives both figures; buy against the per-pier one.
A pier that stops above the local frost line gets lifted by frost heave every winter, and the structure on top of it goes with it. Frost depth runs from about 12 inches on the Gulf coast to more than 6 feet in northern Minnesota, and your building department publishes the local figure. The pour depth in this calculator is the total height of concrete — below the frost line to a few inches above grade, not just the buried part.
Builder's tube is sold in 4, 8 and 12 ft lengths in 6 to 12 inch diameters. The calculator gives total linear feet, which assumes perfect cutting with no offcut waste — real buying means working out how many 12 ft tubes cover six 4 ft piers, and the answer there is two, with nothing left over. Get that wrong and the offcuts cost more than an extra tube would have.
Vertical rebar, usually two to four bars tied in a cage for anything carrying real load, and a post base or anchor bolt set into the wet concrete at the top. Setting the anchor after the concrete has stiffened is a common shortcut and it produces a pier that cannot resist uplift — the whole point of anchoring at all. For anything more than a light deck, how much concrete the pour needs on the total pour and check the bar schedule against your plan.
Plain tubes, no bell, no pad.
36 bags of 80 lb, six per pier
Dividing the grand total instead would suggest 35, which is the number that leaves you one bag short on the last hole. Round up per pier, then multiply.
Each tube sits on an 8 in thick pad to spread the load.
1.42 yd³ — past the point where bags make sense
The pads nearly double the pour. This is the moment most deck projects should be pricing a short-load delivery instead of 65 bags.
Bell 12 in high, pier 5 ft deep — frost-heave resistance.
4.0 ft³ — 7 bags
Treating the bell as a 20 in cylinder would give 2.18 ft³ instead of 1.27 — nine bags for this pier instead of seven, and that error multiplied across every pier in a house.
Generated from π r². Matches the manufacturer's published chart.
| Diameter | Per ft | 80 lb bags per ft | 60 lb bags per ft |
|---|---|---|---|
| 6 in | 0.20 ft³ | 0.33 | 0.44 |
| 8 in | 0.35 ft³ | 0.58 | 0.78 |
| 10 in | 0.55 ft³ | 0.91 | 1.21 |
| 12 in | 0.79 ft³ | 1.31 | 1.75 |
| 14 in | 1.07 ft³ | 1.78 | 2.38 |
| 16 in | 1.40 ft³ | 2.33 | 3.10 |
| 18 in | 1.77 ft³ | 2.95 | 3.93 |
| 20 in | 2.18 ft³ | 3.64 | 4.85 |
| 24 in | 3.14 ft³ | 5.24 | 6.98 |
Rounded up per pier, because each pier is its own pour.
| Diameter | 3 ft deep | 4 ft | 5 ft | 6 ft |
|---|---|---|---|---|
| 8 in | 2 | 3 | 3 | 4 |
| 10 in | 3 | 4 | 5 | 6 |
| 12 in | 4 | 6 | 7 | 8 |
| 16 in | 7 | 10 | 12 | 14 |
Tube costs $2–$5 per linear foot in the common 8 to 12 inch sizes, and 80 lb bags of concrete mix run $6–$9 in 2026. A typical six-pier deck foundation is therefore $250–$400 in materials — against $150–$400 per pier if a contractor digs, forms and pours it, where the digging is most of what you are paying for.
The crossover sits around a cubic yard, or roughly 45 bags. Below it, bags are cheaper and there is nothing to schedule. Above it, you are moving over a ton and a half of material by hand, and a short-load delivery — typically $140–$200 a yard plus a $50–$150 short-load fee — costs less than the day it saves.
Digging is where pier money goes. A two-man auger handles 8 and 10 inch holes in reasonable soil; 12 inches and up in clay or rock wants a machine, and a rented skid-steer auger runs $300–$500 a day. On sites where a machine cannot reach, hand-digging six frost-depth piers is genuinely a weekend.
Buying tube in 12 ft lengths and cutting to suit, rather than in 4 ft lengths per pier. Choosing 10 inch over 12 inch where the load allows — that is a third less concrete per foot. And digging accurately: an oversized hole gets filled with concrete you pay for, and a 14 inch hole for a 12 inch tube is 36% more pour if the tube is not used at all.
The formula needs the radius in feet. A 12 inch tube has a 0.5 ft radius, not 1 ft. Getting this wrong quadruples or quarters the answer, and it is by far the most common pier estimating error.
A flared base is a cone, and a cone holds a third of its bounding cylinder. Treating a 20 in bell over a 10 in tube as a plain 20 in cylinder overstates that section by about 70% — two extra bags on a single pier, and far more once it is multiplied out.
Each pier is poured separately, so partial bags do not carry over. Round up per pier, then multiply. The difference is small per hole and awkward when you are one bag short on the last one.
A pier that does not reach below frost depth will be jacked upward every winter regardless of its diameter. More concrete does not fix insufficient depth — it just makes a heavier thing for the frost to lift.
Post bases and anchor bolts go into wet concrete and get worked in so the concrete closes around them. Pushed into a stiffening pour, they sit in a void and pull straight out under uplift.
Cardboard above ground holds water against the concrete and looks unfinished. Strip the exposed portion once the concrete has set — the buried part can stay.
A 12 inch tube takes about 1.31 bags of 80 lb mix per foot of depth, so a 4 ft pier needs 6 bags. A 10 inch tube is 0.91 bags per foot — 4 bags at 4 ft deep. Round up per pier rather than across the job, because each pier is mixed and poured separately.
0.79 cubic feet per foot of depth. A 4 ft pier is 3.14 ft³, and six of them come to 18.85 ft³ or 0.70 cubic yards — just under the point where a ready-mix delivery starts to make sense.
Most residential decks use 10 or 12 inch tubes, but the correct answer comes from the load and your soil, not from convention. Larger decks, hot tubs and roof loads need bigger piers or a footing pad underneath to spread the load — the pier itself does not spread anything.
The base must sit below your local frost line, which runs from about 12 inches in the deep south to over 6 feet in northern Minnesota. A pier stopping above frost depth is lifted every winter no matter how much concrete is in it, and no diameter compensates for insufficient depth.
The bell is a frustum, not a cylinder: V = πh/3 × (R² + Rr + r²), where R is the bell radius and r the tube radius. A 20 inch bell 12 inches high over a 10 inch tube is 1.27 ft³; calculating it as a 20 inch cylinder gives 2.18, about 70% too high.
Builder's Tube covers 6, 8, 10 and 12 inch diameters in 4, 8 and 12 ft lengths; commercial round forms go up to 48 inches. Plan your cuts against those stock lengths — six 4 ft piers come out of two 12 ft tubes with nothing wasted.
The buried portion stays; strip the part above grade once the concrete has set. Cardboard left above ground holds moisture against the concrete and looks unfinished.
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This calculator uses published constants from named industry sources, each asserted in an automated test that runs on every build. The formula is printed with the result above, and any correction we make is logged on the changelog.
It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.