Sonotube, pier and column-footing concrete calculator
The calculator above turns a tube form's diameter, its pour height and how many piers you are casting into the numbers that matter on site — the concrete volume in cubic feet or metres, a side-by-side 40/60/80 lb (or 20/25/30 kg) bag table, the ready-mix figure in cubic yards, and an optional rebar-cage steel takeoff — for a whole project of pier groups at once, in your browser.
This guide explains how the concrete volume for a round or square tube form is worked out, how a belled base adds a cone of concrete at the foot, how bag counts come from the mix yield rather than guesswork, and how the optional rebar cage is turned into a steel weight.
How much concrete do I need for a sonotube?
A tube form is a cylinder, so the concrete for one pier is π × (diameter ÷ 2)² × height. Multiply by the number of piers, add a waste allowance, then divide by how much one bag yields to get the bags. A 12 in tube 4 ft tall holds about 3.14 ft³; four of them come to roughly 12.6 ft³, or about 24 × 80 lb bags with 10% waste.
In short: volume = π × radius² × height × number of piers; bags = volume × (1 + waste) ÷ bag yield, rounded up. The calculator does every step as you type, shows all three bag sizes together so you can pick the one that is cheapest to carry, and flags when the total is large enough to order ready-mix instead.
How this calculator works
You describe the piers; the calculator handles the arithmetic. Add a pier group for each set of identical piers, choose a round tube or a square form, and enter the diameter (or the two sides) and the pour height, plus how many piers are in the group. Set the concrete grade and a waste allowance once, and they apply to every group.
Each group returns its own volume and bag count, and the project totals the concrete, the six-size bag table, the ready-mix figure and the rebar-cage steel across every group. Turn on a belled base or a rebar cage per group when you need them. Switch between metric (m, mm, kg, m³) and imperial (ft, in, lb, ft³/yd³) at any time and every value converts.
Results export to CSV or print as a clean takeoff. Everything is calculated on your device — nothing you enter is uploaded, and your project is saved only in your own browser.
Estimate your pier concrete in six steps
- Choose unitsPick metric (m, mm, kg, m³) or imperial (ft, in, lb, ft³); every value converts when you switch.
- Add a pier groupAdd a group for each set of identical piers and choose a round tube or a square form.
- Enter the dimensionsType the diameter (or the two sides) and the pour height, then the number of piers in the group.
- Add a bell or cageTurn on a belled base or a rebar cage for the group when you need them; both feed the totals.
- Check the waste allowanceAdjust the waste percentage for spillage and over-excavation; 10% is a safe default.
- Read the results, export or printCheck the volume, bag table, ready-mix and steel, then export to CSV or print the takeoff.
Volume formulas
The concrete for a pier is a straight body plus, if you flare the foot, a cone of concrete under it. The calculator uses these formulas, then multiplies by the number of piers in the group.
The concrete for one pier comes from these formulas; the project multiplies by the number of piers in each group.
Round tube
V = π × (diameter ÷ 2)² × height- The cross-section is a circle, so volume grows with the square of the diameter.
- Doubling the tube diameter quadruples the concrete.
Square / rectangular form
V = side a × side b × height- Sides are entered in millimetres or inches; height in metres or feet.
- Covers square Sonotube-style forms as well as round tubes.
Belled base (cone)
V = (π × h ÷ 3) × (R² + R·r + r²)- R is the bell's bottom radius, r the tube radius and h the flare height.
- Added to the straight body; the flare's top matches the tube by construction.
Bags, grade and waste
Premix bags are sold by weight, so the bag count depends on how much mixed concrete one bag yields. The calculator derives a default yield from the bag weight and grade — about 0.6 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb (about 0.012 m³ for a 25 kg bag) — and shows all three sizes together so you can compare.
A waste allowance protects against under-ordering from spillage, over-excavated holes and a tube that is cut a little long; 10% is a sensible default. Beyond about half a cubic metre of concrete in total, the calculator flags ready-mix and estimates the truck loads, because mixing that many bags by hand stops being practical.
Bags and the ready-mix steer come from the volume, the bag yield and the waste allowance.
Bags to buy
bags = ⌈ volume × (1 + waste) ÷ bag yield ⌉- Volume is the project total; waste defaults to 10%; bags always round up.
- Bags are totalled from the summed volume, so per-pier rounding never inflates the order.
Bag yield
yield ≈ bag weight (kg) × 0.000468 × grade factor- 40 / 60 / 80 lb bags yield about 0.30 / 0.45 / 0.60 ft³ of mixed concrete.
- Higher grades carry a factor slightly below 1; override the yield to match your product.
Ready-mix
trucks = ⌈ volume × (1 + waste) ÷ truck load ⌉- Flagged once the total passes about 0.5 m³ (0.65 yd³); ordered by volume, not by bag.
- Suppliers often set a minimum load or a short-load fee — compare against the bag cost first.
Rebar cage and cover
Piers that carry uplift or bending are usually reinforced with a cage: several vertical bars tied at intervals with horizontal hoops, held a clear distance in from the tube wall. When you turn a cage on for a group, the calculator lays out the vertical bars over the pour height and the tie hoops up the pier, then weighs the steel from the standard bar tables — the same engine the rebar-weight calculator uses.
The diagram below labels the parts: the tube diameter, the pour height, the flared bell at the foot, and the vertical bars and tie hoops of the cage. Clear cover is the distance from the steel to the concrete face.
A round tube (Ø 12 in, 4 ft) on a 20 in belled base, reinforced with four vertical bars and tie hoops held a clear cover in from the tube wall. Bell and cage are optional per pier group.
Concrete per tube (quick reference)
The table below gives the concrete and 80 lb bag count for one round tube at common diameters and heights, so you can size a job at a glance before opening the calculator. Multiply by the number of piers and add your own waste allowance.
Concrete and 80 lb bag count for one round tube (no waste). Multiply by the number of piers and add your own allowance.
| Diameter | 2 ft | 3 ft | 4 ft |
|---|---|---|---|
| 6 in | 0.39 ft³1 × 80 lb | 0.59 ft³1 × 80 lb | 0.79 ft³2 × 80 lb |
| 8 in | 0.70 ft³2 × 80 lb | 1.05 ft³2 × 80 lb | 1.40 ft³3 × 80 lb |
| 10 in | 1.09 ft³2 × 80 lb | 1.64 ft³3 × 80 lb | 2.18 ft³4 × 80 lb |
| 12 in | 1.57 ft³3 × 80 lb | 2.36 ft³4 × 80 lb | 3.14 ft³6 × 80 lb |
| 16 in | 2.79 ft³5 × 80 lb | 4.19 ft³7 × 80 lb | 5.58 ft³10 × 80 lb |
Volumes are the bare geometry; the top figure is cubic feet and the lower is whole 80 lb bags. The calculator adds waste, other bag sizes, the belled base and the rebar cage.
Straight vs belled base
A belled (flared) base widens the foot of a pier for more bearing area and uplift resistance. Because the flare is a cone of concrete added under the straight tube, it can add a surprising amount to the pour — worth knowing before you order.
Straight tube
3.14 ft³
Belled base
4.33 ft³
A straight 12 in × 4 ft tube holds about 3.14 ft³. Adding a 20 in belled base takes it to about 4.33 ft³ — the flare alone adds roughly 1.19 ft³, over a third more concrete.
Worked examples
Each example reproduces what the calculator returns, so you can check the method by hand. Figures use grade C25 with a 10% waste allowance unless stated.
Each example reproduces the calculator's output for the given inputs.
Four 12 in tubes, 4 ft tall (deck footings)
Imperial · 80 lb bags · C25- Given
- Round tube Ø 12 in × 4 ft
- 4 piers
- 80 lb bags (yield ≈ 0.6 ft³), 10% waste
- Formula
V = π × (Ø ÷ 2)² × H × count; bags = ⌈ V × 1.1 ÷ yield ⌉- Steps
- One pier: π × 0.5² × 4 = 3.14 ft³
- Four piers: 3.14 × 4 = 12.57 ft³ (≈ 0.47 yd³)
- With 10% waste: 12.57 × 1.10 = 13.82 ft³
- 80 lb bags: ⌈ 13.82 ÷ 0.6 ⌉ = 24 bags
- Result
- About 24 × 80 lb bags (or 31 × 60 lb / 47 × 40 lb) for the four piers
Four 300 mm tubes, 1.2 m tall
Metric · 25 kg bags · C25- Given
- Round tube Ø 300 mm × 1.2 m
- 4 piers
- 25 kg bags (yield ≈ 0.0117 m³), 10% waste
- Formula
V = π × (Ø ÷ 2)² × H × count; bags = ⌈ V × 1.1 ÷ yield ⌉- Steps
- One pier: π × 0.15² × 1.2 = 0.0848 m³
- Four piers: 0.0848 × 4 = 0.339 m³
- With 10% waste: 0.339 × 1.10 = 0.373 m³
- 25 kg bags: ⌈ 0.373 ÷ 0.0117 ⌉ = 32 bags
- Result
- About 32 × 25 kg bags (or 27 × 30 kg / 40 × 20 kg) for the four piers
A 12 in pier on a 20 in belled base
Imperial · belled base- Given
- Round tube Ø 12 in × 4 ft
- Belled base Ø 20 in, 10 in high
- 1 pier
- Formula
V = body + (π × h ÷ 3)(R² + R·r + r²)- Steps
- Straight body: π × 0.5² × 4 = 3.14 ft³
- Bell frustum: R = 0.83 ft, r = 0.5 ft, h = 0.83 ft
- Frustum: (π × 0.83 ÷ 3)(0.69 + 0.42 + 0.25) = 1.19 ft³
- Total: 3.14 + 1.19 = 4.33 ft³
- Result
- The belled pier needs about 4.33 ft³ — roughly 1.19 ft³ more than the straight tube
Common mistakes to avoid
Forgetting the belled base. The flare at the foot is extra concrete — a 12 in pier with a 20 in bell needs over a third more than the straight tube alone. Turn the bell on so it is counted.
Estimating by volume but forgetting waste. A hole is rarely the exact size of the tube, and concrete spills — always carry an allowance so you are not one bag short.
Ordering by a single bag size out of habit. The three sizes cover very different volumes, so the cheapest and easiest to carry depends on the job; compare all three before buying.
Reading the tie spacing or cover off this tool as a code requirement. The cage is a takeoff of the bars you specify, not a structural design — confirm bar sizes, spacing and cover with your local code or an engineer.
Frequently asked questions
Short answers to the questions people most often ask about estimating concrete, bags and rebar for tube-form piers with this calculator.
How much concrete does a sonotube need?
A tube form is a cylinder, so the concrete is π × (diameter ÷ 2)² × height. A 12 in tube 4 ft tall holds about 3.14 ft³; a 300 mm tube 1.2 m tall about 0.085 m³. Enter the diameter, height and number of piers and the calculator works out the total, then the bags and ready-mix.
How many bags of concrete for a sonotube?
Divide the volume (including waste) by how much one bag yields and round up. An 80 lb bag yields about 0.6 ft³, so a 12 in × 4 ft tube (≈ 3.14 ft³) needs about 6 × 80 lb bags with 10% waste — or 8 × 60 lb or 12 × 40 lb. The calculator shows all three sizes at once.
How much concrete per foot of tube?
Per foot of height, a round tube holds π × (diameter ÷ 2)² cubic feet: about 0.35 ft³/ft for an 8 in tube, 0.79 ft³/ft for 12 in and 1.40 ft³/ft for 16 in. Multiply by the pour height and the number of piers. The reference table above lists common sizes.
What size sonotube do I need?
That is a structural decision — pier size depends on the load, the soil and frost depth, so follow your plans, local code or an engineer. This calculator does not size piers; once you know the diameter and depth, it tells you the concrete, bags and rebar you will need to buy.
How much extra concrete does a belled base add?
A belled base is a cone of concrete under the tube, worked out with the frustum formula (π × h ÷ 3)(R² + R·r + r²). A 12 in tube with a 20 in bell 10 in high adds about 1.19 ft³ — over a third more than the straight tube. Turn the bell on for a group and it is added to the volume and bags automatically.
How much rebar goes in a sonotube pier?
Reinforced piers commonly use four vertical bars tied with horizontal hoops, held a clear cover in from the tube wall, but the sizes and spacing are an engineering decision. Turn on the rebar cage for a group, enter the bars you plan to use, and the calculator lays them out and weighs the steel using standard bar tables.
Should I use bags or ready-mix for piers?
Bags suit a handful of piers; once the total passes about half a cubic metre (0.65 yd³) — roughly six 12 in × 4 ft piers — ready-mix delivered by truck is usually easier and cheaper than mixing dozens of bags. The calculator flags the crossover and estimates the truck loads, but check the supplier's minimum load first.
Does it work for square or rectangular forms?
Yes. Switch a pier group to the square/rectangular shape and enter the two sides and the height; the concrete is side a × side b × height. Everything else — the bag table, ready-mix, cost and CSV export — works the same. The belled base applies to round tubes only.
Does it work in metric and imperial?
Yes. Enter diameters and sides in millimetres or inches, heights in metres or feet, and read the volume in cubic metres or cubic feet, with cubic yards for ready-mix. The bag table swaps between 20/25/30 kg and 40/60/80 lb. Switch the unit toggle at any time and every value converts.
Is the calculator private?
Yes. Every calculation runs in your browser — nothing you enter is uploaded to a server, and your project is saved only in your own browser's storage. You can export a CSV or print the takeoff, both generated on your device.