This sonotube calculator sizes the concrete for round tube-form piers — the cardboard cylinders that form deck, fence, pole-barn and sign footings. Enter tube diameter, depth and how many piers, and get the volume per pier and for the whole job in cubic feet and yards, plus the bag count if you’re mixing rather than ordering a truck. Pier math fools intuition (a modest tube holds more than you think), so this page exists to make the order right the first time.

Concrete per pier –
Total for all piers –
Bags needed –

Volume is the exact cylinder (π r² × depth). Real holes bell out and over-dig, so the 10% extra default is honest planning, not padding — post holes and piers nearly always drink more than the cylinder on paper.

The Cylinder That Holds More Than You Think

Tube-form piers are pure cylinders, and cylinder volume grows with the square of the radius — the fact that wrecks estimates. A 12-inch tube 4 feet deep holds π × 0.5² × 4 = 3.14 cu ft. Step up to a 16-inch tube (which looks only “a bit wider”) and the same depth holds 5.59 cu ft — 78% more concrete for 33% more diameter. Step to 24 inches and it’s 12.57 cu ft per pier, nearly half a yard each. Diameter decisions made casually at the store (“the 16 was on sale”) quietly double a concrete order; run this calculator with the diameter your footing design actually specifies, which is driven by the load above and the soil below, not by what’s in stock.

The depth input deserves the same respect: it is the depth of concrete in the tube, which for frost-country footings means below the frost line — 36 to 48 inches across much of the northern US, set by local code — plus whatever the tube extends above grade (typically 2–6 inches to shed water away from the post or bracket). Add a flared or bell base (a wider footing pad poured at the bottom, common on deck piers in soft soil) and that pad is its own extra volume: a 24-in diameter × 8-in thick bell under a 12-in pier adds about 2.1 cu ft — fold it into the waste allowance or run the numbers again for the bell as a short, fat cylinder and add the results.

Worked Example (Verified)

  • Six 12-inch piers, 48 inches deep (defaults — a mid-size deck): 3.14 cu ft each, 18.85 cu ft total = 0.70 cu yd, and 35 bags of 80 lb mix with the 10% over-dig allowance. This is the exact crossover where a towable mixer or a short-load delivery starts competing with bags — the bag count is honest either way.
  • Fence corner posts, 8-inch tubes 36 inches deep, ×4: 1.05 cu ft each, 4.19 total → 8 bags of 80 lb. Small tubes are why bag mixing survives: the whole corner system is a two-trip wheelbarrow job.
  • Pole-barn piers, 18-inch × 54 inches, ×8: 7.95 cu ft each, 63.6 total = 2.36 cu yd (2.59 with allowance) — truck territory, and the reason barn builders plan pier day around the ready-mix schedule, not the bag aisle.

Choosing Diameter and Depth: The Load Path

A pier transfers a point load — a deck beam’s end, a post, a column — into the soil at a depth where frost can’t heave it and soil can bear it. Sizing logic runs top-down: the structure’s load per pier (deck codes commonly design around 1,500 psf soil bearing for typical soils) sets the required footing area at the base; the tube diameter provides that area in round form (12 in = 0.79 sq ft of bearing, 16 in = 1.40, 24 in = 3.14). Soft clay bears less, sand and gravel more, and local code tables govern — when a permit is involved, the inspector’s table, not this page, is the authority. What this page guarantees is that once diameter and depth are chosen, the concrete order matches the geometry.

Above the footing, the tube’s job is formwork and frost protection: smooth cardboard sides give frost less to grip than rough-dug earth (heaving soil slides on the tube rather than jacking it), and the pour rises cleanly to a level top for the post bracket. Cut tubes to length with the top 2–6 inches above grade, brace them plumb before the pour — wet concrete will find any lean and keep it — and strip or leave the above-grade cardboard to weather off after curing.

Pour Discipline

  • Dig, then measure the real hole. Augers wander and walls slough; if the hole belled wider than the tube, the concrete outside the tube still counts — the waste allowance absorbs normal over-dig, but walk the holes with a tape before ordering.
  • Gravel or a footing pad first. A few inches of compacted gravel (or a poured bell) at the base gives the tube a level seat and drainage under the pier; dry-laid tubes on soft bottom soil pour uneven.
  • Fill in lifts and consolidate. Rod or vibrate each 12-inch lift to chase air pockets — honeycombed pier sides are where freeze-thaw starts its work.
  • Set hardware while it’s wet. Post bases and J-bolts go in within the working window, checked against string lines in both directions; a bracket ½ inch off in wet concrete is a shim forever after.
  • Crown the top. Trowel a slight dome so water sheds off the pier instead of pooling around the post base — the cheapest rot prevention in the whole structure.
  • Cure before loading. Posts can typically be set after 48–72 hours; full structural loads wait for the 7-day strength milestone, with 28 days to design strength.

Cost Notes

Tubes themselves are cheap ($8–$20 per 12-ft length depending on diameter, cut to size on site) — the money is the concrete: at the defaults, 35 bags ≈ $175–$280 in mix, versus a short-load delivery that may carry a $150+ fee under a yard. The breakeven flips with pier count and diameter exactly as the examples show, which is why the calculator reports yards and bags side by side. Add post brackets ($8–$25 each), gravel, and rebar where code requires vertical reinforcement in the pier.

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Frequently Asked Questions

How much concrete do I need for a 12-inch sonotube?

Per foot of depth, a 12-inch tube holds π × (0.5 ft)² = 0.785 cu ft. At the common 48-inch footing depth that’s 3.14 cu ft per pier — about 5.2 bags of 80 lb mix (at 0.60 cu ft per bag), call it 6 per pier with allowance, or 35 bags for six piers with the calculator’s 10% extra included. Scale linearly with depth: 36 inches is 2.36 cu ft, 54 inches is 3.53. Diameter changes are not linear — see the next answer before substituting sizes.

How much more concrete does a bigger tube use?

Volume follows diameter squared: going from 12 to 16 inches multiplies volume by 1.78, from 12 to 24 inches by 4.0, and from 8 to 12 inches by 2.25 — a 24-inch pier at 4 feet deep holds 12.57 cu ft, nearly half a cubic yard in a single hole. This is why “free” upgrades to a larger tube at the store are never free, and why footing diameter should come from the load and soil bearing design, with this calculator pricing the geometry afterward.

How deep should sonotube piers be?

Below your local frost line, full stop — the entire point of the pier is to found the structure on soil that doesn’t move seasonally. Frost depths run from ~12 inches in the deep South to 48+ inches in the northern tier and are set by local code (ask the building department; it’s a one-line answer). Above grade, extend the tube 2–6 inches above finished ground so water sheds away from the post. Enter total concrete depth — below-grade plus above-grade portion — into the calculator.

Do I need a wider footing at the bottom of the tube?

Often, yes — the tube provides the column, but bearing area at the base is what keeps the pier from punching into the soil under load. Options: a pre-cast footing pad dropped in the hole, a belled-out excavation (flared with a spade or a bell auger), or a larger-diameter bottom section. A 24-inch × 8-inch bell under a 12-inch pier adds about 2.1 cu ft of concrete and more than doubles the base bearing area (3.14 sq ft vs 0.79). Your local code table states required footing sizes per load; the calculator handles the tube, and the bell can be added as a second short-cylinder calculation.

Bags or ready-mix for piers?

Follow the bag count. Under ~20 bags, hand or drum mixing beside the holes is efficient. The default six-pier deck at 35 bags is the gray zone — a drum mixer and a helper make it a morning’s work, and a towable 1-yard mixer rental splits the difference. Past about 1.5–2 cu yd total (the pole-barn example at 2.4–2.6 yd), delivery wins outright on cost, consistency and the working-window problem: eight holes must all be ready when the truck arrives, but the pour itself takes twenty minutes instead of a day. Piers are unusually bag-friendly as concrete work goes, because each hole is an independent pour — no cold-joint panic between holes.

Can the cardboard tube stay in the ground?

Yes — below grade, the tube is typically left in place; it biodegrades slowly without affecting the pier, and its smooth surface is actually working for you (frost gripping the soil slides against the tube rather than jacking a rough earth-formed pier). Above grade, peel the tube off after the concrete cures a day or two for a clean finish, or let weather strip it over a season. In wet holes, plastic-coated or fiberglass tubes resist the cardboard softening that can oval a tube before the pour.

Do sonotube piers need rebar?

Code-dependent. Many residential deck piers are plain concrete by local amendment; others require one or more vertical bars (often #4) running the pier’s length, especially in seismic zones, tall piers, or where the pier extends well above grade (unbraced length invites lateral failure). The concrete volume the bar displaces is negligible — don’t subtract it. Check the permit set; when in doubt, a vertical #4 bar and a few inches of cover is cheap insurance that no inspector has ever objected to.

Is this sonotube calculator free?

Yes — free, instant, no sign-up. Enter diameter, depth and pier count for per-pier and total volume in feet and yards, plus the bag count with over-dig allowance.