Round Sonotube and rectangular columns — volume, bags, rebar cage, and tie schedule for deck piers, fence posts, and structural columns.
Column Estimator · v3.2 · US
Quick pickCommon Sonotube sizes
Dimensionsround column
Typical useSets rebar layout
Reinforcement
Materials Order Live
Order this much— cu yd— per column × — qty
Raw volumebefore waste—
Cubic metressecondary unit—
80 lb bags~0.022 yd³ each—
60 lb bags~0.017 yd³ each—
Vertical rebar——
Ties / stirrups——
Sonotubes——
Cost——
Columns total—
Per column—
Weight—
Cage layout—
This calculator estimates materials, not structural capacity.
Column sizing for load-bearing applications depends on axial load, slenderness ratio, soil bearing, and code-prescribed reinforcement minimums. For any structural column carrying a building, deck, or roof — a licensed engineer specifies the rebar and ties.
— How the column calculator works
From a Sonotube to a full materials list.
The concrete math is the simpler part. The real value of a proper column calculator is the rebar cage: the vertical bars, the tie spacing, and the extra length needed to anchor the bars into the footing below and the structure above.
Volume. Round columns use V = π × r² × H. Rectangular columns use V = W × D × H. Multiply by quantity, add waste, and that's your concrete order.
Vertical bars. Each vertical bar runs the full column height, plus extra length to dowel into the footing below and project into the structure above. The calculator handles this with the "extra dowel length" field.
Ties or stirrups. Horizontal bars spaced at fixed intervals — 12" o.c. is the residential standard, tighter near the top and bottom. Each tie wraps around the vertical cage with hooks to prevent the bars from buckling outward under load.
Tie length. For round columns, ties are spiral or circular hoops sized to fit inside the column with rebar cover. For rectangular, they're rectangular stirrups bent to match the cross-section minus cover.
The "typical use" chips pre-load sensible defaults: a deck pier uses 4 × #4 bars at 12" tie spacing, a fence post needs no rebar (just concrete around the post), a structural column uses 6–8 bars at tighter spacing. You can override any of it.
For load-bearing UK columns, design follows Eurocode 2 (BS EN 1992). Minimum cover per BS 8500 Table A.5 depends on exposure class — 40 mm nominal cover for XC2 internal foundations.
Heavy support350–400 mmCarport posts, large pergolas, patio roofs.
Structural400–600 mmVerandah posts, AS 3600 designed columns.
Concrete volume per metre of column height (AU)
Diameter
Volume / m
20 kg bags / m
Volume × 1.2 m
150 mm
0.018 m³
1.8
0.021 m³ (2.1 bags)
200 mm
0.031 m³
3.1
0.038 m³ (3.8 bags)
250 mm
0.049 m³
4.9
0.059 m³ (5.9 bags)
300 mm
0.071 m³
7.1
0.085 m³ (8.5 bags)
400 mm
0.126 m³
12.6
0.151 m³ (15.1 bags)
500 mm
0.196 m³
19.6
0.236 m³ (23.6 bags)
600 mm
0.283 m³
28.3
0.339 m³ (33.9 bags)
Structural columns in Australian residential and commercial work follow AS 3600. Cover varies by exposure class — 30 mm A1, 40 mm B1, 50 mm B2 coastal.
Canadian columns follow CSA A23.3 for structural design. Cover per CSA A23.1: 75 mm cast against earth, 50 mm against forms.
— Rebar cage
What goes inside the column.
The rebar cage is the load-resisting heart of a structural column. Concrete handles compression beautifully; rebar handles the tension that develops near supports, at column tops, and from any moment loading. Three pieces matter:
Vertical bars. 4 × No. 4 bars is the residential deck-pier default. Bars run the full column height plus a 12" extension into the footing below and 3–4" projection at the top to dowel into the framing or post base.
Ties or stirrups. No. 3 bars bent into hoops or rectangular stirrups, wrapping around the vertical cage. Spacing per ACI 318-19 §10.7.6: max 16 bar diameters (8" for #4 verticals), 48 tie diameters (18" for #3 ties), or the least column dimension.
Cover. 1.5" cover for cast-against-forms (above ground portion), 3" cover when cast against earth (below grade). Use plastic chairs or dobies to hold the cage off the soil and centred in the form.
Vertical bars. 4 × 12 mm H10 bars is the residential default for a small column. Bars run column height plus 300 mm extension into the footing below and 100 mm projection at the top.
Links (ties). 8 mm bars bent into rectangular links or hoops, wrapping the vertical cage. Spacing per Eurocode 2 §9.5.3: the lesser of 12 vertical bar diameters, the smaller column dimension, or 300 mm — so 200 mm typical for 12 mm bars.
Cover. Per BS 8500 Table A.5 — typically 40 mm nominal cover for XC2 internal foundations, 50 mm for XC3/XC4 exterior exposure.
Vertical bars. 4 × N12 bars is the residential default. Bars run full column height plus 300 mm extension into the footing below and 100 mm projection at top.
Ligatures (ties). R6 or R8 bars bent into rectangular ligatures or helical ties, wrapping the vertical cage. Spacing per AS 3600 §10.7: the lesser of the column dimension, 15 × longitudinal bar diameter (180 mm for N12), or 600 mm — typically 200–300 mm.
Cover. AS 3600 exposure class — 30 mm A1, 40 mm B1, 50 mm B2 coastal, 65 mm C1/C2 tidal splash.
Vertical bars. 4 × 15M bars is the residential default. Bars run column height plus 300 mm extension into the footing below and 100 mm projection at top.
Ties. 10M bars bent into rectangular ties or helical ties, wrapping the vertical cage. Spacing per CSA A23.3 §10.10: the least of 16 longitudinal bar diameters (240 mm for 15M), 48 tie diameters (480 mm for 10M), or the column dimension — typically 240–300 mm.
Cover. CSA A23.1: 75 mm cast against earth, 50 mm against forms, 40 mm if exterior exposure with no salt contact.
Tighter ties at the ends.
Ties (or ligatures) should be spaced more closely at the top and bottom of the column — typically half the normal spacing for a distance equal to the larger column dimension or one-sixth the height. This confines the concrete in the high-stress regions where the column transfers load into the footing and into the structure above.
JM
Reviewed by Jordan Mireles, P.E.Licensed civil engineer · 14 years residential and light commercial concrete. Column dimensions, rebar minimums, and tie schedules verified against ACI 318-19 Chapter 10 (Columns), ACI 332-20 residential code, and ASTM A615 rebar specifications. Last reviewed May 2026.
— Common column questions
Quick answers.
How much concrete for a 12-inch Sonotube?
A 12-inch diameter Sonotube at 4 feet deep needs 0.12 cubic yards (3.14 cubic feet) of concrete — about 6 bags of 80-pound pre-mix or 7 bags of 60-pound. Multiply by the number of piers for the total order.
How many bags of concrete in a Sonotube?
An 8-inch diameter Sonotube needs about 1.4 cubic feet per foot of height — roughly 2.3 80-pound bags per foot. A 10-inch needs 2.2 cu ft per foot (3.7 bags). A 12-inch needs 3.14 cu ft per foot (5.2 bags). A 14-inch needs 4.3 cu ft per foot (7.1 bags).
How much rebar in a concrete column?
A typical residential deck pier uses 4 vertical No. 4 bars in a cage, with horizontal ties at 12–16 inches on centre. Structural columns supporting buildings use 4–8 vertical bars depending on size and load, with closer-spaced ties near the top and bottom.
What size Sonotube for a deck post?
Most residential decks use 10 or 12-inch diameter Sonotubes for deck posts, sized to spread the load onto soil below the frost line. For heavier decks, hot tubs, or carports, 14–16 inch diameter is common. Always check local code and the deck plans for required size.
How deep does a column need to be poured?
Below the local frost line and on undisturbed soil. Southern US: 12–18 inches. Northern US: 36–48 inches. UK: 750 mm for stable ground, deeper near trees on clay. Canada: 1.2 m in southern Ontario, 1.5–1.8 m across the Prairies. Australia: usually above frost depth except alpine zones, with site-class footings under AS 2870. The column itself can extend above ground; the concrete fill extends to required frost depth.
Do I leave the Sonotube on or strip it off?
For below-grade portions, leave it on — it doesn't matter and removing it risks disturbing soil. For above-grade portions (the visible part of a deck post or column), strip it off after 3–7 days of curing for a cleaner appearance and to allow proper inspection. Peel the cardboard away from top to bottom; if it sticks, score it lengthwise with a utility knife.