What a sonotube is for
A 'Sonotube' (the brand that became the generic word) is a heavy cardboard cylinder set in the ground as a form, so a deck footing or column comes out round, smooth and plumb. Typical deck footings are 10–12 inch tubes sized to the beam load, poured down to the frost line with a footing pad or flared base at the bottom.
Size the tube to the load, not the hole
The tube diameter follows the load: a typical residential deck footing for one post is 10–12 inches; a ground-level porch post may only need 8. Your local deck code table (based on tributary area and soil bearing) is the authority — the tube should also be a couple inches wider than the post base hardware so anchors have room.
Pouring tips
Wet the tube inside so it does not steal water from the mix, pour in lifts and rod each lift to push out air, and set the anchor bolt while green. Leave the tube on through curing — stripping it too early in freeze country exposes green concrete.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
Figures, formulas and reference tables last verified: 19 September 2026.