How this sonotube calculator works
A sonotube is a cylinder, so the concrete you need is circle area times depth. The calculator answers in the unit tube pours actually use — bags of concrete per tube — and shows the ready-mix equivalent for bigger sets:
bags per tube = V × (1 + waste%) ÷ bag yield, rounded up
Worked example: how much concrete for a 12 in sonotube
A 12 in tube 4 ft deep holds π × 0.5² × 4 = 3.14 ft³. With 10% waste that's 3.46 ft³ — at 0.60 ft³ per 80 lb bag, 6 bags per tube. The default four-tube deck job totals 24 bags (about $174), or 0.75 yd³ as ready-mix.
How many bags of concrete per sonotube?
Six bags of 80 lb concrete per 4 ft of 12 in tube is the number to remember — smaller tubes take far less, big piers far more. Engine-computed counts, waste included:
| Tube diameter | Volume at 4 ft | Bags — 3 ft deep | Bags — 4 ft deep |
|---|---|---|---|
| 8 in | 1.40 ft³ | 2 | 3 |
| 10 in | 2.18 ft³ | 3 | 4 |
| 12 in | 3.14 ft³ | 5 | 6 |
| 16 in | 5.59 ft³ | 8 | 11 |
| 18 in | 7.07 ft³ | 10 | 13 |
| 24 in | 12.57 ft³ | 18 | 24 |
How deep should sonotube footings go?
- Depth: below the local frost line and at least 12 in into undisturbed soil (IRC R403.1.4) — frost governs almost everywhere it freezes.
- Bearing: soft trench bottom? Bell the base or use a pre-formed footing form to spread the load before the tube goes in.
- The pour: brace the tube plumb, pour in lifts, rod each lift, and set your post base or anchor bolt while the top is wet — crowning the top sheds water.
Sonotube FAQ
How many bags of concrete per sonotube?
A 12 in tube takes about 6 bags of 80 lb concrete per 4 ft of depth (3.14 ft³ plus waste). An 8 in tube takes 3 bags per 4 ft; a 10 in tube takes 4. Deeper or wider climbs fast — the calculator recomputes per-tube and total counts as you type.
What sizes do sonotubes come in?
Common stock diameters are 8, 10, 12, 16, 18 and 24 in, in lengths of 4 and 12 ft that you cut to height. As a rule of thumb, the tube diameter should be about 3× the post width it will carry — a 4×4 post on a 12 in tube, a 6×6 on an 18 in.
How deep should sonotube deck footings go?
Below the local frost line, bearing on undisturbed soil — the IRC floor is 12 in below grade, but frost depth usually governs and can mean 36–48 in in cold climates. Widen the base with a bell or pre-formed footing form where soil is soft.
Can you mix concrete in the sonotube?
No — mix in a wheelbarrow or mixer, then pour in continuous lifts and rod or vibrate each lift to knock out air pockets. Dry-pouring into the tube leaves weak, honeycombed concrete at the bottom, exactly where the load is.
Is ready-mix worth it for a sonotube pour?
Usually not below about 1 yd³ — roughly 45 bags — because small ready-mix orders carry a short-load fee that erases the per-yard saving. The default four-tube job is 24 bags: a bag-and-mixer morning. A dozen big piers is a different story; price both lines above.
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