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
The per-tube figure is what you batch by; the total drives the ready-mix decision. At the defaults — four 12 in tubes 4 ft deep — that’s 6 bags per tube and 24 bags total, or 0.75 yd³ as ready-mix; the math is in the pour math section below.
The parts of a tube-form pier
The tube is just the form — the pier it shapes has a few parts that carry the load and tie it to what’s above:
The tube form
A waxed cardboard cylinder braced plumb in the hole; it holds the wet concrete round and peels away (or stays buried) after cure. Diameter sets the column size; you cut the length to the hole depth plus a few inches proud of grade.
The footing base
Soft soil wants more bearing than a bare tube gives, so the base is belled out or set on a pre-formed plastic footing form — a wide foot under the round leg. That’s extra concrete this calculator doesn’t count; add it as a pad in the footing calculator.
Rebar
Vertical bars in the tube tie the pier together and resist uplift, often hooked into the base. What’s required is a local-code and load question, not a volume one.
Post base or anchor bolt
Set while the top is still wet: a galvanized post base to hold a 4×4 or 6×6 off the wet ground, or an anchor bolt for a column. Crown the top so water sheds off the pier instead of pooling at the post.
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 to size and pour tube piers
Diameter, depth and count are the whole order — the pour is where the care goes.
- Pick the diameter — roughly 3× the post width — and set the depth below the local frost line, at least 12 in into undisturbed soil.
- Count the tubes into the count field; the calculator gives per-tube bags and the ready-mix total together.
- Bell or foot the base where soil is soft, and add that concrete separately — the tube volume alone assumes a straight cylinder.
- Brace each tube plumb and pour in continuous lifts, rodding or vibrating each lift to drive out air pockets.
- Set the post base or anchor while wet, crown the top, and leave it to cure before loading — vehicles and structure off it for the full window.
Worked example: concrete for a 12 in sonotube
The default, worked line by line — a 12 in tube 4 ft deep, ×4 tubes, 10% waste, 80 lb bags:
waste = 3.14 × 1.10 = 3.46 ft³
bags/tube = 3.46 ÷ 0.60 = 5.76 → 6 bags
four tubes = 4 × 3.14 = 12.57 ft³ → 24 bags total
ready-mix = (12.57 × 1.10) ÷ 27 = 0.51 → order 0.75 yd³
Notice bags round per tube, not on the total: 5.76 becomes 6 at every tube, so four tubes buy 24 bags even though the raw total is 23. Batching a whole bag per hole is also just easier at the mixer — round up and pour full.
What this sonotube estimate doesn’t include
- The tube forms. A few dollars a foot by diameter — the cardboard itself, cut to length.
- Base footing concrete. The bell or pre-formed footing form under the tube, added separately.
- Rebar and post bases. Vertical steel, anchor bolts and galvanized post saddles.
- Digging. Post-hole auger or hand digging to frost depth, plus spoil haul-off.
- Bracing lumber. Stakes and scraps to hold every tube plumb during the pour.
- Inspection. Deck and structural piers are commonly inspected before pour — open holes.
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.
What size sonotube for a 6x6 post?
A 10–12 in tube carries a 6×6 deck post structurally, but many builders step up to a wider tube — or bell the base — so the footing area, not the concrete column, meets the frost-depth bearing the load needs. The soil under the pier decides the diameter as much as the post on top.
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.
Sources and standards
- IRC R403.1.4: footing and pier depth — the 12 in bearing and frost-line requirement.
- IRC R507 (decks): pier and footing sizing for the deck loads most tube piers carry.
- Cylinder geometry: volume is π r² h exactly — the table is computed, not tabulated.
- Published bag yields: 0.60 ft³ per 80 lb bag, the maker’s figure behind the per-tube counts.
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