Sonotube Calculator
Calculate concrete for Sonotube round forms: pick the tube diameter and depth, tell it how many footings, and get cubic yards plus 80 lb and 60 lb bag counts — the standard way to size deck footings and post piers.
Frost depth + 2–6″ above grade
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You might also need:Concrete Calculator·Rebar Calculator
Tube volume math
80 lb bags = volume ÷ 0.60 cu ft yield per bag
Six 12-inch tubes at 4 feet deep hold 18.8 cubic feet of concrete — 35 bags of 80 lb mix with 10% waste, or 0.77 cubic yards if you'd rather order a small ready-mix load. Past about 30 bags, pricing a u-cart or short load is worth the phone call.
Bags per foot of depth by tube size
| Tube diameter | Volume per ft | 80 lb bags per ft | Typical use |
|---|---|---|---|
| 8″ | 0.35 cu ft | 0.6 | Fence posts, mailboxes |
| 10″ | 0.55 cu ft | 0.9 | Light deck posts |
| 12″ | 0.79 cu ft | 1.3 | 4×4 deck posts |
| 16″ | 1.40 cu ft | 2.3 | 6×6 posts, pergolas |
| 18″ | 1.77 cu ft | 2.9 | Heavy decks |
| 24″ | 3.14 cu ft | 5.2 | Piers, additions |
Pour tips
- Rod as you fill: plunge a rebar rod every 6 inches of fill to release trapped air pockets.
- Reinforce tall piers: 1–2 vertical rebar in tubes over 3 ft — size it with the rebar calculator.
- Gravel base: 4–6 inches of compacted gravel under the tube improves drainage — the gravel calculator covers it.
- Pouring a slab too? Combine loads with the concrete calculator to clear ready-mix minimums.
Frequently asked questions
How many bags of concrete fill a Sonotube?+
A 12-inch tube takes about 1.3 × 80 lb bags per foot of depth; a 10-inch tube 0.9 bags per foot; an 8-inch tube 0.6 bags per foot. A typical 12 inch × 4 ft deck footing needs six 80 lb bags.
What size Sonotube do I need for a deck post?+
Rule of thumb: tube diameter three times the post width — 12 inches for a 4×4 post, 16–18 inches for a 6×6. Heavier decks and cold climates may need a wider bell footing at the base; check your local code and frost depth.
How deep should Sonotube footings go?+
Below your local frost line — 12 inches in the warm South, 36–48 inches across the northern US and Canada. The footing should also bear on undisturbed soil, with the tube extending 2–6 inches above grade to keep the post dry.
Can I mix concrete in the hole instead of using a form?+
For fence posts, pouring dry mix around the post and wetting it works. For structural deck footings, use the tube: it gives a consistent cross-section, keeps soil out of the mix, and produces the clean above-grade pier code inspectors want to see.
How long before I can build on a Sonotube footing?+
Wait 24–48 hours before light framing loads and 7 days before full deck loads. Set post anchors into the wet concrete at pour time — drilling later is weaker and harder to align.