How much concrete does a Sonotube take?
A form tube is a cylinder, so its volume is π × radius² × depth. A 12″ tube has a 0.5 ft radius, so each foot of depth holds 3.1416 × 0.5² = 0.79 cu ft, or about 1.3 bags of 80 lb mix. Multiply by the depth and the number of tubes, then divide by what one bag makes.
| Tube | Cu ft per foot | 80 lb bags per foot | 60 lb bags per foot | 50 lb fast-set per foot |
|---|---|---|---|---|
| 6″ | 0.20 | 0.33 | 0.44 | 0.52 |
| 8″ | 0.35 | 0.58 | 0.78 | 0.93 |
| 10″ | 0.55 | 0.91 | 1.21 | 1.45 |
| 12″ | 0.79 | 1.31 | 1.75 | 2.09 |
| 14″ | 1.07 | 1.78 | 2.38 | 2.85 |
| 16″ | 1.40 | 2.33 | 3.10 | 3.72 |
| 18″ | 1.77 | 2.95 | 3.93 | 4.71 |
| 24″ | 3.14 | 5.24 | 6.98 | 8.38 |
The calculator rounds up the total for all tubes, not each tube, because leftover mix from one tube goes into the next.
How deep should footing tubes go?
The bottom of every footing must sit below your local frost line, or frost heave will lift it over the years. Most tubes also stick up a few inches above grade so the wood post stays dry. Count that part in the depth. Your deck plans or local building department set the exact depth and diameter for your load. The post depth calculator has a frost-depth guide by region.
What size Sonotube do I need?
Diameter depends on how much weight each footing carries and on your soil, so it's set by your deck plans or local code, not by a rule of thumb. Larger decks and second-story decks need wider footings or a flared base at the bottom. This calculator covers straight tubes only, so add the base volume separately if you use one.
Bags or ready-mix?
Bags are practical for a typical deck with 6–12 footings. Above about a cubic yard (45 bags of 80 lb mix), ask about ready-mix. A truck with a chute or a small pump can fill every tube in one go.
Related calculators
Pouring a patio or pad too? Try the concrete slab calculator. Setting fence posts instead? Use the fence post concrete calculator.