Sonotube Calculator
Figure concrete for round form tubes. Enter tube diameter in inches, height in feet, how many tubes you are pouring, and a waste factor to get total cubic yards, cubic feet, and how many 80 lb or 60 lb bags to buy.
Example: with Tube diameter (inches) 12 · Tube height (feet) 4 · Number of tubes 4 · Waste factor (%) 10 → Concrete needed: 0.51 cu yd.
- Cubic feet13.82 cu ft total (3.14 per tube before waste)
- 80 lb bags24 bags
- 60 lb bags31 bags
Computed by the calculator below using its default values. Change any input to see your own numbers.
Volume per tube is πr²h. One 80 lb bag of concrete mix yields 0.60 cu ft; a 12-inch tube uses one 80 lb bag per 7 inches of height, roughly.
How much concrete a form tube takes
A form tube (Sonotube is the common brand) is just a cylinder, so its volume is πr²h. The catch is units: diameter is sold in inches while height is measured in feet, and mixing them up throws the answer off by a factor of 144. This calculator converts for you — a 12-inch tube holds 0.785 cubic feet per foot of height, so a typical 4-foot deck footing takes 3.14 cubic feet, about six 80 lb bags with a little waste.
Remember to measure the full tube length, not just what shows above grade. A footing that extends 12 inches above grade but sits 42 inches deep to get below frost line is a 54-inch pour. Frost depth varies by region — check your local building department.
Bags or ready-mix?
Bagged mix makes sense up to roughly a cubic yard — that is 45 or so 80 lb bags, which is real labor to mix. Past that, short-load ready-mix usually wins on both effort and consistency. The bag counts here use the standard published yields: 0.60 cubic feet per 80 lb bag and 0.45 cubic feet per 60 lb bag. Counts round up to whole bags, and the default 10% waste covers spillage, over-excavation at the bottom of the hole, and slightly bellied tubes.
How it’s calculated
Volume per tube = π × (diameter ÷ 24)² × height, with diameter in inches and height in feet, giving cubic feet. Total = per-tube volume × quantity × (1 + waste%). Cubic yards = cubic feet ÷ 27. Bag counts round up using standard yields: 80 lb bag = 0.60 cu ft, 60 lb bag = 0.45 cu ft (manufacturer-published yields for concrete mix).
Assumes a straight cylinder — belled bases, oversized augured holes, and rocky over-dig all add concrete beyond the tube volume, which is what the waste factor is for.
Concrete per foot of tube height
| Tube diameter | Cu ft per ft of height | 80 lb bags per ft |
|---|---|---|
| 8 in | 0.35 | 0.6 |
| 10 in | 0.55 | 0.9 |
| 12 in | 0.79 | 1.3 |
| 16 in | 1.40 | 2.3 |
| 24 in | 3.14 | 5.2 |
Computed with π(d/24)² per foot and 0.60 cu ft per 80 lb bag; rounded.
Common mistakes
- Entering diameter in feet — a '1' in the diameter box should be 12 (inches), or the answer comes out 144× too small.
- Measuring only the above-grade height and forgetting the below-frost-line depth, which is usually most of the pour.
- Buying exact bag counts with zero waste — an under-mixed last footing means a cold joint or a second store run.
- Ignoring the belled or over-augured bottom of the hole, which can swallow half a bag per footing on its own.
Frequently asked questions
What is the formula for sonotube concrete volume?
Volume = π × r² × h. With diameter d in inches and height h in feet, cubic feet = π × (d ÷ 24)² × h. A 12-inch tube 4 feet tall is π × 0.5² × 4 = 3.14 cubic feet.
How many bags of concrete fill a 12-inch sonotube 4 feet deep?
About 6 bags of 80 lb mix. The tube holds 3.14 cubic feet, an 80 lb bag yields 0.60 cubic feet, and 3.14 × 1.1 waste ÷ 0.60 rounds up to 6 bags.
When should I order ready-mix instead of bags?
Around 1 cubic yard and up. That is roughly 45 bags of 80 lb mix — mixing that by hand or in a small mixer is hours of work, and pouring footings in stages risks cold joints.
Does rebar in the tube change how much concrete I need?
Not meaningfully. Two or three vertical #4 bars displace well under 1% of the tube volume, far less than normal spillage. Keep the waste factor and ignore the rebar displacement.