Sonotube Calculator — Concrete Bags per Tube
Filling round tube forms for deck footings, fence posts, or piers? Enter the tube diameter, depth, and how many tubes, and this calculator gives you the concrete volume and bag count.
How to use this calculator
- Pick the tube diameter — 10 inches is common for deck footings, 8 inches for lighter loads.
- Enter how deep the concrete will fill each tube (often down to the local frost line).
- Enter the number of tubes (footings) in your project.
- Choose your bag size; the calculator shows bags per tube and the total.
The formula
A tube form is a cylinder, so we use the cylinder volume.
Volume per tube (cubic feet) = π × (diameter ÷ 2 ÷ 12)² × depth (ft). Dividing by 12 converts the diameter from inches to feet.
Total volume = volume per tube × number of tubes. Bags = volume ÷ bag yield, rounded up (80 lb ≈ 0.6 cu ft).
Worked example
Six 10 in tubes, each filled 4 ft deep, using 80 lb bags.
Radius = 10 ÷ 2 ÷ 12 = 0.417 ftPer tube = π × 0.417² × 4 = 2.18 cubic feetTotal = 2.18 × 6 = 13.1 cubic feetBags = 13.1 ÷ 0.6 = 21.8 → 22 bags (4 per tube)
Six 10-inch footings 4 ft deep need about 22 bags of 80 lb concrete (4 per tube).
Frequently asked questions
How many bags of concrete per Sonotube?
A 10-inch tube filled 4 ft deep holds about 2.2 cubic feet — roughly 4 bags of 80 lb concrete. An 8-inch tube at the same depth needs about 3 bags.
How deep should deck footings be?
Footings should extend below the local frost line so they do not heave — commonly 36–48 inches in cold climates, less in warm ones. Check your local building code.
What diameter tube for a deck footing?
Sizing depends on the load and soil, but 10–12 inch tubes are common for residential decks. Larger spans and beams may need bigger footings or a flared base.