Volume of a round column
The single most common mistake is squaring the diameter instead of the radius, which overshoots by four times. Halve the diameter first, then square. Keep every dimension in the same unit — work in feet and the answer is cubic feet, which you divide by 27 to get cubic yards.
Concrete per foot of depth, by tube size
| Tube diameter | Per foot of depth | 4 ft deep | 80 lb bags (4 ft) |
|---|---|---|---|
| 8 in | 0.35 ft³ | 1.40 ft³ | ~3 |
| 10 in | 0.55 ft³ | 2.18 ft³ | ~4 |
| 12 in | 0.79 ft³ | 3.14 ft³ | ~6 |
| 16 in | 1.40 ft³ | 5.59 ft³ | ~10 |
| 18 in | 1.77 ft³ | 7.07 ft³ | ~12 |
27 cubic feet make one cubic yard. Bag counts assume an 80 lb bag yields about 0.6 ft³; a 60 lb bag yields about 0.45 ft³, so multiply the bag count by roughly 1.3.
Before you order
- Add a waste margin. Spillage, an uneven bottom and a slightly bulged tube mean real usage runs 3 to 5 percent over the math. Round up, and buy one spare bag.
- Bags vs ready-mix. Bagged mix is fine up to roughly half a cubic yard total. Past that, ordering ready-mix by the yard is usually cheaper and faster than opening dozens of bags.
- Weight, if you care. Concrete is about 150 lb per cubic foot (2,400 kg/m³), so a 12-inch pier 4 feet deep weighs near 470 lb once cured.
Common questions
How much concrete does a 12-inch Sonotube need per foot?
A 12-inch (1-foot) diameter tube holds about 0.785 cubic feet of concrete per foot of depth. A 4-foot deep footing is about 3.14 cubic feet, or 0.116 cubic yards.
How many bags of concrete fill a Sonotube?
Divide the tube volume in cubic feet by the yield of one bag: an 80 lb bag makes about 0.6 cubic foot, a 60 lb bag about 0.45. A 12-inch tube 4 feet deep (3.14 cubic feet) takes about six 80 lb bags. Buy one extra.
Should I use the tube size or measure it?
Use the labeled size for the diameter. Sonotube sizes are the inside diameter, so the finished pier matches the number on the tube. Just confirm the pour depth, since that is what you actually control on site.
