An 8-inch tube holds about 0.35 cubic feet per foot of depth. At a typical 4-foot depth that's 1.4 cubic feet (0.05 yd³) per tube, roughly 3 eighty-pound bags each once you add 10% waste. The calculator above is pre-set to a 8″ tube; change the depth and tube count to match your job.
Common questions
How many bags of concrete per 8" sonotube?
Figure about 0.6 eighty-pound bags per foot of depth, waste included. A 4-foot 8-inch tube takes roughly 3 bags. Round up. Running short mid-pier is the worst place to run short.
How much does an 8" sonotube hold per foot?
About 0.35 cubic feet per foot of depth (π × r² with a 8-inch diameter). Multiply by your depth in feet, then by the number of tubes.
How deep should the tube go?
Below your local frost line, on undisturbed soil. That's a code and climate question, not a calculator question. Typical frost depths run from 12 inches in the South to 48+ in the North.