Working out concrete for round footings
A Sonotube footing is just a cylinder, so the volume is π × radius² × depth. Working in inches, a 10 inch tube 48 inches deep holds 3.14 × 5² × 48 = about 3,770 cubic inches. Divide by 1,728 to get cubic feet (roughly 2.2 ft³ per footing) and by 27 more to reach cubic yards. Bags are then a simple division by the yield printed on the bag: an 80 lb bag of standard concrete mix makes about 0.60 ft³, a 60 lb bag about 0.45 ft³ and a 50 lb bag about 0.375 ft³. Notice how fast this adds up — six footings at that size is roughly 22 bags, close to half a ton of material to carry and mix by hand.
Two things decide whether the footing actually works. Depth must take the bottom below the local frost line, which ranges from almost nothing in the far south to 48 inches or more in cold climates, because a footing sitting in soil that freezes will be lifted every winter and will take your deck with it. Diameter is set by the load: bigger tubes spread the weight over more soil, and the required size is usually specified in your deck plan or local code, not guessed. Once you are past about 1 cubic yard total — roughly 40 bags — price out a ready-mix delivery or a trailer mixer before committing to bagged concrete, since mixing that volume by hand is a long, hard day and the pours will not be consistent.
Bag yields vary between products and brands; confirm the figure on the label. Footing size and depth should follow local code or your permit drawings.