How Footing Volume Is Calculated
Round holes and sonotubes use the cylinder formula: π × radius² × depth. Rectangular footings are simply length × width × depth. The calculator works per hole, multiplies by your count, and converts to cubic yards and standard bag counts (60 lb bags yield 0.45 ft³, 80 lb bags 0.60 ft³). Results include no waste factor — for augered holes it's common to add one extra bag per 4–6 holes to cover irregular sides.
Quick Reference: Bags per Round Hole
| Hole size (dia × depth) | Volume | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 8″ × 24″ | 0.70 ft³ | 2 | 2 |
| 10″ × 30″ | 1.36 ft³ | 4 | 3 |
| 12″ × 36″ | 2.36 ft³ | 6 | 4 |
| 16″ × 42″ | 4.89 ft³ | 11 | 9 |
Depth is usually dictated by your local frost line, not by preference — footings must sit below it so freeze-thaw cycles can't heave the structure. Frost depth ranges from a few inches in the South to 4+ feet in the northern U.S., so check with your local building department before digging. Post spacing and diameter for decks are code items too; when in doubt, a 12″ sonotube at frost depth is the common residential deck default.
Tip: Bell-bottomed or hand-dug holes are wider at the base than the top. Measure the widest diameter — or add roughly 10–15% — or you will run short on the last hole.