How this calc works

This is one of the calcs people get wrong most often, and it is worth understanding why. A post hole is a cylinder, not a box. The volume is pi times the radius squared times the depth — not length times width times depth. The calc takes your hole diameter in inches, halves it and converts to feet for the radius, then runs the cylinder formula and converts to cubic yards (plus 10% waste and a bag-count estimate so you can decide truck-or-bags).

For a fence line, multiply one hole by your number of posts. The app does the whole run at once; here, run one hole and do the multiplication, or check the related deck-post calc.

Common mistakes

The classic mistake is using square-box volume for a round hole, which overshoots by about 21% — you buy a quarter more concrete than you need. Most generic calculators do exactly this. This one uses the true cylinder area.

The second is forgetting the post itself displaces concrete. For a chunky 6x6, subtract its volume from the hole; for a metal or 4x4 post the difference is small enough to leave in as cushion.

Code & regional notes

Hole depth is driven by your frost line, not preference. Cold-climate codes commonly require footings below the frost depth (often 36-48 inches in northern states) so the post does not heave in winter. Call your local building office for the frost depth before you dig.

RELATED FORMULAS