How to calculate it yourself
Volume in ft³ = π × (radius in ft)² × depth in ft. A 60 lb bag yields 0.45 ft³; an 80 lb bag yields 0.6 ft³. Bags needed = volume ÷ yield, rounded up. Always order one extra bag per 4 posts for spillage.
Common scenarios
6-foot wood privacy fence (10×30 hole)
Each post takes 1.5 ft³ of concrete = 2 bags of 80 lb. A 100-foot fence with 8-ft post spacing needs 13 posts × 2 bags = 26 bags. Add 2-3 spare bags for the gate posts and any over-dig.
Chain-link fence (8×24 hole)
Lighter fences with smaller posts: 0.66 ft³ each = 1.5 bags of 80 lb. The terminal/gate posts get the heavier 10×36 spec — 2.5 bags each.
Frost-zone fence (12×48 hole)
In northern climates the hole must extend below frost line. Each post takes 3.4 ft³ = 5 bags of 80 lb. This is the project where ready-mix delivery may be cheaper than bagged for a long fence run.