Post Hole Concrete Formula
The amount of concrete needed for a fence post hole depends on three measurements: the hole diameter, the hole depth, and the cross-sectional area of the post itself (which displaces some concrete). The formula is:
Hole area (sq ft) = Ο Γ (hole radius in ft)Β²
Post area (sq ft) = post widthΒ² (square post) or Ο Γ (post radius in ft)Β² (round post)
Concrete volume (cu ft) = (hole area β post area) Γ depth (ft)
Bags needed = β total cu ft Γ· bag yield β
Bag yields: 50 lb = 0.375 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft. Always round up β partial bags still count as a full purchase. Order 10% extra for spillage and uneven holes.
Quick Reference: Bags Needed by Post Count
Assuming a 10β³ diameter hole, 3 ft depth, 4Γ4 post, and 80 lb bags (0.60 cu ft each):
| Posts | Total Cu Ft | 80 lb Bags | +10% Buffer |
|---|---|---|---|
| 5 posts | 3.2 cu ft | 6 bags | 7 bags |
| 10 posts | 6.3 cu ft | 11 bags | 12 bags |
| 15 posts | 9.5 cu ft | 16 bags | 18 bags |
| 20 posts | 12.6 cu ft | 21 bags | 24 bags |
| 25 posts | 15.8 cu ft | 27 bags | 30 bags |
Note: actual bag counts shown in table are calculated independently and may differ slightly from the calculator above due to the 10β³ hole / 4Γ4 post assumption. Use the calculator for your exact inputs.
Frequently Asked Questions
How deep should fence posts be set?+
The standard rule is to bury 1/3 of the total post length. For a 6-foot fence with 8-foot posts, set 2β2.5 feet deep. In cold climates, dig below the local frost line (often 36β48 inches in northern states) to prevent heaving. A minimum of 2 feet is recommended regardless of post height.
What mix of concrete is best for fence posts?+
Fast-setting concrete (such as Quikrete Fast-Setting or Sakrete Fast-Setting) is the most popular choice. Pour dry mix directly into the hole and add water β no mixing needed. It sets in 20β40 minutes, letting you continue work the same day. Standard 80 lb bags are the most economical option per cubic foot.
Fast-set vs regular concrete for fence posts β which is better?+
Fast-setting concrete sets in 20β40 minutes and can be poured dry β ideal for fence posts since you don't need a mixer. Regular concrete takes 24β48 hours to cure but is slightly stronger (5,000 psi vs 4,000 psi). For typical residential fences, fast-set is the practical choice. Use regular concrete for heavy gates or structural posts bearing significant load.
How long should I wait before attaching fence panels to new posts?+
With fast-setting concrete, posts are stable enough for light work within 4 hours and ready for fence panel installation after 24 hours. For regular concrete, wait at least 24β48 hours before attaching panels and 7 days for full working strength. Avoid applying significant lateral force for at least 28 days.