How to Calculate Concrete for Fence Posts
Hole volume minus post volume, multiplied by the number of posts
The formula: hole volume minus post volume
A fence-post hole is typically a cylinder — π × radius² × depth. But the post itself occupies part of that cylinder and displaces concrete, so the actual concrete volume per hole is the hole's volume minus the post's own volume (also calculated as a cylinder or a rectangular prism, depending on whether the post is round or square).
Standard hole sizing
A common rule of thumb is a hole diameter roughly 3 times the post width and a depth of about ⅓ to ½ the post's above-ground height (deeper in frost-prone regions, to get below the frost line). Local building codes and frost-depth requirements vary significantly by region — check your local code rather than relying on a single national rule.
Multiply by the number of posts
Once you have the per-hole volume, multiply by the total number of posts in the fence run to get the total concrete needed. Corner and gate posts are sometimes set in larger holes than line posts — calculate those separately if their dimensions differ.
Worked Example
A 10-inch diameter hole, 2 ft deep, with a 4×4 in (nominal) post, for 8 posts.
Hole volume: π × (5 in)² × 24 in ≈ 1,885 in³
Post volume (using actual ~3.5×3.5 in): 3.5 × 3.5 × 24 = 294 in³
Concrete per hole: 1,885 − 294 = 1,591 in³ ≈ 0.034 yd³
Total for 8 posts: 0.034 × 8 ≈ 0.27 yd³
Assumptions & Limitations
- Hole diameter and depth follow a common rule of thumb — always confirm against local frost-depth and code requirements.
- Post volume uses actual (not nominal) dimensions, since a nominal 4×4 is slightly smaller in practice.
Related Calculators
Related Guides
Sources
NIST SP 811
Unit conversion reference (cubic feet, cubic yards, cubic metres).
NIST SP 811 — reference