How to Use This Calculator
Start by selecting your post shape—either square/rectangular (cuboid) or cylindrical (round). Enter the post's width and thickness for rectangular posts, or diameter for round posts. Next, specify the depth to which you'll bury the post in the ground.
If you're setting multiple posts, enter the total quantity. The calculator will compute the concrete needed per post and multiply by the number of posts to give your total volume. Results display in cubic yards or cubic meters depending on your unit preference.
Once you have the concrete volume, use a cement calculator to determine how many bags of cement, sand, and gravel you'll require.
Concrete Volume Formulas
Concrete fills the annulus (ring-shaped space) between the post and the hole wall. The calculation subtracts the post's cross-sectional volume from the hole's larger cross-section, then multiplies by burial depth.
For round posts:
Concrete = (π/4 × (3d)² − π/4 × d²) × h
For rectangular posts:
Concrete = (3w × 3t − w × t) × h
Total = Single post volume × Number of posts
d— Post diameter (round posts only)w— Post width (rectangular posts only)t— Post thickness (rectangular posts only)h— Depth of post burial in groundπ— Mathematical constant, approximately 3.14159
Practical Considerations for Post Hole Concrete
Avoid common mistakes when calculating and pouring concrete for fence posts.
- Burial depth should be at least one-third of post height — A 6-foot post needs at least 2 feet underground. Shallow-set posts are prone to leaning or shifting in freeze–thaw cycles. Check local frost lines; northern regions may require deeper burial.
- Oversized holes provide stability and drainage — The standard rule is to make the hole three times the width of the post in each direction. This creates space for concrete consolidation and allows water drainage around the post base, preventing rot.
- Dry concrete works but requires moisture — Pre-mixed dry concrete can be placed directly in the hole and will hydrate from groundwater. However, monitor the hole for the first week—if it doesn't rain, water it lightly to ensure proper curing.
- Account for settling and top-off — Concrete may settle 10–15% as it cures. Plan to overfill slightly and level the surface after curing to prevent water pooling around the post collar.
Common Post and Hole Size Combinations
For a standard 4×4 inch post, a 12×12 inch hole at 3 feet depth requires approximately 0.099 cubic yards (about 240 lbs) of concrete—roughly three 80 lb bags or two 94 lb bags plus a small bag.
For 6×6 posts, expect roughly double that volume. Round posts 4 inches in diameter buried 3 feet deep need about 0.08 cubic yards. Always verify local building codes; some regions require additional concrete depth or reinforcement in frost-prone areas.
When estimating material costs, account for waste and compaction variability. Most professionals order 10–15% extra to avoid running short mid-project.