Fence Post Concrete Calculator
Determine exact concrete volume per post hole, total bags needed, and waste allowance — with a live cross-section that updates as you type.
Project inputs
Cross-section preview
Live scale — updates with your inputs.
Concrete required
Bags needed
Rounded up to whole bags
0
Volume per hole is calculated from the annular space between the post and the hole wall. Waste allowance accounts for spillage, uneven holes, and settling.
Hole area
0 in²
Concrete / post
0 ft³
Total bags (no waste)
0
How the fence post concrete calculator works
When setting a fence post in concrete, the hole must be wider than the post to allow a concrete collar. The volume of concrete required is the volume of the hole minus the volume of the portion of the post that sits inside the hole. Our calculator uses the annulus (ring) formula: V = π × (Rhole² − Rpost²) × depth, then multiplies by the number of posts and adds your selected waste allowance.
Hole dimensions & post size
Hole diameter
Typically 3× the post width. A 4×4 post needs a 12″ diameter hole; a 6×6 post needs an 18″ hole. Larger holes require more concrete but provide better frost heave resistance.
Hole depth
Rule of thumb: 1/3 of the post's above-ground height, minimum 24″. In cold climates, extend below the frost line — often 36″ or more. Deeper holes need more concrete.
Post dimensions
Standard fence posts are 4×4, 5×5, or 6×6 inches. The post displaces concrete, so a larger post means less concrete per hole.
Post depth in hole
How far the post extends into the hole. This is usually the full hole depth minus a few inches of gravel at the bottom for drainage.
Concrete volume, bag yield & waste
Volume per hole
Measured in cubic feet (ft³) for imperial or cubic meters (m³) for metric. Our calculator shows ft³ by default. The annulus volume is the key number — not the full hole volume.
Bag yield
A standard 80 lb bag of concrete mix yields about 0.60 ft³. 60 lb bags yield ~0.45 ft³, and 40 lb bags yield ~0.30 ft³. Always check your bag's label — fast-setting mixes and high-strength mixes can vary.
Waste allowance
Real-world holes are never perfectly cylindrical. Add 10–15% for spillage, uneven excavation, and settling. For rocky or irregular soil, go up to 20–25%.
Real-world example: 10-post privacy fence
Suppose you're installing 10 posts, each 4×4 inches, into holes 12″ diameter × 30″ deep. The post sits 24″ into the hole (6″ of gravel at the bottom). With an 80 lb bag yielding 0.6 ft³ and a 10% waste allowance:
- Hole volume: π × (6″)² × 30″ = 3,393 in³ = 1.96 ft³
- Post volume: 4″ × 4″ × 24″ = 384 in³ = 0.22 ft³
- Concrete per hole: 1.96 − 0.22 = 1.74 ft³
- Total (10 posts): 17.4 ft³ → with 10% waste: 19.14 ft³
- Bags needed: 19.14 ÷ 0.6 = 31.9 → 32 bags
Excess: 32 × 0.6 − 19.14 = 0.06 ft³ — negligible.