Post Hole Concrete Calculator
Estimate how much concrete fence post holes need. Enter the hole size and post shape, and the tool subtracts the embedded post volume to count only the concrete that fills the gap.
Results
Enter your measurements to see an estimate.
Calculated: — EasyBuildTools, www.easybuildtools.com
How to use the Post Hole Concrete Calculator
Enter the number of holes, the hole diameter and depth in inches, and the post shape. For square posts enter the post width; for rectangular posts enter width and thickness; for round posts enter the diameter. Set the embedded depth (it defaults to the full hole depth), a waste allowance, and a bag size. The tool subtracts the embedded post volume from the hole volume so you count only the concrete that fills the gap.
Formula and methodology
- Hole volume (cylindrical) = π × (hole diameter ÷ 2)² × hole depth, with all dimensions converted to feet, in cubic feet.
- Square post volume = post width × post width × embedded depth (inches converted to cubic feet by dividing by 1728).
- Rectangular post volume = post width × post thickness × embedded depth.
- Round post volume = π × (post diameter ÷ 2)² × embedded depth.
- Concrete per hole = hole volume − embedded post volume (never below zero).
- Total = concrete per hole × number of holes × (1 + waste% / 100). Cubic yards = total cubic feet ÷ 27.
Bag counts use approximate yields: a 40 lb bag ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, and 80 lb ≈ 0.60. These are planning figures — always confirm the yield printed on the bags you buy.
Worked example
Line of posts: 10 holes, 12 in diameter, 24 in deep, 4×4 square posts embedded the full 24 in, 10% waste, 60 lb bags. Hole volume = π × (0.5 ft)² × 2 ft = 1.571 cu ft. Post volume = (4 × 4 × 24) ÷ 1728 = 0.222 cu ft. Concrete per hole = 1.349 cu ft. Total = 1.349 × 10 × 1.10 = 14.84 cu ft = 0.55 cu yd. At 0.45 cu ft per 60 lb bag: 14.84 ÷ 0.45 = 33 bags. Enter the values above to confirm.
Practical post-setting tips
- Dig holes wider than the post — a 12-inch hole for a 4-inch post leaves room for concrete and a clean pour.
- Add a few inches of gravel at the bottom for drainage before setting the post.
- In frost-prone areas, dig to the local frost line so heaving does not lift the posts.
- Keep a waste allowance: hole walls are rarely perfectly straight and some concrete stays on the shovel and in the mixer.
- For the full fence material list — posts, panels, rails — pair this with the Fence Calculator, and read How Much Concrete Do I Need for Fence Posts? and How to Calculate Concrete for a Post Hole.