Concrete
How to Calculate Concrete for Fence Posts
Calculate how much concrete you need per fence post, including hole size, post displacement and gravel base, with examples for 4×4 and 6×6 posts.
3 min read
- Dual US / Metric
- +5% Waste Allowance
- Free · Runs in Your Browser
Fence Post Concrete Calculator
Setting posts in concrete is one of the most common DIY concrete jobs. This calculator subtracts the volume of the post and any gravel base from each hole, then tells you how many bags you need in total and per post.
Enter your measurements
Enter your measurements and press Calculate to see quantities, a material breakdown and an optional cost estimate.
How the calculation works
- Hole volume = π × (Hole diameter ÷ 2)² × (Hole depth − Gravel depth)
- Post volume = Post width × Post thickness × (Hole depth − Gravel depth)
- Concrete per hole = Hole volume − Post volume
The post displaces concrete, so subtracting it gives a more accurate bag count, especially for large posts in narrow holes.
Assumptions used
- Holes are cylindrical with straight sides. Hand-dug holes are often wider at the top, which is why a small waste allowance is included.
- Concrete fills the hole to ground level.
Example calculation
10 posts, each a 4×4 (3.5 × 3.5 in actual) in a 10 in diameter hole 24 in deep, no gravel base, no waste.
- 1Hole: 3.1416 × 5² × 24 = 1,885 in³
- 2Post: 3.5 × 3.5 × 24 = 294 in³
- 3Concrete per hole: 1,591 in³ = 0.92 ft³
- 460 lb bags (0.45 ft³): 0.92 ÷ 0.45 = 2.05 bags per hole
- 5All 10 posts: 9.2 ft³ ÷ 0.45 = 20.5 → 21 bags
Result: 21 bags if you share leftover concrete between holes, or 30 bags if you mix whole bags hole by hole (3 per post).
How to measure your project
- Use the actual post size, not the nominal size: a 4×4 is 3.5 × 3.5 in and a 6×6 is 5.5 × 5.5 in.
- A hole roughly three times the post width is a common guideline, e.g. 10–12 in for a 4×4.
- Hole depth is commonly one-third to one-half of the above-ground post height, and below the frost line in cold regions.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole 2 ft deep, about two 60 lb bags or 1.5 80 lb bags. Larger holes and deeper posts need more, so use the calculator with your sizes.
Should I put gravel under fence posts?
Many installers place a few inches of gravel at the bottom of the hole for drainage, which helps reduce rot at the base of wooden posts. Enter that depth in the gravel field.
Can I use fast-setting concrete for posts?
Yes. Fast-setting mixes are popular for posts and are often poured dry and then soaked with water. Their yield can differ from standard mixes, so check the bag.
Related guides
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Understand concrete bag yields, convert volume into bags, and compare 40, 60 and 80 lb bags (and 20–30 kg bags) with worked examples and a quick reference table.
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Results are estimates based on the measurements and assumptions you enter. Always confirm product specifications, local building codes and final quantities with your supplier or a qualified professional. See our disclaimer.