How much concrete for fence posts?
Estimate concrete for fence posts from post-hole count, hole diameter, hole depth, mixed hole sizes, waste, bag size, and bag yield.
Reviewed - May 19, 2026Short answer
For each fence post hole, calculate cylindrical volume from hole diameter and depth, multiply by the number of holes, add waste, then divide by the concrete bag yield.
An 18-post fence with 10 in diameter by 30 in deep holes needs about 27.0 cu ft of concrete with 10% waste, or 45 standard 80 lb bags in the concrete calculator.
If gate, corner, end, or terminal posts need wider or deeper holes than line posts, calculate those hole groups separately, add the raw cubic feet, then apply waste and bag rounding once.
Fence post concrete method
- Count the post holes that will actually receive concrete, including line posts, corners, end posts, and gate posts.
- Calculate larger gate, corner, or terminal holes as their own group when they differ from ordinary line-post holes.
- For each hole, convert diameter and depth to feet and calculate cylinder volume: pi x radius x radius x depth.
- Multiply one-hole volume by the post-hole count to get project volume before waste.
- For mixed hole sizes, add each group's raw cubic feet before applying the waste percentage.
- Add a waste buffer for uneven holes, loose soil, spillage, and slight over-digging.
- Divide the waste-adjusted cubic feet by the selected bag yield and round up to whole bags.
Quick examples
These examples use post-hole or custom-volume mode with the current U.S. default concrete assumptions: 0.60 cu ft per 80 lb bag and a 10% waste buffer.
Worked example
18 post holes, 10 in diameter by 30 in deep.
Starter shopping list
- 80 lb concrete bags 45 bags
- Post-hole digger or auger, gravel, bracing As needed
- Mixer, water source, finishing and curing supplies As needed
This example is generated from the same calculator logic used on the Concrete slab calculator page.
Hole size
- Hole diameter and depth drive the bag count more than post count alone, so measure the actual auger size and target depth.
- Frost depth, fence height, wind exposure, gate loads, and local code can require deeper or wider holes than a generic estimate.
Mixed hole sizes
- Gate, corner, hinge, latch, and end posts may use wider or deeper holes than line posts, so avoid one average hole size when loads differ.
- When line posts and terminal posts differ, calculate each group separately or enter the combined raw cubic feet in custom-volume mode before applying waste.
Bag size and waste
- This worked example uses 80 lb bags. Smaller 60 lb or 40 lb bags cover less volume and increase the bag count.
- Keep a waste buffer because post holes are rarely perfect cylinders and the bottom may need gravel or cleanup before concrete.
When to compare ready-mix
- A small fence is usually practical with bags, but dozens of deep holes can mean a lot of hand mixing.
- If the calculator shows many bags, compare ready-mix delivery, a rented mixer, or a short-load supplier before buying.
Common mistakes
- Using post count without measuring hole diameter and depth.
- Forgetting corner, end, and gate posts when counting holes.
- Using one average hole size when gate, corner, or terminal posts need larger footings.
- Using an 80 lb bag yield after deciding to buy 40 lb or 60 lb bags.
- Skipping waste for loose soil, over-digging, and spillage.
- Choosing hole depth without checking frost line, post size, fence height, wind exposure, and local code.
FAQ
How do I calculate concrete for fence posts?
Calculate one post hole as a cylinder, multiply by the number of holes, add waste, then divide by the concrete bag yield and round up to whole bags.
How many 80 lb bags does one fence post need?
It depends on hole diameter and depth. A 10 in diameter by 30 in deep hole is about 1.36 cu ft before waste, so it commonly plans around three 80 lb bags when rounding per post.
Should I estimate bags per post or total project volume?
For buying, total project volume usually gives a tighter estimate. For staging at each hole, per-post rounding is easier but can be more conservative.
What if gate posts need bigger holes?
Calculate line-post holes and larger gate or terminal holes separately, add their raw cubic feet, then apply waste and divide by bag yield. The calculator's custom-volume mode can handle the combined raw volume.
Do I need gravel in the bottom of fence post holes?
Many installations use gravel for drainage or leveling, but the right detail depends on soil, post material, fence system, and local practice.
When should I use ready-mix instead of bags?
Compare ready-mix or a rented mixer when the estimate reaches many bags, the holes are deep, or the site needs faster, more consistent placement.
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