Post Hole Concrete Calculator

A post hole concrete calculator returns the bags of concrete needed to set your posts. Each hole's cylinder volume minus the post's own volume gives the net concrete per hole. Multiply that by the post count and the waste allowance, divide by the bag's yield, and round up to whole bags.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Bags Needed

Includes a 5% waste allowance.

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Concrete mix22 bags

Breakdown
Bags Needed	22
Concrete per Hole	0.921 cu ft
Total Concrete Volume	9.21 cu ft

How to Calculate Post Hole Concrete

Each hole is a cylinder. Its volume is `π × (diameter ÷ 2)² × depth`. The post itself occupies part of that cylinder for its full embedded depth. So a square post's volume (`width² × depth`) is subtracted out, since concrete doesn't fill the space the post takes up. That net volume, times the number of posts, plus a waste allowance, gives total concrete volume needed. QUIKRETE's Concrete Mix No. 1101 data sheet gives a 60 lb bag a yield of about 0.45 cu ft (0.3 cu ft for 40 lb, 0.6 cu ft for 80 lb). Divide total volume by the bag's yield and round up to get the bag count.

Example: 10 posts, 10 in. diameter holes, 24 in. deep, 3.5 in. (4x4) posts

10 posts, 10 in. diameter holes, 24 in. deep, 3.5 in. (4x4) posts, 60 lb bags, 5% waste: 1. Hole volume: π × (0.833 ÷ 2)² × 2 = π × 0.174 × 2 = 1.091 cu ft 2. Post volume: 0.292² × 2 = 0.170 cu ft 3. Net volume per hole: 1.091 − 0.170 = 0.921 cu ft 4. Total volume: 0.921 × 10 = 9.21 cu ft 5. With 5% waste: 9.21 × 1.05 = 9.67 cu ft 6. Bags needed: 9.67 ÷ 0.45 = 21.5, rounded up to 22 bags

Post Hole Concrete: Compare Scenarios

Scenario	Bags Needed	Concrete per Hole	Total Concrete Volume
Small (4 posts)	9	0.921 cu ft	3.68 cu ft
Medium (10 posts)	22	0.921 cu ft	9.21 cu ft
Large (30 posts)	65	0.921 cu ft	27.62 cu ft

What Is Post Hole Concrete?

A post hole concrete calculator converts hole dimensions, post size, and post count into the number of concrete bags needed to set them. It correctly subtracts the post's own volume from each hole, rather than treating the hole as if it were being filled solid.

Post Hole Concrete: Pros and Cons

Pros

Subtracts the post's own volume from each hole, rather than overestimating by treating the hole as solid concrete
Bag yield is grounded in a real manufacturer data sheet (QUIKRETE Concrete Mix No. 1101), matching the same figures used in the Concrete Mix and Concrete Slab calculators
Works for any hole size and post size combination, rather than assuming one standard fence post dimension

Cons

Doesn't tell you how deep to dig. Hole depth (often tied to your local frost line) is a code and climate question for your jurisdiction, not something this calculator determines
Assumes a square post cross-section. A round post needs its own cylinder-volume subtraction instead of the width-squared approximation used here
Doesn't account for a fast-setting, water-only concrete product's specific mixing instructions. Check your product's data sheet for its exact water ratio and cure time

Post Hole Concrete: What a Pro Checks

Subtract the post's own volume from each hole rather than treating it as solid concrete. Skipping this step overestimates material, sometimes significantly for a larger post in a narrower hole.
Dig to your local frost line, not a fixed national figure. Check your local building code or an experienced local contractor for your area's actual required depth.
Bag yield here is grounded in QUIKRETE's Concrete Mix No. 1101 data sheet (about 0.3/0.45/0.6 cu ft for 40/60/80 lb bags). These are the same figures used across this site's other concrete calculators.
Fast-setting, water-only concrete products are commonly used for setting posts, poured dry into the hole and then saturated with water. Check your specific product's data sheet for its exact instructions.

Frequently Asked Questions

How many bags of concrete do I need per fence post?

It depends on your hole size. A common setup (10 in. diameter, 24 in. deep, 4x4 post) needs a little over 2 bags of 60 lb concrete per post once the post's own volume is subtracted out. That comes to about 22 bags for 10 posts in the example above.

How deep should a fence post hole be?

Hole depth is typically driven by your local frost line depth (to prevent frost heave) plus enough embedment for stability. It is commonly a third of the post's above-ground height. This varies significantly by climate and jurisdiction, so check your local building code or an experienced local contractor rather than using a single national figure.

Do I need to subtract the post volume from the concrete calculation?

Yes. Skipping this step overestimates how much concrete you need, sometimes significantly for a larger post in a narrow hole. The post occupies real volume inside the hole for its entire embedded depth, and concrete only fills what's left.

Can I use fast-setting concrete for fence posts?

Yes. Many fast-setting concrete products are specifically marketed for setting posts. They're often poured dry into the hole around the post, then saturated with water, skipping traditional mixing. Bag yield (cu ft covered per bag) is usually similar to standard concrete mix, but always check your specific product's data sheet.

What diameter should a post hole be?

A commonly used rule of thumb is roughly 3 times the post's width. A 4x4 post (3.5 in. actual) often gets a 10-12 in. diameter hole. Wider holes give more concrete for stability but need more material. Check your local code or fence manufacturer's installation instructions for a specific requirement.

Sources

QUIKRETE Concrete Mix No. 1101 Data Sheet (bag yield by size) (opens in a new tab)

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