Fence and Post Concrete Estimator

Concrete Post Hole Calculator

Calculate concrete for fence, deck, gate, mailbox, and sign post holes by diameter, depth, gravel allowance, hole count, and bag size. Get cubic feet, cubic yards, 40/60/80 lb bag counts, waste, and material-only cost.

Round Holes

Uses diameter and concrete-filled depth.

Hole Count

Multiply one hole across a fence or deck run.

Bag Choice

Compare 40, 60, and 80 lb bag pressure.

Quick answer

One 12 in diameter post hole that is 36 in deep needs about 2.59 ft3 of concrete with 10% waste. That rounds up to about 5 common 80 lb bags, 6 common 60 lb bags, or 9 common 40 lb bags.

Enter Post Hole Details

Results

Live estimate

Concrete Volume

0.096 yd³

2.5918 ft³ / 0.0734 m³

Bags Needed

5 bags

Rounded up to whole bags

Estimated Cost

$25.00

Based on bag price

Waste Factor

10%

Included in total volume

Post hole formula

One round hole

Volume per hole = pi x (diameter / 2)^2 x concrete-filled depthVolume per hole = pi x (1.000 / 2)^2 x 3.000

Total holes

Total before waste = 2.3562 x 1 holesFinal volume = total x (1 + 10%/100)

Bags

Bags = ceil(final volume / 0.6)

Quick Post Hole Bag Counts

Project	Hole size	Concrete with 10% waste	80 lb bags	60 lb bags	40 lb bags
1 hole	8 in x 24 in	0.77 ft3	2	2	3
1 hole	10 in x 30 in	1.50 ft3	3	4	5
1 hole	12 in x 36 in	2.59 ft3	5	6	9
8 holes	10 in x 30 in	11.97 ft3	20	27	40
12 holes	12 in x 36 in	31.15 ft3	52	70	104

These examples use common bag yields and no gravel deduction. Use the calculator above when holes are over-dug, gravel replaces concrete, or the job has mixed post sizes.

Example Post Hole Estimates

One 12 in x 36 in post hole

Common fence or gate planning example with 10% waste.

Result: 2.5918 ft3 / 0.096 yd3 / 5 bags

Eight 10 in x 30 in fence holes

Small fence run using 60 lb bags for a carryable option.

Result: 11.9991 ft3 / 0.4444 yd3 / 27 bags

Twelve 12 in x 36 in holes

Larger run where bag count should be planned before buying.

Result: 31.1018 ft3 / 1.1519 yd3 / 52 bags

Before You Buy Bags

Measure the actual dug diameter, not only the planned auger size.
Subtract gravel depth if the bottom of the hole is not filled with concrete.
Separate regular line posts from larger gate, corner, deck, or sign posts.
Check utility marking, frost depth, permit, and structural requirements before digging.

Post Hole Planning Notes

This calculator estimates material quantity only. It does not decide post size, embedment depth, frost depth, wind load, deck code, utility location, or permit requirements.

For the full written guide, see the concrete post hole calculator guide. If the project is a continuous wall or beam footing, use the concrete footing calculator.

Frequently Asked Questions

How do I calculate concrete for post holes?

Use the round-hole formula: pi times radius squared times concrete-filled depth. Then multiply by the number of holes, add waste, and divide by the bag yield.

How many 80 lb bags for a 12 inch by 36 inch post hole?

One 12 inch diameter by 36 inch deep hole needs about 2.59 cubic feet with 10% waste. With a common 0.60 cubic foot 80 lb bag yield, that rounds up to about 5 bags.

Should I subtract gravel depth from a post hole?

Yes. If gravel sits at the bottom of the hole, enter only the depth that will actually be filled with concrete or subtract the gravel depth before calculating.

Can I use this for fence posts and deck posts?

Use it for material quantity after the approved hole diameter and depth are known. It does not decide frost depth, post size, gate load, deck code, or structural design.

Should I use 40 lb, 60 lb, or 80 lb bags?

Use the bag yield printed on the product. 80 lb bags reduce bag count, while 40 lb and 60 lb bags may be easier to carry for small or scattered holes.

When should I compare ready-mix for post holes?

Compare ready-mix when the job reaches dozens of bags, holes are grouped together, or consistency and placement speed matter. Bags are often practical for scattered fence posts.

Move to the calculator that matches your next estimating step.

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Disclaimer: This calculator is for concrete material planning only. Site conditions, base preparation, reinforcement, drainage, joints, form accuracy, and local building rules can change actual requirements. Consult a qualified contractor or engineer for structural work.