How many bags of concrete for a fence post?

A standard 4×4 post in a 10" diameter hole at 24" deep needs about 1.5 bags of 80 lb concrete. At 36" deep, plan for 2-3 bags per post. The exact amount depends on hole diameter, depth, and post size.

How deep should a fence post hole be?

The standard rule is 1/3 of the total post length underground. For a 6-foot fence, use 8-foot posts buried 24-30 inches. In frost-prone areas, go below the frost line (typically 36-42 inches). Gate posts and end posts should be deeper for extra stability.

How wide should a post hole be?

The hole diameter should be 3 times the post width. For a 4×4 post (actual 3.5"), use an 8-10" hole. For a 6×6 post (actual 5.5"), use a 12" hole. Wider holes provide more concrete mass and better stability.

Can I use fast-setting concrete for fence posts?

Yes. Fast-setting concrete like Quikrete Fast-Setting can be poured dry into the hole and then wetted — no mixing required. It sets in 20-40 minutes. Regular concrete mix works too but requires mixing and takes 24-48 hours to set.

Should I use 60 lb or 80 lb bags for post holes?

Either works. 80 lb bags are more cost-effective but heavier to carry. 60 lb bags are easier to handle at remote post locations. Coverage: 60 lb bag = 0.45 cu ft, 80 lb bag = 0.60 cu ft.

How do I calculate concrete volume for a post hole?

Calculate the cylinder volume of the hole (π × radius² × depth) minus the post volume (width × depth × depth for square posts). Post holes are cylinders, so the formula is: π × (hole_radius)² × depth − post_width² × depth, all in consistent units.

Post Hole Concrete Calculator — Bags Per Post & Total

What Is the Post Hole Concrete Calculator?

The Post Hole Concrete Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your post size, hole diameter, hole depth, you get instant results including concrete / hole, total concrete, bags needed. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.

Why This Calculation Matters

Getting concrete / hole right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.

When Should You Use This Calculator?

This tool is most useful when you know your post size and need to find the right concrete / hole. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.

Post Hole Concrete Calculator

Post Size

Hole Diameter (inches)

Hole Depth (inches)

Number of Posts

Bag Size

Concrete / Hole

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Total Concrete

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Bags Needed

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Bags per Post

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Total Weight

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Hole Volume

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Configure post and hole settings to calculate concrete needed.

How to Use This Calculator

Enter Your Post Size: Start by entering your post size — this is the primary input for the calculation.
Fill In Additional Details: Complete the remaining fields: hole diameter, hole depth, number of posts, bag size. Each value refines the calculation for greater accuracy.
Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
Review Your Results: Check your concrete / hole, total concrete, bags needed. Use these figures to inform your next decision or compare against alternative scenarios.

How to Calculate Post Hole Concrete

The calculator finds the cylinder volume of the hole, subtracts the volume occupied by the post, and divides by the yield per bag.

Concrete = π × r² × depth − post_area × depth

Hole volume: π × (hole_diameter/2)² × depth
Post volume: post_width × post_depth × hole_depth
Concrete needed: hole volume − post volume
Bags: concrete volume ÷ yield per bag (0.60 cu ft for 80 lb)
Hole diameter should be 3× the post width
Depth should be 1/3 of total post length or below frost line

Tips & Considerations

Double-check your post size before calculating — even small input errors can significantly change your results.
Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
Pay attention to both concrete / hole and total concrete — they work together to give you the full picture.
Bookmark this page for quick access next time you need to plan your projects.
If you're unsure about your bag size, start with a conservative estimate and adjust from there.

Frequently Asked Questions

How many bags of concrete per fence post?

A 4×4 post in a 10" hole at 24" deep needs about 1-2 bags of 80 lb concrete. At 36" deep, plan for 2-3 bags. Larger 6×6 posts displace more concrete so need fewer bags per hole.

How deep should post holes be?

Bury 1/3 of total post length. For a 6 ft fence with 8 ft posts, go 24-30" deep. In frost areas, dig below the frost line (36-42"). Gate and end posts should be deeper.

How wide should post holes be?

3× the post width. For 4×4 posts (3.5" actual), use 8-10" holes. For 6×6 posts (5.5" actual), use 12" holes.

Can I use fast-setting concrete?

Yes. Pour dry into the hole and add water — no mixing needed. Sets in 20-40 minutes. Regular mix works but takes 24-48 hours.

60 lb or 80 lb bags?

80 lb bags are more economical. 60 lb bags are easier to carry to remote post locations. Coverage: 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft.

How is post hole volume calculated?

Cylinder volume (pi × radius² × depth) minus the rectangular post volume. All converted to cubic feet, then divided by bag yield.