How much extra should I order?

A common rule of thumb is to order 5–15% more concrete than the bare minimum to account for irregular holes, soil collapse, and waste while mixing and placing.

Does it handle belled holes?

The calculator assumes straight cylindrical holes. For belled or flared holes, you can approximate by using the larger diameter at the bell, or you can add an extra allowance on top of the cylindrical estimate.

Can I use this for deck or pergola footings?

Yes, as a volume estimate. However, footing size and depth for structural elements must follow local building codes and engineering requirements, which this tool does not enforce.

How do I convert this to bags of concrete?

Divide the total cubic feet by the volume per bag shown on your concrete mix (for example, about 0.6 cu ft per 80‑lb bag or 0.45 cu ft per 60‑lb bag) and round up to the next whole bag.

construction calculator

Post Hole Concrete Calculator

Estimate concrete volume for fence or deck post holes by entering hole size and count.

Results

Cubic feet: 14.14
Cubic yards: 0.52

Overview

Setting fence, deck, pergola, or mailbox posts usually means digging a row of cylindrical holes and then trying to guess how many bags of concrete you need. Under‑ordering leads to extra trips; over‑ordering leaves you with wasted material.

This post hole concrete calculator turns your hole dimensions into a clear volume estimate so you can plan concrete orders with more confidence. By treating each hole as a simple cylinder, it converts diameter, depth, and hole count into total cubic feet and cubic yards of concrete.

Use it as a fast job‑site or planning tool alongside local code requirements and manufacturer recommendations for post sizing and embedment depth.

How to use this calculator

Count how many posts you plan to set and enter that value in Number of holes.
Measure or decide on the hole diameter in inches, using the widest expected diameter if the hole will be slightly irregular.
Measure or plan the hole depth in inches from finished grade down to the bottom of the hole or footing.
Review the calculated total cubic feet and cubic yards of concrete needed for all holes combined.
Use the cubic yards figure to price out ready‑mix delivery, or convert cubic feet to bag counts using your preferred bag size (for example, 0.6 cu ft per 80‑lb bag).
Add a bit of extra volume to your order to cover over‑digging, soil collapse, and waste.

Inputs explained

Number of holes: The total count of post holes you plan to pour—include all fence posts, corner posts, gate posts, pergola supports, deck posts, sign posts, or mailboxes for the project.
Hole diameter (in): The diameter of each post hole in inches. If you are using cardboard forms (sonotubes), use the tube size. If holes are hand‑dug and irregular, enter the average or maximum expected diameter.
Hole depth (in): The depth of each hole in inches from finished grade down to the bottom. Check local codes for minimum depth below frost line and any specific embedment requirements for structural posts.

Outputs explained

Cubic feet: The total concrete volume for all holes expressed in cubic feet. This is useful for converting to bag counts using the volume per bag printed on the packaging.
Cubic yards: The same total volume expressed in cubic yards (1 cubic yard = 27 cubic feet), which is how ready‑mix concrete orders are typically quoted.

How it works

Each post hole is approximated as a cylinder, using your diameter and depth in inches.

We convert inches to feet, then calculate the volume per hole using the standard cylinder formula: volume = π × radius² × depth.

We multiply the volume per hole by the total number of holes to get total cubic feet of concrete required.

We then divide cubic feet by 27 to convert to cubic yards, which aligns with how ready‑mix orders are typically quoted.

The outputs give you both cubic feet and cubic yards, which you can then translate into bag counts or short‑load truck orders based on your supplier’s options.

Formula

We model each post hole as a cylinder.

Convert inches to feet:
Diameter_ft = Diameter_in ÷ 12
Depth_ft = Depth_in ÷ 12
Radius_ft = Diameter_ft ÷ 2

Volume per hole (cubic feet) = π × (Radius_ft)^2 × Depth_ft

Total cubic feet = Volume per hole × Number of holes
Total cubic yards = Total cubic feet ÷ 27

When to use it

Sizing concrete for fence or privacy screen posts before a weekend project so you can buy enough bags in a single trip.
Estimating concrete volume for deck or pergola posts when comparing the cost of bagged mix versus a short‑load truck.
Planning material needs for sign posts, mailbox posts, or light posts in residential or light commercial settings.
Helping DIYers and small contractors quickly sanity‑check concrete volume when laying out post spacing and hole dimensions.
Using on job bids to estimate material quantities and costs for projects that involve repeating post footings.

Tips & cautions

Add 5–15% extra volume to your concrete order to account for over‑excavation, uneven holes, soil cave‑ins, and waste during mixing and placement.
Check local building codes or engineering guidelines for minimum hole diameter and depth relative to post size, especially for decks, pergolas, and structures that carry significant loads.
If you plan to bell the bottom of the hole (wider at the base), use the larger diameter in the calculator or add additional volume manually.
Using sonotubes or forms makes hole diameters more consistent, which improves the accuracy of this volume estimate.
Remember that posts displace a small amount of concrete; the recommended extra margin usually covers this displacement.
Models holes as perfect cylinders and does not directly account for belled bottoms, flares, or irregular shapes.
Does not subtract the volume of the posts themselves from the hole volume; in practice, a small volume margin usually covers this effect.
Does not automatically convert volume to bag counts; you’ll need to divide cubic feet by the volume per bag shown on your chosen concrete mix.
Does not check structural adequacy or code compliance for post sizing, spacing, or footing dimensions; it focuses solely on concrete volume.
Ground conditions, frost heave, uplift forces, and structural loads are not modeled—those must be addressed with local codes and engineering guidance.

Worked examples

Six 12" × 36" fence post holes

Diameter = 12 in → 1.0 ft; radius = 0.5 ft.
Depth = 36 in → 3.0 ft.
Volume per hole ≈ π × (0.5)^2 × 3 ≈ 2.36 cu ft.
Total cubic feet ≈ 2.36 × 6 ≈ 14.16 cu ft.
Total cubic yards ≈ 14.16 ÷ 27 ≈ 0.52 cu yd.

Ten 9" × 30" light‑duty posts

Diameter = 9 in → 0.75 ft; radius = 0.375 ft.
Depth = 30 in → 2.5 ft.
Volume per hole ≈ π × (0.375)^2 × 2.5 ≈ 1.10 cu ft.
Total cubic feet ≈ 1.10 × 10 ≈ 11.0 cu ft.
Total cubic yards ≈ 11.0 ÷ 27 ≈ 0.41 cu yd.

Eight 10" × 48" deck posts

Diameter = 10 in → 0.833 ft; radius ≈ 0.4167 ft.
Depth = 48 in → 4.0 ft.
Volume per hole ≈ π × (0.4167)^2 × 4 ≈ 2.18 cu ft.
Total cubic feet ≈ 2.18 × 8 ≈ 17.44 cu ft.
Total cubic yards ≈ 17.44 ÷ 27 ≈ 0.65 cu yd.

Deep dive

Estimate concrete volume for post holes by entering the number of holes, their diameter, and depth to see total cubic feet and cubic yards required.

Use this post hole concrete calculator to plan fence, deck, or pergola projects and decide how many bags or how much ready‑mix to order.

FAQs

How much extra should I order?: A common rule of thumb is to order 5–15% more concrete than the bare minimum to account for irregular holes, soil collapse, and waste while mixing and placing.
Does it handle belled holes?: The calculator assumes straight cylindrical holes. For belled or flared holes, you can approximate by using the larger diameter at the bell, or you can add an extra allowance on top of the cylindrical estimate.
Can I use this for deck or pergola footings?: Yes, as a volume estimate. However, footing size and depth for structural elements must follow local building codes and engineering requirements, which this tool does not enforce.
How do I convert this to bags of concrete?: Divide the total cubic feet by the volume per bag shown on your concrete mix (for example, about 0.6 cu ft per 80‑lb bag or 0.45 cu ft per 60‑lb bag) and round up to the next whole bag.

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This post hole concrete calculator provides approximate volume estimates only. Actual concrete needs can vary due to irregular hole shapes, soil conditions, post size, and placement technique. It does not verify structural adequacy, frost‑line requirements, or code compliance. Always confirm dimensions on site, follow local building codes, and consult with a qualified contractor or engineer for critical structural work before ordering materials or building.