Post Hole Concrete Calculator
Estimate the concrete required around square or round fence posts using a practical three-times-post-size hole rule.
Project inputs
Concrete estimate
Volume breakdown
| Component | Per post (ft³) | All posts (ft³) |
|---|---|---|
| Excavated hole | 3.0000 | 24.0000 |
| Buried post | 0.3333 | 2.6667 |
| Concrete fill | 2.6667 | 21.3333 |
How to use the post hole concrete calculator
What this calculator does
This calculator estimates the volume of concrete that occupies the space between a buried fence post and its excavated hole. It applies a common planning rule in which the hole is three times the post width and three times the post thickness, or three times the diameter for a round post. The result is a material-volume estimate, not a structural design, footing specification, frost-depth determination, or substitute for local code requirements.
When to use it
Use it when budgeting concrete for a new fence, checking whether leftover mix is enough for a repair, comparing square and round post layouts, or preparing a purchase quantity before digging. For installation guidance beyond volume, consult local requirements and the QUIKRETE guide to setting posts.
How to calculate
- The calculator opens with a complete demonstration: eight square 4 × 4 inch posts buried 36 inches deep. Results and a validated Excel workbook are available immediately.
- Select Post shape. Choose square or rectangular for posts with two side dimensions, or round for posts described by diameter.
- Select the Length unit. Switching between inches and centimeters converts the dimensions already entered, so the physical size remains the same.
- Enter the Number of posts, the post width or diameter, the post thickness when applicable, and the post depth. Results update live.
- Read the total first, then review the per-post amount, hole opening, and volume breakdown. Select Download Excel to export the current typed model. Reset clears the demonstration and results; Excel remains unavailable until a complete valid state is entered again.
Input guide
Post shape is required and determines the cross-section formula. A square or rectangular post uses width × thickness; a round post uses the circle area from its diameter. Length unit is required and accepts inches or centimeters. Mixing dimensions from different unit systems is a common mistake, so convert them first or use the selector.
Number of posts is a required whole number from 1 to 10,000. The example uses 8. Doubling this value doubles the project total but does not change the per-post amount. Decimal counts, zero, and negative values are rejected.
Post width is required for square and rectangular posts and becomes Post diameter for round posts. Enter a positive decimal without scientific notation; 4 inches is a typical example. Post thickness is required only for square or rectangular posts and uses the same accepted format. A smaller actual dressed lumber dimension produces less displaced post volume than a nominal dimension, so use the size you intend to place in the hole. Post depth is required, must be positive, and represents the buried length. The example uses 36 inches. Increasing depth increases both hole and post volume in direct proportion.
Output guide
Concrete for all posts is the primary estimate and is displayed in cubic yards, cubic feet, and liters. Concrete per post is the same fill volume for one hole. Hole opening shows the calculated three-times-post-size plan dimensions. Hole volume per post is the full excavated prism or cylinder, while Buried post volume is the portion occupied by the post. The table shows those same values per post and for all posts. Zero is not a meaningful valid result here because all required dimensions and the post count must be positive.
Worked example
For eight square posts measuring 4 × 4 inches and buried 36 inches, the planned hole opening is 12 × 12 inches. One hole has a volume of 12 × 12 × 36 = 5,184 cubic inches, or 3.0000 cubic feet. The buried post occupies 4 × 4 × 36 = 576 cubic inches, or 0.3333 cubic feet. Subtracting gives 2.6667 cubic feet of concrete per post. Multiplying by eight gives 21.3333 cubic feet, which equals 0.7901 cubic yards and about 604.11 liters. Those values match the first-open display and workbook.
How the volume model works
The core identity is simple: concrete volume equals excavated hole volume minus buried post volume. For a rectangular post, both shapes are rectangular prisms. For a round post, both are cylinders. Because the recommended hole diameter or side lengths are three times the post dimension, the hole cross-sectional area is nine times the post area. The concrete therefore occupies eight times the buried post volume when the hole and post share the same depth.
The NIST explanation of SI volume units provides context for cubic meters and liters, while the NIST guide to length units explains the linear measurements from which volume is derived.
Planning allowances and practical limits
Real holes are rarely perfect prisms or cylinders. Loose soil, bell-shaped bottoms, over-excavation, water, gravel bases, and spillage can all increase the amount used. Treat the calculated figure as the geometric minimum under the stated three-times-size assumption, then apply a project allowance suited to your digging method and site. Premixed concrete products also vary by yield; use the manufacturer's stated yield per bag rather than assuming every bag size produces the same volume.
Post depth is often governed by wind exposure, fence height, soil, frost conditions, and local rules. The FEMA coastal construction guidance illustrates why foundation and embedment decisions depend on environmental loads and site conditions. For ordinary residential work, verify local permit and utility-marking requirements before excavation.