Fence Post Concrete Calculator
Estimate bagged concrete after subtracting square fence post displacement.
- Concrete before allowance
- 14.0066 cu ft
- With allowance
- 15.4072 cu ft
- Post count
- 10 posts
- Material subtotal (entered price only)
- Not supplied
Whole packages round up. Prices include only the items you enter. Delivery, tax and installation may be extra.
Different hole sizes: one concrete shopping list
A gate post and a line post can have different measured holes. Enter each group separately; this worksheet sums the unrounded concrete volume before rounding the shared bag order. It is independent of the quick calculator.
Measure actual hole diameter, actual square-post width and only the concrete-filled depth. Subtract any gravel layer beneath it from total hole depth. These examples do not specify how deep or wide to dig.
Why combine volume before rounding?
The editable example has eight 12 in holes around 3.5 in posts, filled 24 in deep, plus two 16 in holes around 5.5 in posts, filled 30 in deep. With 10% extra and 0.6 cu ft per bag, the shared purchase is 32 bags. Rounding the two groups independently gives 21 + 11 = 32 here; rounding each individual hole instead gives 8 × 3 + 2 × 6 = 36. Pooling avoids four bags only if the same mix can actually be shared across the placement. Do not assume partially used mixed batches remain usable for a later pour.
Round, constant-diameter holes with square posts occupying the full concrete-filled depth. Gravel beneath concrete is excluded. All groups use the same mix and bag yield; pooling assumes usable mixed material can be shared during the planned placement. Separate pours may need separate rounding. Not a footing-depth, frost, wind, soil or structural design. Tax, labor, equipment and unentered costs excluded.
Confirm exact product yield, mixing and placement instructions, bag availability, return terms and delivery before ordering. A nominal bag weight is not a volume. A zero delivery entry means no charge entered, not verified free delivery.
How to calculate
The hole is round and the post is square. Concrete-filled depth excludes any gravel below the concrete. The post is assumed to occupy the full concrete depth. Bags round up across the whole project.
Worked example
Number of posts: 10 · Round hole diameter (in): 12 · Square post actual width (in): 3.5 · Concrete-filled depth (in): 24 · Bag yield (cu ft): 0.6 · Extra material (%): 10.
These inputs produce 26 concrete bags. Change the measurements above to match your project.
Measure the right surfaces
Enter the number of identical round holes, actual hole diameter, actual square post width and depth occupied by concrete. Do not include gravel below the concrete in this depth. If holes differ, use the Different hole sizes worksheet to record each group and pool unrounded volume before buying whole bags of the same mix.
A second example you can check
A 12 in round hole around a 3.5 in square post, filled 24 in deep, needs (π × 6² − 3.5²) × 24 ÷ 1728 = 1.4007 cu ft. Ten holes and 10% extra need 15.4072 cu ft. At 0.6 cu ft yield per bag, the pooled order rounds to 26 bags, rather than rounding each individual hole to three bags.
Where estimates go wrong
The post displaces concrete, so forgetting to subtract it overstates volume. An oversized or irregular excavated hole adds concrete, however, so nominal auger size may not match the real cavity. Product yield describes mixed volume; bag weight is not itself a volume.
Build your supplier order
Confirm mix suitability, label yield, hole geometry and post embedment requirements for your design and soil. The calculation does not set footing depth or validate resistance to wind, frost or overturning. Plan mixing and placement according to the product instructions.
Fence Post Concrete: practical questions
Does this work for a round post?
Not directly. The current displacement model is a square post; using its diameter as square width subtracts the wrong area.
Why must the post diagonal fit?
All corners of the square must be inside the circular hole. The validation checks width × √2 against the hole diameter.
Inputs and references
QUIKRETE Concrete Mix No. 1101 data sheet — This product lists approximately 0.60 cu ft yield for an 80 lb bag. Check the yield for your actual mix and bag size.
YardTakeoff · Revised October 22, 2026 · Formula checks and scope · Report a calculation error
Plan the whole order
Use the quote checklist to record materials outside this calculator’s scope.