Practical guide
How Much Concrete for Fence Posts?
Estimate the concrete-filled part of each post hole by subtracting the embedded post cross-section from the matching hole shape, then multiplying by concrete depth and count.
Updated 15 October 2026
Reviewed by the 99Tools team. Editorial and corrections policy.
Quick answer: subtract the post volume
For a round hole around a round post, volume = pi ÷ 4 × (hole diameter² − post diameter²) × concrete depth. Use metres for every dimension. Multiply by the number of identical holes, then apply only an agreed allowance.
A geometry example, not a post design
For four hypothetical holes of 300 mm diameter around 100 mm diameter posts, each with 600 mm of concrete depth, measured volume is approximately 0.150796 m³. Adding an illustrative 5% gives approximately 0.158336 m³, or 158.336 litres. These sizes demonstrate the arithmetic and are not installation recommendations.
Rectangular holes and posts
For matching rectangular shapes, subtract post length × post width from hole length × hole width, then multiply by concrete depth and count. The post must fit within the hole. The tool does not approximate an incompatible post shape by blindly subtracting a diameter from a width.
Use the part actually filled with concrete
Enter the embedded depth occupied by the concrete, not automatically the complete excavation depth. A separate material layer or void would need its own measured treatment. Irregular holes, rough excavation and displaced soil can change actual usage.
Convert to premix bags only after measuring
Take the required finished volume to the bag result and enter the selected product’s declared yield. Do not substitute bag mass or mixing-water litres. Use zero extra allowance if the transferred volume already contains it.
Size and safety come from the design
The tool does not determine hole depth, diameter, spacing, soil capacity, durability, wind loading or a suitable concrete product. Confirm the installation requirements for the fence, post material and site before using a quantity estimate.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
Frequently asked questions
Do I count the volume occupied by the post?
Subtract its embedded volume when it displaces the concrete in the shape being measured.
Does this calculator choose a safe post-hole depth?
No. Hole dimensions and concrete suitability must come from the installation specification and site requirements.
What if my holes are not identical?
Add separate calculator sections for the different measurements instead of applying one repeated count.