Cylindrical Hole Volume Formula
For round post holes, multiply the squared radius by π and the depth. This gives you the total volume of earth removed and the base for concrete calculations.
Volume = π × radius² × depth
radius— Distance from the hole's centre to its edge, measured in metres or feetdepth— Vertical distance from ground surface to the bottom of the holeπ— Mathematical constant approximately equal to 3.14159
Rectangular Hole Volume Formula
For square or rectangular holes—such as those for deck footings—multiply length, width, and depth together. This straightforward calculation treats the hole as a simple rectangular prism.
Volume = length × width × depth
length— The longer horizontal dimension of the hole in your chosen unitwidth— The shorter horizontal dimension of the holedepth— Vertical distance from ground level to the hole bottom
Calculating Concrete Requirements
Post holes rarely fill exactly with concrete due to spillage, absorption, and settling. The calculator adds a 7% waste factor to account for these losses. If you're setting multiple posts—fence sections, pergola supports, or deck corners—the tool multiplies the concrete volume by the number of holes, then applies the waste percentage.
- Single hole: Multiply the hole volume by 1.07 (the 7% waste allowance)
- Multiple holes: Calculate total hole volume, multiply by 1.07, then multiply by the quantity of posts
- Material cost: Divide the concrete volume (in cubic metres or yards) by the yield of your concrete mix to determine bags needed
Adjust the waste percentage if your site conditions differ—sandy soil or high wind may increase losses.
Cylindrical vs Rectangular Holes: Practical Differences
Circular holes are most common for fence posts and lamp standards because they minimise soil disturbance and create uniform bearing. Rectangular holes suit deck ledger boards, retaining wall anchors, and foundation work where alignment matters.
When measuring diameter, use the full width across the hole's centre rather than relying on auger size alone—augers can enlarge or compress soil. For rectangular holes, square corners are rare in practice: measure the longest diagonals to ensure your concrete volume estimate covers actual ground conditions. Deeper holes in unstable soil may need wider excavation for safety shoring, which affects real-world concrete consumption.
Common Calculation Pitfalls
Avoid these frequent mistakes when estimating hole volumes and concrete needs.
- Confusing radius and diameter — Many contractors input the auger bit size (diameter) into the radius field, doubling the calculated volume. Remember: diameter is twice the radius. If your auger is 300 mm wide, the radius is 150 mm, not 300 mm.
- Ignoring ground settlement and absorption — Fresh concrete shrinks slightly and settles in loose soil. The 7% waste figure covers spillage but may be conservative in sandy or clay soil. Monitor actual concrete use on your first few holes and adjust the waste factor accordingly.
- Measuring depth inconsistently — Holes drilled to different depths across a site—especially in sloping ground—can skew material estimates. Always measure from a reference level (finished grade) rather than eyeballing. A 10 cm difference per hole adds up quickly across multiple posts.
- Forgetting bulk order minimums — Concrete suppliers sell in fixed quantities (typically 0.5 m³ or 1 m³ bags). If you calculate 2.3 m³, you'll likely order 3 m³ worth, generating left-over material. Factor supplier batch sizes into your budget.