Round-hole volume
Gross volume = π × (diameter ÷ 2)² × depth × hole count. Use metres for all lengths to get cubic metres. Measure the hole itself. A hole wider than the auger needs more concrete.
Worked example: six holes
Take six holes, each 300 mm across and 600 mm deep. These are example sizes, not a footing design.
- Diameter: 0.30 m. Radius: 0.15 m. Depth: 0.60 m.
- One hole: π × 0.15² × 0.60 = 0.04241 m³.
- Six holes: π × 0.15² × 0.60 × 6 ≈ 0.25447 m³, or 254.47 L gross. Keep full precision until the final result.
Allow for the post
A post takes up some of the space in the hole. Say each square post is 100 mm × 100 mm and sits 600 mm deep. Each post takes up 0.10 × 0.10 × 0.60 = 0.006 m³. Six posts take up 0.036 m³. Subtract that from the gross hole volume. This leaves about 0.21847 m³ for concrete.
An example 10% allowance brings it to 0.24032 m³, or 240.32 L. For a product with a stated mixed yield of 10 L per bag, buy 25 bags. Round up once at the end of the sum.
Use the real post shape and the length below ground. The web tool gives gross hole volume. Note the space taken by posts on the Chippy Tools concrete sheet.
Choose the hole size from the design
Soil, loads, wind and the structure all affect the footing design. A volume sum does not tell you how deep to dig. Find footing guides for your country.
Basis and product source
The cylinder formula gives the full hole volume. For bag counts, use the mixed yield on the exact product’s label or data sheet. Follow the specified mix, placing and curing steps. Confirm the final order with your supplier.