Estimator notes
How this estimator works
Inputs are normalized to a common measurement system before the formula runs. Intermediate values keep their precision, and the result rounds only where a material must be purchased in a whole unit.
Typical planning scenarios
- Ten 12-inch diameter holes, 30 inches deep.
- A gravel base excluded from concrete depth.
- Round bags up after allowing for waste.
Prices are optional. The quantity result still works when every price field is left empty.
Five solved numerical examples
These documented examples use fixed inputs so you can compare the displayed result with the written arithmetic. Comparable calculator evidence is documented when available; MaterialNest does not claim qualified domain-expert approval.
1. Single metric hole without displacement
Expected result
- Gross volume (L)
- 35.3
- Post displacement (L)
- 0
- Net concrete before waste (L)
- 35.3
- Waste allowance (L)
- 0
- Concrete volume with waste (L)
- 35.3
- Concrete bags
- 2
- Gravel volume (L)
- 7.1
Manual derivation
- Concrete depth = 600 - 100 = 500 mm; π × 150² × 500 = 35.34 L.
- No displacement or waste applies; ceil(35.34 / 20) = two bags. Gravel volume is 7.07 L.
2. Two holes with rectangular-post displacement
Expected result
- Gross volume (L)
- 226.2
- Post displacement (L)
- 18
- Net concrete before waste (L)
- 208.2
- Waste allowance (L)
- 20.8
- Concrete volume with waste (L)
- 229
- Concrete bags
- 23
- Gravel volume (L)
- 25.1
Manual derivation
- Two cylindrical holes at 900 mm concrete depth contain 226.19 L gross; two 100 × 100 mm posts displace 18 L.
- Net 208.19 L plus 10% is 229.01 L; ceil(229.01 / 10) = 23 bags.
3. Four holes with round-post displacement
Expected result
- Gross volume (L)
- 141.4
- Post displacement (L)
- 15.7
- Net concrete before waste (L)
- 125.7
- Waste allowance (L)
- 12.6
- Concrete volume with waste (L)
- 138.2
- Concrete bags
- 7
- Gravel volume (L)
- 28.3
Manual derivation
- Four 300 mm holes at 500 mm concrete depth contain 141.37 L; four 100 mm round posts displace 15.71 L.
- The 125.66 L net plus 10% is 138.23 L; ceil(138.23 / 20) = seven bags.
4. Ten holes with five-percent waste
Expected result
- Gross volume (L)
- 392.7
- Post displacement (L)
- 0
- Net concrete before waste (L)
- 392.7
- Waste allowance (L)
- 19.6
- Concrete volume with waste (L)
- 412.3
- Concrete bags
- 17
- Gravel volume (L)
- 0
Manual derivation
- Ten cylinders give 10 × π × 125² × 800 = 392.70 L.
- Five-percent waste adds 19.63 L, for 412.33 L; ceil(412.33 / 25) = 17 bags.
5. Deep holes with rectangular posts
Expected result
- Gross volume (L)
- 477.1
- Post displacement (L)
- 36
- Net concrete before waste (L)
- 441.1
- Waste allowance (L)
- 66.2
- Concrete volume with waste (L)
- 507.3
- Concrete bags
- 17
- Gravel volume (L)
- 95.4
Manual derivation
- Three 450 mm holes at 1000 mm concrete depth contain 477.13 L; the three rectangular posts displace 36 L.
- Net 441.13 L plus 15% is 507.30 L; ceil(507.30 / 30) = 17 bags. Gravel volume is 95.43 L.
Common mistakes to avoid
- Using the full hole depth after adding gravel.
- Using a bag yield from a different concrete product.
- Allowing post displacement to exceed the hole volume.
Frequently asked questions
Does this confirm a code or permit requirement?
No. This calculator provides a material-planning estimate only. It does not provide structural engineering, permit advice, building-code interpretation, footing-depth approval, installation instructions, or guaranteed quantities.
Can I use metric measurements?
Yes. Switch to metric mode and enter millimetres, centimetres, or metres. The formula uses the same normalized measurements.
Estimate disclaimer
Results are planning estimates. Actual material requirements vary by product, installation method, site conditions, waste, and local rules. Verify product instructions and local requirements. This tool does not determine structural safety, footing depth, permit requirements, installation requirements, or code compliance. Have critical work reviewed by a qualified professional.
Last calculation review: July 16, 2026. Published by MaterialNest; no qualified domain-expert approval is claimed.