Auditable calculation record
The volume-to-bag record
The bag total comes from ideal cylindrical holes, entered post displacement, waste, and the yield printed on your chosen mix. Field holes and products can differ, and this calculation does not choose safe footing size, embedment, or installation practice. Check local requirements and the product label, with qualified review for critical posts.
- Formula version
- 1.2.0
- Last reconciled
- September 13, 2026
- Measurement datum
- Hole depth, gravel depth, and any post displacement all use the user-entered cylindrical-hole geometry.
- Rounding
- Volume retains intermediate precision. Only the final purchase quantity rounds up to a whole bag after the selected waste allowance.
- Known exclusions
- Hole diameter, depth, embedment, footing design, soil conditions, and bag yield are not recommended.
Follow the volume from excavation to whole bags
- 01Depth occupied by concrete
- The gravel base is reported separately. Gross concrete volume uses hole depth minus gravel depth, before any post is subtracted.
- 02Displacement and waste
- When enabled, displacement removes the post volume only through the concrete-filled depth. Net volume and the selected waste allowance stay visible as separate steps.
- 03Convert the combined order once
- The final bag count uses the unrounded total volume with waste and the exact yield you entered. Displayed volume is rounded for reading; dividing that displayed number can differ at a whole-bag boundary.
Compare a hole allowance and post displacement
1. Single metric hole without displacement
Complete inputs
Post-hole geometry
- Number of holes
- 1
- Hole diameter
- 300 mm
- Hole depth
- 600 mm
- Gravel base depth
- 100 mm
Concrete assumptions
- Post displacement
- Not subtracted
- Post cross-section
- Round
- Concrete bag yield
- 20 L
- Waste or overage
- 0%
Expected result
- Gross volume
- 35.3 L
- Post displacement
- 0 L
- Net concrete before waste
- 35.3 L
- Waste allowance
- 0 L
- Concrete volume with waste
- 35.3 L
- Concrete bags
- 2
- Gravel volume
- 7.1 L
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
Complete inputs
Post-hole geometry
- Number of holes
- 2
- Hole diameter
- 400 mm
- Hole depth
- 1000 mm
- Gravel base depth
- 100 mm
Concrete assumptions
- Post displacement
- Subtracted
- Post cross-section
- Square / rectangular
- Post width
- 100 mm
- Post depth
- 100 mm
- Concrete bag yield
- 10 L
- Waste or overage
- 10%
Expected result
- Gross volume
- 226.2 L
- Post displacement
- 18 L
- Net concrete before waste
- 208.2 L
- Waste allowance
- 20.8 L
- Concrete volume with waste
- 229 L
- Concrete bags
- 23
- Gravel volume
- 25.1 L
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.
Check the hole profile and bag label
- 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.
Gravel depth and product yield
- Does gravel count as concrete depth?
- No. The entered gravel base is reported separately and removed from the depth filled with concrete.
- Why must I enter bag yield?
- Bag sizes and stated yields differ. The calculator uses the yield from the exact product label instead of assuming one universal bag.
On September 13, 2026, MaterialNest reconciled the post-hole volume and bag-rounding record; hole design and site suitability are not calculated. Editorial and correction policy.