Reading the post-setting estimate
Hole diameter and depth are typically set by local code, frost depth and post load requirements rather than by this material calculator.
- Minimum post-hole depth is commonly governed by local frost-line depth and building code for structural or fence posts — check local requirements rather than relying on the depth entered here as a design recommendation.
- This calculator assumes a square post cross-section; for round posts, enter an equivalent width or accept a small overestimate of concrete needed.
Why does post size matter for concrete volume?
When setting a post in concrete, the post itself occupies part of the hole's volume — concrete only fills the space around the post, not the post's own footprint. This calculator subtracts the post's cross-sectional volume (assumed square) from the cylindrical hole volume to get the actual concrete volume needed.
Getting hole depth right matters beyond just concrete quantity: local frost-depth and code requirements typically set a minimum post-hole depth for fence and deck posts, which is a site- and jurisdiction-specific decision separate from this material estimate.
How to use this post hole concrete calculator
- Enter the hole diameter in centimeters.
- Enter the hole depth in centimeters.
- Enter the post's width in centimeters, assuming a square post cross-section (enter 0 for a very thin post or if concrete volume without post displacement is preferred).
- Enter the number of posts being set.
- Read the total concrete volume, the volume per post, and the number of 20 kg bags needed.
The formula behind the post hole concrete estimate
Hole volume equals π × (hole diameter ÷ 2)² × hole depth, the standard cylinder-volume formula. Post volume equals post width squared multiplied by hole depth, treating the post as a square prism running the full depth of the hole. Concrete volume per post is hole volume minus post volume, and total volume multiplies by the number of posts.
Worked example: a 30 cm diameter hole, 60 cm deep, with a 9 cm square post, gives a hole volume of π × (0.15)² × 0.6 ≈ 0.0424 m³ and a post volume of (0.09)² × 0.6 ≈ 0.0049 m³, for a net concrete volume of about 0.0375 m³ per post. Six posts need about 0.225 m³ total, or roughly 24 bags of 20 kg pre-mixed concrete.
Common mistakes
- Ignoring the post's own volume and ordering concrete for the full hole volume, which overstates the quantity needed.
- Digging holes shallower than the local frost-depth or code-required minimum to save on concrete, risking frost heave or instability.
- Forgetting to multiply per-post volume by the total number of posts before ordering.
- Using dry-set (poured dry, then watered) fence-post concrete for a structural application where a properly mixed and placed pour is required.
Perguntas frequentes
How much concrete do I need for a fence post hole?
Subtract the post's own volume from the cylindrical hole volume; a typical 30 cm diameter, 60 cm deep hole with a 9 cm square post needs roughly 0.0375 cubic meters of concrete per post.
How deep should a post hole be?
Minimum post-hole depth is commonly set by local frost-line depth and building code requirements for the specific post type and load — check local code rather than using a generic rule of thumb.
Do I need to subtract the post's volume from the concrete estimate?
Yes — the post occupies part of the hole, so concrete only fills the remaining space around it; skipping this subtraction overstates the concrete quantity needed, especially for larger posts.
How many bags of concrete does one fence post need?
It depends on hole size, but a typical 30 cm diameter, 60 cm deep hole with a standard post needs roughly 4 bags of 20 kg pre-mixed concrete per post — use the calculator's per-post result for the specific hole size being used.
Referências
- American Wood Council (AWC) — post-frame and fence-post setting guidance.
- International Residential Code (IRC) — frost-protected footing depth provisions vary by jurisdiction and climate zone.
- QUIKRETE technical data sheets — published yield per bag for fence-post concrete mix.