Post Hole Concrete Calculator
Work out how much ready-mix concrete you need to backfill a cylindrical post hole—ideal for fence lines, pergolas, deck footings, and sign posts.
Soil conditions, wind loads, and building codes may require engineered footings or rebar reinforcement. Confirm your design with local authorities or a structural professional before pouring concrete.
Examples
- 0.35 m diameter × 0.75 m depth ⇒ 0.072 m³ (≈2.5 ft³, about 4 × 40 kg bags)
- 0.25 m diameter × 0.90 m depth ⇒ 0.044 m³ (≈1.6 ft³, roughly 3 bags)
- 0.40 m diameter × 1.00 m depth ⇒ 0.126 m³ (≈4.4 ft³, around 7 bags)
- 0.30 m diameter × 0.60 m depth ⇒ 0.042 m³ (≈1.5 ft³, plan for 3 bags)
FAQ
Does the calculation subtract the volume of the post?
No. It assumes you are filling the entire hole. If you want the net concrete after inserting the post, compute the post volume (π × radius² × depth for round posts, or length × width × depth for square posts) and subtract it.
How can I convert the result to premix bag counts?
Divide cubic meters by the yield per bag: about 0.018 m³ for a 40 kg bag or 0.011 m³ for an 80 lb bag. Always round up and buy an extra bag for contingencies.
Do I need a gravel base or drainage layer?
Most outdoor posts benefit from 10–15 cm of compacted gravel at the bottom of the hole to promote drainage. Subtract that depth from the concrete fill height if you include it.
What if my hole flares wider at the bottom?
Bell-shaped footings require segmenting the hole into two cylinders (shaft and bell) or using a cone formula. Calculate each section separately and sum the volumes.
Additional Information
- Volume formula: π × radius² × depth. Radius equals diameter ÷ 2, so doubling the diameter quadruples the concrete required.
- To switch to imperial, multiply the cubic-meter result by 35.3147 for cubic feet or by 1,000 to obtain liters for mixing on site.
- Add 10–15% contingency to cover gravel displacement, post adjustments, and surface finishing. Round up to whole bags when purchasing material.