Post hole calculator
Two errors dominate post hole estimating: forgetting that the post itself displaces concrete, and buying gravel at the depth it compacts to rather than the depth it is poured. This calculator handles both and shows the volume it subtracted.
Your holes
How this is calculated
- Concrete column = π × (diameter ÷ 2)² × (depth − gravel), in cubic inches.
- Post displacement = post cross-section × (depth − gravel), subtracted because the post occupies that space.
- Bags = volume in ft³ ÷ 0.60 for 80 lb, ÷ 0.45 for 60 lb, rounded up.
- Gravel = π × (diameter ÷ 2)² × gravel depth, then × 1.15 because aggregate settles.
Worked example
Six holes, 12″ diameter, 36″ deep, 6″ of gravel, 6×6 posts:
- Concrete column: π × 36 × 30 = 3,393 in³ = 1.963 ft³
- Post: 30.25 in² × 30″ = 907 in³ = 0.525 ft³
- Net per hole: 1.963 − 0.525 = 1.438 ft³
- Six holes: 8.63 ft³ → 15 bags of 80 lb (0.6 ft³ each)
- Gravel: π × 36 × 6 = 679 in³ × 6 = 2.36 ft³, × 1.15 = 2.7 ft³
Skipping the displacement step would have ordered 17 bags instead of 15 — a 12% overbuy, which is not fatal but is money in the ground.
Depth is a code question, not a volume question
This calculator will happily compute the concrete for a hole that is too shallow. Depth is set by frost line in cold climates and by bearing capacity everywhere else, and both are published locally. A footing that stops above the frost line will heave, and no amount of concrete fixes it.
The one rule that is nearly universal: the footing must bear below the frost depth, and the post must not sit on soil. That is why the gravel layer exists — it drains water away from the post base and gives a compacted bearing surface that is not mud.
Post in concrete, or on a base?
Embedding a post directly in concrete is common and it works, provided the concrete crowns above grade so water runs off, and the post is rated for ground contact. The alternative is a standoff post base on top of the concrete, which keeps the wood out of the wet zone entirely and is the better detail where appearance allows.
What fails is a post set in a hole with soil at the bottom and concrete poured around it: water collects in the soil, the post base stays wet, and the post rots off at ground level eight years later. The gravel layer and the crowned concrete are what prevent it.
What a hole volume will not tell you
- It does not tell you how deep to dig. Frost depth and bearing capacity are local published values, and the tool flags rather than guesses.
- It does not size the footing. Diameter follows from post load and soil bearing — see the footing calculator for the same arithmetic from the other direction.
- It does not include a bell or flare. A belled footing holds more concrete than a straight cylinder; add it manually if your detail calls for one.
- It assumes a dry mix. If you are adding water on site, bag yield is approximate — check the bag you bought.
Questions people actually ask
How many bags of concrete for a 12 inch post hole?
At 36″ deep with 6″ of gravel and a 6×6 post, about 2.4 bags of 80 lb per hole. Without subtracting the post, the same hole computes to 3.3 bags — the difference is real money across six holes.
How deep should a deck post hole be?
Below the frost line where frost occurs, and deep enough for the load where it does not. Published frost depths range from a few inches in the south to 60″ or more in the north, so this is a local number, not a rule.
Do I need gravel at the bottom of a post hole?
It helps. A compacted gravel base drains water away from the post end and provides a firm bearing surface, which is the difference between a post that lasts and one that rots at the base.
Can I pour concrete around a treated post?
Commonly done, and it works if the post is rated for ground contact and the concrete is crowned above grade so water runs away. A standoff base is the more durable detail where the design allows it.
How much concrete does a 60 lb bag make?
About 0.45 cubic feet, against 0.60 for an 80 lb bag. Yields vary slightly by mix, so buy one spare bag per ten when the count is close.