Post hole formula
A post hole is estimated as a cylinder: pi times radius squared times depth. Diameter is entered in inches and depth can use feet and inches.
Multiply by the number of holes, then add waste and round the bag count up.
Posts and footings
Post Hole Concrete Calculator
Estimate concrete for fence posts, deck footings and round holes.
US units
10% default waste
Bag and yard estimates
Concrete estimate
Enter dimensions
Printable reference
Post Hole Concrete Bag Chart
Use this chart to sanity-check common round fence-post holes and deck-footing tube forms before buying bagged concrete.
Source note: counts assume a full round hole, 10% waste, and approximate bag yields. Confirm hole size, footing requirements and bag yield before checkout.
Estimate assumptions
- Estimates are for planning and ordering only.
- Bag counts are rounded up to whole bags.
- Ready-mix yards are rounded up to the nearest 0.25 cubic yard.
- Default waste is 10%. Increase it for uneven excavation, spillage, or complex forms.
- Verify structural work with a contractor, engineer, or supplier before pouring.
Fence-post examples
For a 4x4 fence post in a 12 in diameter hole that is 36 in deep, the calculator estimates about 2.36 cubic feet before waste. With 10% waste, that rounds to 5 bags if you choose 80 lb bags.
For a run of eight line-post holes at 10 in diameter and 30 in deep, the estimate is about 10.91 cubic feet before waste. With 10% waste, that rounds to 20 bags of 80 lb concrete.
Deck-footing examples
A 12 in diameter round footing or tube form at 48 in deep is about 3.14 cubic feet before waste. With 10% waste, that rounds to 6 bags of 80 lb concrete.
An 18 in diameter round footing at 48 in deep is about 7.07 cubic feet before waste. With 10% waste, that rounds to 13 bags of 80 lb concrete.
Post hole reminders
- DiameterMeasure the hole diameter, not the post width.
- DepthFollow local frost-depth, deck-code and structural guidance where required.
- QuantityUse the number of holes with the same diameter and depth.
- Post displacementThis calculator estimates a full concrete cylinder and does not subtract the post or a gravel base.
Quick example table
- 4x4 fence post: 12 in dia x 36 in deep2.36 cu ft before waste; 5 80 lb bags with 10% waste for 1 hole.
- Line-post run, 8 holes: 10 in dia x 30 in deep10.91 cu ft before waste; 20 80 lb bags with 10% waste for 8 holes.
- Deck footing tube: 12 in dia x 48 in deep3.14 cu ft before waste; 6 80 lb bags with 10% waste for 1 hole.
- Large pier form: 18 in dia x 48 in deep7.07 cu ft before waste; 13 80 lb bags with 10% waste for 1 hole.
Common next purchases
Concrete mix
Rebar
Tube forms
Mixing tools
Trowels and floats
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FAQ
How much concrete per fence post?
It depends on hole diameter and depth. Enter both dimensions and the number of posts to get a bag estimate.
Can I use this for deck footings?
Yes for a volume estimate, but deck footings may require code-specific sizing, frost-depth checks and inspections.
Should I include gravel at the bottom?
This calculator estimates concrete only. Any gravel base should be estimated separately.
Does the calculator subtract the post volume?
No. It estimates the full round hole as concrete. That is conservative for bag planning, but check the actual hole and post layout before buying.