Concrete for Fence Posts Calculator
Calculate how many bags of concrete you need to set your fence posts.
Bags needed
How concrete volume for fence posts is calculated
A round post hole is a cylinder, and its volume is π × radius² × depth. Since the fence post itself displaces part of that cylinder, the post's own volume (approximated here as a square post: width² × depth) is subtracted to get the net volume of concrete actually needed to fill the space around the post.
That net volume, calculated in cubic inches, is converted to cubic feet by dividing by 1,728 (the number of cubic inches in a cubic foot), then multiplied by the number of post holes on the job to get a total project volume.
Finally, total volume is divided by the yield of a single bag of pre-mixed concrete — commonly around 0.30 cubic feet for a 40 lb bag, 0.375 cubic feet for a 50 lb bag, or 0.60 cubic feet for an 80 lb bag — and rounded up, since bags are sold whole.
Where these numbers come from
The formula and default values in this calculator are based on:
- Cylinder-minus-post volume method with standard pre-mixed concrete bag yields (per bag manufacturer specifications, e.g. Quikrete/Sakrete published yields) is standard fence-post concrete estimating practice.
These are planning estimates. Check your local building code, product specifications, and a licensed contractor before you buy materials or build.
Frequently asked questions
How much concrete yield does a bag of Quikrete or Sakrete actually give?
Yield varies slightly by brand and product, but commonly a 40 lb bag yields about 0.30 cubic feet, a 50 lb bag about 0.375 cubic feet, and an 80 lb bag about 0.60 cubic feet of mixed concrete. Check the yield printed on your specific bag, since formulations differ.
Should I subtract the post's volume from the hole volume?
Yes -- the post occupies real space inside the hole, so the concrete only fills the space around it. Skipping this step overestimates the concrete you need, sometimes significantly on larger posts in smaller-diameter holes.
What hole diameter should I dig for a given post size?
A common rule of thumb is a hole diameter roughly three times the width of the post, which gives enough surrounding concrete for holding strength -- for a 4-inch post, that's about a 10 to 12-inch diameter hole, though local code or engineered designs may specify otherwise.
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