Post Hole Concrete Calculator
Work out how many 60 or 80 lb bags of concrete fill a cylindrical post hole around your post, from the hole size and post dimension.
How it works
The concrete fills the gap between the round hole and the post sitting in it. So the calculator finds the volume of the cylindrical hole — pi times the radius squared times the depth — then subtracts the volume the post itself takes up, treating the post as a square section like the 3.5-inch face of a nominal 4x4.
That leftover volume is your concrete, converted from cubic inches to cubic feet. Each bag has a known yield: a 60-pound bag of premixed concrete makes about 0.45 cubic feet and an 80-pound bag about 0.60, so the tool divides the fill volume by the yield and rounds up to whole bags.
Set the post side to zero if you're pouring a solid pad or footing with nothing in the middle. For a run of fence posts, work out one hole and multiply by your post count, and keep a spare bag or two on hand since holes never come out perfectly to size.
Frequently asked questions
Why subtract the post volume?
Because the post is already in the hole taking up space — you only pour concrete into the ring around it. Subtracting the post's volume from the hole's volume gives the true amount of concrete you need, so you don't over-order.
How much does a 60 or 80 lb bag cover?
A 60-pound bag of premixed concrete yields roughly 0.45 cubic feet once mixed, and an 80-pound bag about 0.60 cubic feet. The calculator uses those figures, so switching bag size changes the count without you redoing any math.
How deep should a post hole be?
A common guideline is to bury about a third of the post's above-ground height and to get below the frost line in cold climates, often 24 to 36 inches. Deeper, wider holes need more concrete — enter your actual depth and diameter to see the effect.