Fence Post Concrete Calculator
Round holes and tube forms. Pick fence, deck, mailbox, or Sonotube, then enter how many you are filling.
This many holes is about a cubic yard or more. A mixer or a small ready-mix order may beat mixing every bag by hand.
How the numbers are figured
Each hole is a cylinder: π × radius² × depth. Radius is half the diameter. Inches are converted to feet before multiplying. The total is that volume times the number of posts. Bag counts use the same US pre-mix yields as the slab calculator and round up to whole bags.
The wood post is not subtracted. That keeps a little extra in the hole and in the cart.
Common questions
How much concrete for one fence post?
A 10 inch wide hole that is 24 inches deep takes about 1.1 cubic feet, or 2 bags of 80 lb mix, before waste. Deeper deck holes use more.
How do you calculate concrete for a round post hole?
Treat the hole as a cylinder: pi times radius squared times depth. Convert inches to feet first. Multiply by the number of holes, then divide by the yield on the bag.
Should I subtract the wood post from the hole?
This calculator does not subtract the post, so the bag count is a little high on purpose. Coming up short on a fence line is worse than carrying one extra bag.
How many 80 lb bags for a 12 inch Sonotube?
A 12 inch tube filled 48 inches deep takes about 3.14 cubic feet, or 6 bags of 80 lb mix, before waste. Use the inside diameter printed on the tube. Change the diameter for 8 or 10 inch tubes.