Fence Post Concrete Calculator

It depends on the hole and post size. A 4×4 post in a 10-inch-wide, 24-inch-deep hole needs about two 60 lb bags. The calculator finds the hole volume, subtracts the post, and divides by the bag's set yield, then rounds up so each hole is covered.

A common rule is to bury one-third of the post's above-ground height plus about six inches, and to dig below your local frost line. For a standard 6 ft fence that's roughly a 24-inch-deep hole. Deeper holes and corner or gate posts use more concrete.

About three times the width of the post — so a 3.5-inch 4×4 post wants a hole around 10 inches in diameter. A wider hole gives the concrete more grip but uses more material; the calculator updates the bag count as you change the diameter.

Larger bags are usually cheaper per cubic foot and mean fewer bags to carry, but they're heavier. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6. Pick the size you can handle and the calculator converts your volume into a bag count.

Yes, and the calculator does it for you. The post takes up space in the hole, so the concrete only fills the gap around it. Subtracting the post volume — square or round — keeps you from over-ordering, especially with larger 6×6 posts.

Buy a few extra bags beyond the calculated number. Holes are rarely perfect, augers oversize them, some concrete spills, and a hole that hits rock or a root may need more. A small buffer keeps the job moving without a second trip.

Yes. Fast-setting fence-post mixes come in the same 50 and 80 lb bags with similar yields, so the bag count holds. Many can be set dry — pour the bag in the hole and add water — but always follow the product's directions.

Yes, it's completely free with no signup required. Your inputs are saved locally in your browser so they persist between visits — nothing is uploaded to a server. Clearing your browser data will erase your saved values.