Post Hole Calculator — Concrete for Fence & Deck Posts

Round holes take less concrete than people guess — until you multiply by a whole fence line. Enter the hole size and count; get bags per hole and for the full job.

Your post holes

Rule of thumb: hole diameter = about 3× the post width.
Below the frost line for structural posts; fences often go 1/3 of the post length.

Concrete per hole: 0 cu ft — results below are for all 1 hole(s).

Always order a little extra. You cannot send back a shortage mid-pour.
Prices — national-average defaults, edit to your local prices
National average; roughly $140–$170 in most areas.

Defaults are planning averages, not quotes. Your supplier's prices are the real ones — type them in.

Concrete to order (with your waste allowance)
0 cu yd
0
80 lb bags
0
60 lb bags
0
50 lb bags
0
40 lb bags
Mix it yourself (bags)
$0
Ready-mix truck
$0

Estimates only. The calculator figures the full cylinder; the post itself displaces some concrete, and a gravel base layer (common: 4–6 inches of gravel in the bottom for drainage) displaces more. Most people still order the full amount — soil crumbles, holes bell out, and a spare bag beats a second trip.

Worked example: one 12-inch diameter hole, 36 inches deep: π × (12/24)² × 3 ft = 2.36 cu ft. That is 4 bags of 80 lb mix per hole (3.93, rounded up). A 10-hole fence run needs 23.56 cu ft — 40 bags exact, 44 with 10% extra.

Sonotubes and forms

For above-grade piers (deck footings that rise out of the ground), cardboard tube forms (commonly sold as Sonotube) set the diameter — use the tube's actual inside diameter in the calculator. Below grade, the hole itself is the form. If your soil is sandy and holes cave in, tubes or a slightly larger diameter with the waste allowance set to 15% will save your pour day.

Frequently asked questions

How many bags per post hole?

For the standard 12″ × 36″ hole: 4 bags of 80 lb, or about 5–6 bags of 60 lb. Smaller 8″ × 24″ holes take about 1 cu ft — 2 bags of 80 lb. The calculator above gives exact numbers for your size.

Should I subtract the space the post takes up?

You can — a 4×4 post removes roughly 0.35 cu ft from a 36″ hole. In practice, holes dig wider than planned and gravel goes in the bottom, which roughly cancels the post's displacement. Ordering the full cylinder amount is the safe call.

Fast-setting or regular mix for posts?

Fast-setting mix (the red bag) is made for posts: pour it in dry around a braced post, add water, and it sets in 20–40 minutes with no mixing. It costs more per bag and yields the same volume. For a long fence line, regular mix in a drum mixer is cheaper; for a few posts, fast-set is worth it.