Post hole concrete calculator

How many bags of concrete to set fence, deck or mailbox posts, with the post itself and the gravel base taken off.

How it works

Each hole is treated as a cylinder: π × radius² × depth. We take off the gravel layer at the bottom and the space the post itself fills, multiply by the number of holes, add your extra allowance, then divide by what one bag makes.

Concrete fills the hole around the post, above a gravel base. Only that ring of concrete needs buying: the post and the gravel take up the rest.

Concrete volume = (π × (hole diameter ÷ 2)² − post cross-section) × (hole depth − gravel depth) × number of holes.

Worked example

Ten holes, 10 in wide and 36 in deep, with 6 in of gravel in each and 4×4 posts (3.5 in actual):

Hole area: π × 5² = 78.5 sq in. Post: 3.5 × 3.5 = 12.25 sq in. Concrete area: 66.3 sq in.
Concrete depth: 36 − 6 = 30 in, so each hole takes 66.3 × 30 = 1,989 cu in, or 1.15 cu ft.
Ten holes: 11.5 cu ft. At 0.60 cu ft per 80 lb bag, that is 20 bags.

Ignore the post and the gravel and the same holes come out at 28 bags. That's why a plain "volume of the hole" figure overbuys.

A metric example: eight 300 mm holes, 600 mm deep, 100 mm square posts and 100 mm of gravel, with 10% extra, need 29 × 20 kg bags.

Sizing the holes

Quikrete's post-setting instructions suggest a hole about three times the post's width, a depth of one-third to one-half of the post's height above ground plus 6 in, and about 6 in of gravel in the bottom for drainage. A 4×4 fence post standing 6 ft out of the ground would then sit in a hole roughly 10–12 in wide and 30–42 in deep. Frost depth and local rules can require deeper holes, so check before you dig.

Pooled or per hole?

The main figure assumes you mix concrete in a barrow and share it across holes, so part-bags aren't wasted. If you pour dry mix straight into each hole, part of the last bag in each hole is left over. The calculator shows that per-hole count too, so you can buy for the way you'll work.

Common questions

How many bags of concrete per fence post?

A 4×4 post in a 10 in hole, 36 in deep with 6 in of gravel, takes about 1.15 cu ft of concrete: just under two 80 lb bags, or just over three 50 lb bags. Uneven holes take more, so allow some extra. Larger holes need more again: use the calculator with your sizes.

Do fast-setting mixes yield the same?

Not always. Check the yield printed on the bag and enter it with Other size if it isn't one of the listed bags.

Reviewed by Alex Rowan, Editor. Last reviewed 17 October 2026. How we calculate · Report a problem