How much concrete for a mailbox post?

Q: How much concrete for a mailbox post?

About 1 bag, including a waste allowance. 0.36 cu ft (concrete per post). 0.36 cu ft (concrete, whole job). 0.01 cu yd (same volume in cubic yards). 0.11 cu ft (volume the posts displace). 0.17 cu ft (gravel for the drainage bases).

About 1 bag, with a waste allowance included.

0.36 cu ft — concrete per post
0.36 cu ft — concrete, whole job
0.01 cu yd — same volume in cubic yards
0.11 cu ft — volume the posts displace
0.17 cu ft — gravel for the drainage bases

Change the numbers to match your project

Hole diameter Three times the post width, per Quikrete’s post-setting guide. A 4x4 wants 10–12 in; a 6x6 wants 12–18 in. Hole depth One-third to one-half of the above-ground post height, plus 6 in for the gravel base — and never shallower than your local frost depth. Post Wood is listed at its dressed size, not the nominal name. A 4x4 is 3.5 in on a side. Gravel in the bottom Quikrete’s guide calls for 6 in of gravel or crushed stone under the post so water drains away from the end grain. Concrete does not fill this. Set to 0 if you are skipping it. Concrete stops below grade The same guide stops the concrete 3–4 in down so you can backfill with soil. Set to 0 if you are instead crowning the concrete at grade to shed water. How many posts Extra for waste % An augered hole is never a clean cylinder. 10% covers ragged sides; use 15% in loose sand or gravel where the hole bells out. Bag Fast-setting mix yields the same 0.0075 cu ft per pound as general mix — but it is sold in a 50 lb bag, so you need more bags for the same hole.

Bags of mix, whole job 1 bags Rounded up. Buy one spare per ten posts, not per post.

Concrete per post 0.36 cu ft

Concrete, whole job 0.36 cu ft

Same volume in cubic yards 0.01 cu yd

Volume the posts displace 0.11 cu ft Already subtracted above. A plain hole-volume figure leaves this in.

Gravel for the drainage bases 0.17 cu ft

Every other hole on this page gets deeper when you want it to hold better. This one has a ceiling on it, written by the Postal Service.

USPS publishes four figures for a curbside box, and two of them decide the concrete. "Bury your post no more than 24″ deep." And: "Position your mailbox 41″ to 45″ from the road surface to the bottom of the mailbox or point of mail entry." The support is specified as well — "A 4″ x 4″ wooden support or a 2″-diameter standard steel or aluminum pipe" — together with the warning that follows it: avoid "unyielding and potentially dangerous supports, like heavy metal pipes, concrete posts, and farm equipment."

That ceiling is a crash rule, not paperwork. The Postal Service's own bulletin credits the Federal Highway Administration with the dimensions, on the grounds that a support of that size and depth "should safely break away if struck by a vehicle", and adds that the box "must also be securely attached to its post to prevent separation if struck." You are building something designed to lose an argument with a car.

So the depth field is a maximum here, not a target. The one-third rule and the frost line, which govern every fence post, both push the other way. Where your frost line is deeper than 24 in you cannot honour both, and 24 in is the one with a federal reason behind it. A heaved mailbox is a nuisance; a mailbox set like a gate post is a bollard at windscreen height.

What the ceiling costs in concrete: about a third of a cubic foot. An 8 in hole 24 in deep, less the 6 in gravel base, the 3 in of soil backfill and the post, leaves roughly 0.36 cu ft — one 80 lb bag, a little over half used. The identical 4x4 in a fence, at 10 in by 30 in, takes two.

Add the two published limits and the post length falls out: 24 in below grade plus 41 to 45 in up to the floor of the box is 65 to 69 in, so a 6 ft 4x4 is the buy, standing 3 to 7 in proud for the mounting board. Treat that as approximate — the 41 in is measured from the road surface, while the post is 6 to 8 in back from the curb, where the ground normally sits higher.

Open the full concrete calculator for post holes for other sizes and the full explanation →

Last reviewed 2026-07-31