Easy Masonry Calc

How many bags of concrete for a mailbox post?

Short answer

A 4×4 mailbox post in a 10 inch hole 2 feet deep takes about 1.01 cubic feet of concrete with 10% extra: 3 bags of 50 lb fast-setting mix (2.7 bags exactly) or 2 of 80 lb standard mix.

How to work it out

  1. Hole 3.1416 × (10 in ÷ 2)² minus the post, × 24 in of concrete
  2. + 10% = 1.01 cu ft per post ÷ 0.375 cu ft per 50 lb bag = 2.7 bags per post
  3. One post: buy 3 bags

A mailbox post is a one-post job, which makes fast-setting mix the easy choice: no mixing, and the post stands on its own within the hour. The question is usually how many bags to grab. Slope the top of the concrete away from the post so rain runs off.

Adjust for your project

in
Quikrete’s rule of thumb: about 3 times the post width (10½ in for a 4×4, so a 10 or 12 in auger)
in
Quikrete: about ⅓ of the post’s total length. Local frost depth and fence rules come first
50 lb fast-setting ≈ 0.375 cu ft · 80 lb standard ≈ 0.60 cu ft
More options: Gravel · Top · Extra …
in
Many builders use 4–6 in of gravel for drainage; Quikrete’s own method uses none. 0 if none
in
Quikrete’s fast-setting method stops 3–4 in below the surface and finishes with soil or sod
%
Hand-dug holes are rarely neat cylinders — 10–15%
ft
$
Optional — an example figure; use your store’s price
Bags to buy3 bags50 lb bag
Bags per post
2.7 bags
If you share batches between holes
3 bags · 50 lb bag
Concrete for all posts
1.01 cu ft (0.0375 cu yd)
Quikrete’s rule of thumb for this post
Hole about 10-1/2 in wide and 24 in deep for a 6 ft post
  • Call 811 (in the US) or your local utility locate service a few days before you dig.
Check local rulesPost depth (often below the local frost line), fence height and setbacks are set by local rules — check with your building department or HOA before you dig.

For this job · affiliate link
Post levelA strap-on post level reads plumb on two faces at once, so you can set the post and walk away while fast-setting mix hardens.
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Mailbox post in a 10 in hole, by depth

Mailbox post in a 10 in hole, by depth
Hole depthBags per postConcrete for all posts
18 in2 bags0.76 cu ft
24 in2.7 bags1.01 cu ft
30 in3.4 bags1.27 cu ft
36 in4.1 bags1.52 cu ft

Three bags or two?

At 2 ft deep the hole needs a little over 1 cu ft with a margin. In 50 lb fast-setting bags that is 2.7, so buy 3 — about 150 lb of mix. In 80 lb bags of standard mix it is 1.7, so 2 bags (160 lb), but you have to mix them wet first. Quikrete puts a fast-setting post ready for strain in 4 hours, versus 24 hours for standard mix, which matters when the carrier comes tomorrow.

If you want the top finished in soil or grass, stop the concrete 3–4 in below the surface as Quikrete’s fast-setting instructions describe; at 4 in that trims the concrete by a sixth.

Mailbox post details

Your local post office can tell you the height and setback it wants for the box, and some areas restrict solid masonry or heavy posts near the road because of the risk to vehicles — check before you build anything sturdier than a wood or breakaway post. Call 811 first: the strip along the road often carries buried cables. For a full fence line, see how much concrete for a 4×4 post; a post set on an 8 in tube form instead is on how many bags of concrete for an 8 inch Sonotube.

Different size? Use the full Fence Post Concrete Calculator.

What you’ll need

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Frequently asked questions

How many 80 lb bags of concrete for a mailbox post?

2 bags of 80 lb for a 10 inch hole 2 feet deep.

How much concrete for a mailbox post with a soil cap on top?

About 0.84 cubic feet, 3 bags of 50 lb, stopping 4 inches below the surface.

Other common sizes

Sources