How many 80 lb bags of concrete for a 4x4 fence post?

Data checked October 2026

Short answer

A 4×4 post set 36 in deep takes about 3.1 × 80-lb bags of Concrete Mix — buy 4 with 10% extra (or 6 × 50-lb Fast-Setting).

Hole: 10.5 in wide (about 3× the 3.5 in post) and 42 in deep — 36 in for the post plus 6 in of dry Concrete Mix.
Concrete per post: 1.85 cu ft, counting the dry-mix plug, with the top flush with the ground (36 in of concrete around the post).
Bags: 6 × 50-lb Fast-Setting (about $47.82) or 4 × 80-lb Concrete Mix (about $25.88).
Mix the 4 bags with 3–5 gal of water before placing the concrete, and brace the post for 24 hours.
Post length: 9' (6' above ground + 36 in buried).
This follows Quikrete's Concrete Mix sheet: 6 in of dry mix in the bottom, the post on top, mixed concrete up to the surface and sloped away from the post. It costs less per cubic foot than Fast-Setting, but the post must stay braced for 24 hours.

Adjust it to your exact job

Your posts

Post size

Lumber is smaller than its name: a 4×4 measures 3½ in, a 6×6 measures 5½ in.

Number of posts

Height above ground

= 6' · 1.83 m

The hole

Post depth in ground⅓ of length

= 3'

Hole diameter3× post

= 10.5"

Bottom of the hole

Quikrete Concrete Mix sheet: 6 in of dry mix in the bottom, then the post, then mixed concrete. The plug counts as concrete.

Concrete

Concrete mix

Patios, walkways, shed bases, steps — the default for most DIY slabs.

Bag size

Extra for waste & spillage

10% covers holes that come out wider than planned and spillage.

You need

4× 80-lb bags

Standard 4000 psi mix: 3.1 bags per post × 1 post, plus 10% extra · 320 lb to carry

Concrete per post: 1.85 ft³3.08 × 80-lb bags, before waste
Hole: 10.5 × 42inwide × deep · 2.1 ft³ of soil to dig out
Post length: 9'above ground + buried; buy the next length up
Mixing water: 3–5galstart low; max weakens the concrete

Fast-Setting or Concrete Mix?

Bags by product for this job
Mix	Per post	Total	Est. cost
Fast-Setting 50 lb	4.9	6	$47.82
Fast-Setting 60 lb	4.1	5	—
Concrete Mix 80 lb	3.1	4	$25.88
Concrete Mix 60 lb	4.1	5	$24.25

Frost line & structural posts

The hole bottom is 42 in below grade. For a fence, burying a third of the post is the manufacturer rule. Posts that hold up a structure — a deck, pergola or carport — must reach at least 6 in below your local frost line and follow local code; your building department can give you the frost depth. Call 811 before you dig.

Timeline for standard 4000 psi mix

Walk on: 24 hours
Light loads / posts: 24 hours
Keep moist (cure): 5 days ≥70°F · 7 days at 50–70°F
Protect from freezing: 48 hours

Shopping list

4 bags Standard 4000 psi mix — 80-lb bags3.1 bags per post + 10% extra$25.88
5 gal Clean water for mixing3–5 gal; start low, never exceed the max

Estimated total$25.88

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
4 bags × $
$25.88

Materials total$25.88

Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.

Did this answer your question?

Compare

4×4 post at other depths (same 10.5 in hole and base)

Post in ground	Hole depth	50-lb Fast-Setting / post	80-lb Concrete Mix / post
24 in	30 in	3.6	2.2
30 in	36 in	4.2	2.7
36 in ←	42 in	4.9	3.1
42 in	48 in	5.6	3.5
48 in	54 in	6.3	3.9

How this was worked out

The hole is a cylinder: π × (10.5 ÷ 2)² = 86.6 sq in. The post takes up 12.3 sq in, leaving 74.3 sq in of concrete for every inch of depth. Concrete runs 36 in around the post: 74.3 × 36 = 2,676 cu in, plus the 6 in dry-mix plug (86.6 × 6 = 520 cu in). 3,196 cu in ÷ 1,728 = 1.85 cu ft.

A 50-lb bag of Fast-Setting makes 0.375 cu ft and an 80-lb bag of Concrete Mix makes 0.6 cu ft, giving 4.93 and 3.08 bags per post.

Filled to the brim the way bag labels count, this hole takes the same 5 bags of Fast-Setting — here the label and the geometry agree.

The full method, material data and tips are on the main post hole concrete calculator page.

Similar jobs

Other sizes people check

Sources

Data behind this answer

Quikrete Fast-Setting No. 1004 — technical data sheet (rev. Nov 2025) — 50 lb = 0.375 cu ft, 20–40 min set, hole 3× post width and 1/3 of post length, ~1 gal water per bag, 30 in lifts, 4 h before loading a post · checked October 2026
Quikrete Concrete Mix No. 1101 — technical data sheet — bag yields (80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30), water per bag, curing times; post method: 6 in of dry mix in the hole bottom, 24 h before strain · checked October 2026
Quikrete — How to set fence posts (how-to page) and Setting Posts project sheet — hole 3× post width, 6 in gravel base, concrete to 3–4 in below grade; structural posts 6 in below the frost line (https://www.quikrete.com/PDFs/Projects/SettingPosts.pdf) · checked October 2026
Quikrete online concrete calculator (calculator.js) — post formula: (hole area − post area) × depth ÷ 1,728 ÷ 0.375, rounded up; post/hole pairs 2/6, 3/9, 4/12, 6/18 in · checked October 2026
Sakrete Fast Setting Concrete Mix — TDS (Rev. 05/18) — post table for a 10 in hole: 12 in 1.25, 18 in 2, 24 in 2.5, 30 in 3 bags (50 lb) · checked October 2026
Sakrete All-Purpose Gravel — product page & TDS — 0.5 cu ft bag (≈3 sq ft at 2 in); 60 lb bag ≈ 0.6 cu ft · checked October 2026
Home Depot & Lowe's listings — Quikrete 80 lb Concrete Mix — $5.98–$6.98 per 80-lb bag; $5.93 bulk tier at 42+ bags; 42 bags per pallet; Fast-Setting 50 lb $7.97 · checked October 2026

Short answer

A 4×4 post set 36 in deep takes about 3.1 × 80-lb bags of Concrete Mix — buy 4 with 10% extra (or 6 × 50-lb Fast-Setting).

Hole: 10.5 in wide (about 3× the 3.5 in post) and 42 in deep — 36 in for the post plus 6 in of dry Concrete Mix.

Concrete per post: 1.85 cu ft, counting the dry-mix plug, with the top flush with the ground (36 in of concrete around the post).

Bags: 6 × 50-lb Fast-Setting (about $47.82) or 4 × 80-lb Concrete Mix (about $25.88).

Mix the 4 bags with 3–5 gal of water before placing the concrete, and brace the post for 24 hours.

Post length: 9' (6' above ground + 36 in buried).

This follows Quikrete's Concrete Mix sheet: 6 in of dry mix in the bottom, the post on top, mixed concrete up to the surface and sloped away from the post. It costs less per cubic foot than Fast-Setting, but the post must stay braced for 24 hours.

Adjust it to your exact job

Your posts

Post size

Lumber is smaller than its name: a 4×4 measures 3½ in, a 6×6 measures 5½ in.

Number of posts

Height above ground

= 6' · 1.83 m

The hole

Post depth in ground⅓ of length

= 3'

Hole diameter3× post

= 10.5"

Bottom of the hole

Quikrete Concrete Mix sheet: 6 in of dry mix in the bottom, then the post, then mixed concrete. The plug counts as concrete.

Concrete

Concrete mix

Patios, walkways, shed bases, steps — the default for most DIY slabs.

Bag size

Extra for waste & spillage

10% covers holes that come out wider than planned and spillage.

You need

4× 80-lb bags

Standard 4000 psi mix: 3.1 bags per post × 1 post, plus 10% extra · 320 lb to carry

Concrete per post: 1.85 ft³3.08 × 80-lb bags, before waste
Hole: 10.5 × 42inwide × deep · 2.1 ft³ of soil to dig out
Post length: 9'above ground + buried; buy the next length up
Mixing water: 3–5galstart low; max weakens the concrete

Fast-Setting or Concrete Mix?

Bags by product for this job
Mix	Per post	Total	Est. cost
Fast-Setting 50 lb	4.9	6	$47.82
Fast-Setting 60 lb	4.1	5	—
Concrete Mix 80 lb	3.1	4	$25.88
Concrete Mix 60 lb	4.1	5	$24.25

Frost line & structural posts

Timeline for standard 4000 psi mix

Walk on: 24 hours
Light loads / posts: 24 hours
Keep moist (cure): 5 days ≥70°F · 7 days at 50–70°F
Protect from freezing: 48 hours

Shopping list

4 bags Standard 4000 psi mix — 80-lb bags3.1 bags per post + 10% extra$25.88
5 gal Clean water for mixing3–5 gal; start low, never exceed the max

Estimated total$25.88

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
4 bags × $
$25.88

Materials total$25.88

Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.

Post in ground

Hole depth

50-lb Fast-Setting / post

80-lb Concrete Mix / post

24 in

30 in

3.6

2.2

30 in

36 in

4.2

2.7

36 in ←

42 in

4.9

3.1

42 in

48 in

5.6

3.5

48 in

54 in

6.3

3.9

How this was worked out

A 50-lb bag of Fast-Setting makes 0.375 cu ft and an 80-lb bag of Concrete Mix makes 0.6 cu ft, giving 4.93 and 3.08 bags per post.

Filled to the brim the way bag labels count, this hole takes the same 5 bags of Fast-Setting — here the label and the geometry agree.

The full method, material data and tips are on the main post hole concrete calculator page.