How much concrete for a 6x6 post 4 feet deep?

Data checked October 2026

Short answer

A 6×6 post set 48 in deep takes about 12.7 × 50-lb bags of Fast-Setting concrete or 8 × 80-lb bags of Concrete Mix — buy 15 or 9 with 10% extra.

Hole: 16.5 in wide (about 3× the 5.5 in post) and 54 in deep — 48 in for the post plus 6 in of gravel.
Concrete per post: 4.78 cu ft with the top 3 in below grade (45 in of concrete around the post).
Bags: 15 × 50-lb Fast-Setting (about $120) or 9 × 80-lb Concrete Mix (about $58.23).
Pour it dry and soak it with about 15 gal of water in all; the post can take rails after 4 hours. The 45 in fill goes in 2 lifts of 30 in or less.
Post length: 12' (8' above ground + 48 in buried).
An 8 ft tall 6×6 — a pergola, carport or tall gate post — goes 4 ft into the ground under the one-third rule. If it holds up a roof, local code and the frost line decide the depth.

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

= 8' · 2.44 m

The hole

Post depth in ground⅓ of length

= 4'

Hole diameter3× post

= 1' 4.5"

Bottom of the hole

Quikrete's how-to: 6 in of gravel under the post lets water drain away from the end grain.

Concrete

Concrete mix

Fence and mailbox posts, small repairs. Too fast for most slabs — mix only what you can place in 20 minutes.

Bag size

Extra for waste & spillage

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

You need

15× 50-lb bags

Sets in 20–40 min: 12.7 bags per post × 1 post, plus 10% extra · 750 lb to carry

Concrete per post: 4.78 ft³12.75 × 50-lb bags, before waste
Hole: 16.5 × 54inwide × deep · 6.68 ft³ of soil to dig out
Post length: 12'above ground + buried; buy the next length up
Water (dry pour): 15gal≈1 gal per 50 lb bag, soil dependent

Fast-Setting or Concrete Mix?

Bags by product for this job
Mix	Per post	Total	Est. cost
Fast-Setting 50 lb	12.7	15	$120
Fast-Setting 60 lb	10.6	12	—
Concrete Mix 80 lb	8	9	$58.23
Concrete Mix 60 lb	10.6	12	$58.20

Why the bag label says more

Filled to the brim with no gravel and no soil cap — the way bag labels and Quikrete's online calculator count — each hole would take 16 × 50-lb bags. The gravel base and the soil cap bring it down to 12.7.

Pour this 45 in column in 2 lifts: fill no more than 30 in of dry mix, soak it, let the water sink in, then add the rest (Quikrete Fast-Setting instructions).

Frost line & structural posts

The hole bottom is 54 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.

Fast-Setting: pour dry, add water

Sets: 20–40 minutes
Load a post: 4 hours
Strength at 24 h: 1,000 psi
Strength at 28 days: 4,000 psi

Shopping list

15 bags Sets in 20–40 min — 50-lb bags12.7 bags per post + 10% extra$120
2 bags All-purpose gravel, 0.5 cu ft bags (6 in per hole)0.74 cu ft in all
15 gal Water to soak the dry mixabout 1 gal per 50 lb bag, depending on the soil

Estimated total$120

Cost estimate

Tap a price to use your store's

50-lb Sets in 20–40 min
15 bags × $
$120

Materials total$120

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

Did this answer your question?

Compare

6×6 post at other depths (same 16.5 in hole and base)

Post in ground	Hole depth	50-lb Fast-Setting / post	80-lb Concrete Mix / post
24 in	30 in	5.9	3.7
30 in	36 in	7.6	4.8
36 in	42 in	9.3	5.8
42 in	48 in	11	6.9
48 in ←	54 in	12.7	8

How this was worked out

The hole is a cylinder: π × (16.5 ÷ 2)² = 213.8 sq in. The post takes up 30.3 sq in, leaving 183.6 sq in of concrete for every inch of depth. Concrete runs 45 in around the post: 183.6 × 45 = 8,261 cu in. 8,261 cu in ÷ 1,728 = 4.78 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 12.75 and 7.97 bags per post.

If you filled the hole to the brim with no gravel or soil cap — how bag labels and Quikrete's online calculator count — it would take 16 bags of Fast-Setting per post. The base and soil cap save 3 bags per hole.

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 6×6 post set 48 in deep takes about 12.7 × 50-lb bags of Fast-Setting concrete or 8 × 80-lb bags of Concrete Mix — buy 15 or 9 with 10% extra.

Hole: 16.5 in wide (about 3× the 5.5 in post) and 54 in deep — 48 in for the post plus 6 in of gravel.

Concrete per post: 4.78 cu ft with the top 3 in below grade (45 in of concrete around the post).

Bags: 15 × 50-lb Fast-Setting (about $120) or 9 × 80-lb Concrete Mix (about $58.23).

Pour it dry and soak it with about 15 gal of water in all; the post can take rails after 4 hours. The 45 in fill goes in 2 lifts of 30 in or less.

Post length: 12' (8' above ground + 48 in buried).

An 8 ft tall 6×6 — a pergola, carport or tall gate post — goes 4 ft into the ground under the one-third rule. If it holds up a roof, local code and the frost line decide the depth.

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

= 8' · 2.44 m

The hole

Post depth in ground⅓ of length

= 4'

Hole diameter3× post

= 1' 4.5"

Bottom of the hole

Quikrete's how-to: 6 in of gravel under the post lets water drain away from the end grain.

Concrete

Concrete mix

Fence and mailbox posts, small repairs. Too fast for most slabs — mix only what you can place in 20 minutes.

Bag size

Extra for waste & spillage

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

You need

15× 50-lb bags

Sets in 20–40 min: 12.7 bags per post × 1 post, plus 10% extra · 750 lb to carry

Concrete per post: 4.78 ft³12.75 × 50-lb bags, before waste
Hole: 16.5 × 54inwide × deep · 6.68 ft³ of soil to dig out
Post length: 12'above ground + buried; buy the next length up
Water (dry pour): 15gal≈1 gal per 50 lb bag, soil dependent

Fast-Setting or Concrete Mix?

Bags by product for this job
Mix	Per post	Total	Est. cost
Fast-Setting 50 lb	12.7	15	$120
Fast-Setting 60 lb	10.6	12	—
Concrete Mix 80 lb	8	9	$58.23
Concrete Mix 60 lb	10.6	12	$58.20

Why the bag label says more

Pour this 45 in column in 2 lifts: fill no more than 30 in of dry mix, soak it, let the water sink in, then add the rest (Quikrete Fast-Setting instructions).

Frost line & structural posts

Fast-Setting: pour dry, add water

Sets: 20–40 minutes
Load a post: 4 hours
Strength at 24 h: 1,000 psi
Strength at 28 days: 4,000 psi

Shopping list

15 bags Sets in 20–40 min — 50-lb bags12.7 bags per post + 10% extra$120
2 bags All-purpose gravel, 0.5 cu ft bags (6 in per hole)0.74 cu ft in all
15 gal Water to soak the dry mixabout 1 gal per 50 lb bag, depending on the soil

Estimated total$120

Cost estimate

Tap a price to use your store's

50-lb Sets in 20–40 min
15 bags × $
$120

Materials total$120

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

5.9

3.7

30 in

36 in

7.6

4.8

36 in

42 in

9.3

5.8

42 in

48 in

6.9

48 in ←

54 in

12.7

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 12.75 and 7.97 bags per post.

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