How much concrete for a 4x6 gate post?

Data checked October 2026

Short answer

A 4×6 post set 36 in deep takes about 9.9 × 50-lb bags of Fast-Setting concrete or 6.2 × 80-lb bags of Concrete Mix — buy 11 or 7 with 10% extra.

Hole: 16.5 in wide (about 3× the 5.5 in post) and 42 in deep — 36 in for the post plus 6 in of gravel.
Concrete per post: 3.72 cu ft with the top 3 in below grade (33 in of concrete around the post).
Bags: 11 × 50-lb Fast-Setting (about $87.67) or 7 × 80-lb Concrete Mix (about $45.29).
Pour it dry and soak it with about 11 gal of water in all; the post can take rails after 4 hours. The 33 in fill goes in 2 lifts of 30 in or less.
Post length: 9' (6' above ground + 36 in buried).
A 4×6 hinge post resists a swinging gate better than a 4×4; the hole is sized from the wide face (5½ in × 3 = 16½ in).

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

= 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

11× 50-lb bags

Sets in 20–40 min: 9.9 bags per post × 1 post, plus 10% extra · 550 lb to carry

Concrete per post: 3.72 ft³9.91 × 50-lb bags, before waste
Hole: 16.5 × 42inwide × deep · 5.2 ft³ of soil to dig out
Post length: 9'above ground + buried; buy the next length up
Water (dry pour): 11gal≈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	9.9	11	$87.67
Fast-Setting 60 lb	8.3	10	—
Concrete Mix 80 lb	6.2	7	$45.29
Concrete Mix 60 lb	8.3	10	$48.50

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 13 × 50-lb bags. The gravel base and the soil cap bring it down to 9.9.

Pour this 33 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 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.

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

11 bags Sets in 20–40 min — 50-lb bags9.9 bags per post + 10% extra$87.67
2 bags All-purpose gravel, 0.5 cu ft bags (6 in per hole)0.74 cu ft in all
11 gal Water to soak the dry mixabout 1 gal per 50 lb bag, depending on the soil

Estimated total$87.67

Cost estimate

Tap a price to use your store's

50-lb Sets in 20–40 min
11 bags × $
$87.67

Materials total$87.67

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

Did this answer your question?

Compare

4×6 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	6.3	3.9
30 in	36 in	8.1	5.1
36 in ←	42 in	9.9	6.2
42 in	48 in	11.7	7.3
48 in	54 in	13.5	8.4

How this was worked out

The hole is a cylinder: π × (16.5 ÷ 2)² = 213.8 sq in. The post takes up 19.3 sq in, leaving 194.6 sq in of concrete for every inch of depth. Concrete runs 33 in around the post: 194.6 × 33 = 6,421 cu in. 6,421 cu in ÷ 1,728 = 3.72 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 9.91 and 6.19 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 13 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.

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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 4×6 post set 36 in deep takes about 9.9 × 50-lb bags of Fast-Setting concrete or 6.2 × 80-lb bags of Concrete Mix — buy 11 or 7 with 10% extra.

Hole: 16.5 in wide (about 3× the 5.5 in post) and 42 in deep — 36 in for the post plus 6 in of gravel.

Concrete per post: 3.72 cu ft with the top 3 in below grade (33 in of concrete around the post).

Bags: 11 × 50-lb Fast-Setting (about $87.67) or 7 × 80-lb Concrete Mix (about $45.29).

Pour it dry and soak it with about 11 gal of water in all; the post can take rails after 4 hours. The 33 in fill goes in 2 lifts of 30 in or less.

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

A 4×6 hinge post resists a swinging gate better than a 4×4; the hole is sized from the wide face (5½ in × 3 = 16½ in).