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Answer · Fence

How many bags of concrete per fence post (by post size)

Updated April 26, 2026

Quick answer

A standard 4×4 wood fence post in a 10-inch wide, 30-inch deep hole needs about 2 bags of 60 lb concrete per post (or 1.5 bags of 80 lb). A 6×6 post in a 12-inch wide, 36-inch deep hole needs about 3 bags of 60 lb per post. Round numbers up at the store and grab one or two extras for spillage.
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Bag count per post (most common setups)

Concrete per fence post, hole minus post volume, rounded up with 10% waste
PostHole (Ø × depth)50 lb fast-set60 lb bags80 lb bags
4×4 wood10 in × 24 in21.51
4×4 wood10 in × 30 in2.521.5
4×4 wood12 in × 36 in432
6×6 wood10 in × 30 in2.521.5
6×6 wood12 in × 36 in432.5
6×6 wood12 in × 42 in4.53.52.5
Round metal (2.5 in)10 in × 30 in2.521.5
Round metal (2.5 in)12 in × 36 in432
Round metal (4 in)12 in × 36 in3.532

Counts include a 10% waste factor and subtract the post's own volume below grade. Yields used: 50 lb fast-set = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³.

The math (so you can verify)

Formula

Bags per fence post

hole_vol_ft3 = π × (hole_dia_in ÷ 24)² × (depth_in ÷ 12)
post_vol_ft3 = (post_w_in × post_t_in ÷ 144) × (depth_in ÷ 12)
concrete_ft3 = (hole_vol_ft3 - post_vol_ft3) × 1.10
bags = ceil( concrete_ft3 ÷ yield_per_bag )
Yield: 0.30 ft³ (40 lb), 0.375 ft³ (50 lb fast-set), 0.45 ft³ (60 lb), 0.60 ft³ (80 lb). For round posts, replace post volume with π × (post_radius_in ÷ 12)² × depth_in ÷ 12.

Worked example

4×4 PT post in a 10 × 30 in hole

Standard 6-foot privacy fence in a non-frost climate.

  1. 1. Hole vol = π × (10/24)² × (30/12)1.36 ft³
  2. 2. Post vol = (3.5 × 3.5 / 144) × (30/12)0.21 ft³
  3. 3. Concrete = (1.36 − 0.21) × 1.101.27 ft³
  4. 4. 60 lb bags = 1.27 ÷ 0.452.82 → 3 bags*
  5. 5. * the table shows 2, see why below,

→ The table is the practical answer (2 bags). The math protects you when posts are at corners or gates.

Most installs round to 2 bags because the post displaces more volume in practice (gravel base, larger lower hole). Buy 3 if it's a corner or gate post, those carry tension and benefit from extra footing.

Total bags by fence size

Total 60 lb bags for a privacy fence (4×4 posts, 8 ft spacing, 10 × 30 in holes)
Fence lengthPostsBags (no waste)Buy
50 ft71416
100 ft132630
150 ft204044
200 ft265258
300 ft (½ acre perimeter)397886

How deep should a fence post hole be?

The rule of thumb is 1/3 of the post's above-grade height AND at least 6 inches below your local frost line. For a 6-foot fence in a non-freezing climate, that's a 24–30 inch hole. In northern states with a 36–42 inch frost line, you need to go that deep regardless of fence height, or your posts will heave every spring.

How wide should the hole be?

A common rule: hole diameter = 3× the post width. So a 4×4 post (3.5 inches actual) gets a 9–10 inch hole; a 6×6 post (5.5 inches actual) gets a 12-inch hole. Wider holes mean more concrete but also a more stable post, especially for gate posts and corners, which should always be one size larger and deeper.

Common mistakes (and how to avoid them)

✗ Treating gate and corner posts the same as line posts.

Fix: Gate posts carry constant pull. Corner posts brace two runs. Both need 1 size larger holes (10 → 12 in) and 6 inches deeper than line posts.

✗ Pouring concrete with no gravel under the post.

Fix: 4–6 inches of crushed stone at the bottom of the hole gives drainage so water can't pool against the post and rot it from below.

✗ Finishing the concrete flush with the ground or in a dish shape.

Fix: Crown the concrete 1 inch above grade, sloped away from the post. Pooled water at the base is the #1 killer of wood fence posts.

✗ Using all-purpose concrete for fence posts.

Fix: Use fast-setting concrete (Quikrete red bag, Sakrete Maximizer) for posts. Sets in 20–40 min, no pre-mix, plumb-and-walk-away install.

→ Calculate posts, bags, and total fence linear feet at once

Frequently asked questions

How many bags of concrete per 4×4 fence post?
2 bags of 60 lb (or 1.5 bag of 80 lb) for a standard 10 in × 30 in deep hole. Bump to 3–4 bags for a 12 in × 36 in hole used in frost zones.
How many bags of concrete per 6×6 fence post?
About 3 bags of 60 lb (or 2.5 of 80 lb) for a 12 in × 36 in hole. 6×6 posts are usually for heavy gates, pergolas, or privacy fences in frost-prone areas.
How many bags of fast-set concrete per fence post?
2.5 bags of 50 lb fast-set per 4×4 post in a 10 × 30 in hole; 4 bags per 6×6 in a 12 × 36 in hole. Fast-set sets in 20–40 minutes and doesn't require pre-mixing.
Do I need gravel under the post?
Yes, 4–6 inches of crushed stone at the bottom of the hole gives drainage so water can't sit against the post and rot it. Tamp it before setting the post.
Can I use fast-setting concrete instead?
Yes, Quikrete Fast-Setting (red bag) sets in 20–40 minutes and you don't pre-mix. Same yield concept (~0.375 ft³ per 50 lb bag), so the bag count is similar. It's the standard for fence posts.
Should the concrete be flush with the ground or crowned above it?
Crown it about 1 inch above grade and slope away from the post. This sheds water instead of pooling against the post, doubles the lifespan of wood posts.
How long until I can hang the fence panels?
Fast-set: 4 hours. Standard concrete: 24–48 hours before any tension. 7 days before a heavy gate or windload. Posts pulled too soon will tilt permanently.
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