Fence Ledger

Fence Ledger

How much concrete for a 4x4 post in a 12 inch hole?

A 12 inch hole is at the wide end of what a 4×4 post needs, and it uses noticeably more concrete than a 10 inch one.

Concrete per post
2.57 bags

Based on a 12 in hole, 30 in deep, on a 4 in gravel base.

What the usual answer misses

The post itself displaces less than you would think. A 4×4 is actually 3.5 in square, so it occupies about 12.25 square inches of a 113 square inch bore — around 11%. The gravel base takes out more than the post does. Using the nominal 4 in figure instead of the real 3.5 in overstates the displacement by nearly a third.

Work out your own

Every assumption above is editable. Change the length, the spacing, the hole size — the numbers and the working update as you go.

Units
Shape
Advanced options

These are typical values, not requirements. Post depth in particular is governed by your local frost line and building code — check before you dig.

Posts
14
Sections
13
Pickets
230
Concrete
36 bags

Estimated cost

Materials only (DIY)
$800 – $2,000
Professionally installed
$2,500 – $5,500

National planning ranges compiled from published US construction cost guides, last checked September 2026 (confidence: medium-low). Prices vary sharply by region, season and contractor. These are estimates for planning, not quotes.

Sources: homeguide.com, calcsummit.com, ergeon.com, homewyse.com

Shopping list

Bill of materials for this fence
MaterialSizeQuantity
Posts4x414
Rails incl. waste2x443
Pickets incl. waste1x6230
Concrete mix36bags
Gravel8bags
Exterior screws≈1,500

Why these numbers

How we got the section count
  1. Run 1: 100 ft ÷ 8 ft = 12.5, rounded up to 13 sections
How we got the post count
  1. 2 posts are fixed by the shape itself
  2. 2 at the open ends
  3. 12 line posts spaced between them
  4. 14 posts in total
How we got the picket count
  1. Each picket covers 5.5 in plus a 0.25 in gap, so they repeat every 5.75 in
  2. 100 ft of fence needs 209 pickets
  3. Plus 10% for offcuts, rounded up to 230
How we got the concrete
  1. Each 12 in hole is 30 in deep, less 4 in of gravel base, leaving 26 in of concrete
  2. The 4x4 post itself takes up part of that, so each hole needs the remainder
  3. Across 14 posts
  4. Divided by the 0.6 cu ft a bag yields: 35.41, so buy 36 bags

Common questions

Is a 12 in hole overkill for a 4×4?
Not necessarily — corner posts, gate posts and tall fences all justify it. For line posts on a standard 6 ft fence, 10 in is common and uses meaningfully less concrete.
Does the post displace much concrete?
About 11% of a 12 in bore. It is worth subtracting, but it is not the dominant term. Hole diameter is.
Should I fill the hole completely?
Most builders stop an inch or two below grade and crown the top so water runs away from the post, then backfill with soil. That slightly reduces the concrete needed compared with filling flush.