Fence Ledger

Fence Ledger

How many bags of concrete do I need per fence post?

The usual answer is two bags per post. That number contains no hole diameter, which is the thing the answer depends on most.

Concrete per post
1.71 bags

Based on a 10 in hole, 30 in deep, on a 4 in gravel base.

What the usual answer misses

Hole width drives the requirement far more than depth does, because volume goes up with the square of the radius. Widening a 10 in hole to 12 in — barely noticeable when you are digging — raises the concrete needed by about 45%. A quantity quoted without its hole size cannot be checked by anyone, including the person who quoted it.

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
24 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 mix24bags
Gravel6bags
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 10 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: 23.27, so buy 24 bags

Common questions

What size hole should I dig?
A common rule is roughly three times the post width, so about 10 to 12 in for a 4×4. Wider holes use markedly more concrete; deeper ones use proportionally more.
Why do two calculators give me different bag counts?
Almost always because they assume different hole sizes and neither says so. Compare the diameter, the depth, whether a gravel base was subtracted, and the bag yield used — get those four to agree and the answers converge. That is the reason all four are shown on this page rather than buried.
How deep should the post go?
Convention is about a third of the fence height, minimum 24 in. But the depth that actually governs is your local frost line, which can be much deeper, and local code. Check before you dig — this is not something to take from a website.
How long before I can hang rails on the posts?
Long enough that it is worth planning around. Fast-setting mix is usually firm within an hour or so but is not at strength for a day or more; standard mix wants longer still. Bracing every post plumb and leaving them overnight is the normal rhythm — building the fence in one continuous session is how posts end up out of line.