Fence Ledger

Fence Ledger

How many bags of concrete for 20 fence posts?

Twenty posts is roughly a hundred and fifty feet of fence at standard spacing, which makes this one of the most commonly ordered quantities there is.

Concrete needed
34 bags

80 lb bags, for 20 posts in 10 in holes 30 in deep on a 4 in gravel base. Change any of those and the count changes.

What the usual answer misses

A bag count for twenty posts that does not state a hole diameter cannot be checked by anyone, including the person who wrote it. The familiar answer of two bags per post gives a round forty, but the true figure for twenty posts ranges from the mid thirties to over sixty depending only on how wide and how deep the holes are. Ordering to the rule of thumb means either a wasted pallet or a second trip.

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
21
Sections
19
Pickets
340
Concrete
35 bags

Estimated cost

Materials only (DIY)
$1,416 – $3,640
Professionally installed
$4,000 – $8,960

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, angi.com, ergeon.com, homewyse.com

Shopping list

Bill of materials for this fence
MaterialSizeQuantity
Posts4x421
Rails incl. waste2x463
Pickets incl. waste1x6340
Concrete mix35bags
Gravel8bags
Exterior screws≈2,200
Walk gate kit1

Why these numbers

How we got the section count
  1. Run 1: 148 ft ÷ 8 ft = 18.5, rounded up to 19 sections
How we got the post count
  1. 3 posts are fixed by the shape itself
  2. 2 at the open ends
  3. 1 carrying a gate
  4. 18 line posts spaced between them
  5. 21 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. 148 ft of fence needs 309 pickets
  3. Plus 10% for offcuts, rounded up to 340
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 21 posts
  4. Divided by the 0.6 cu ft a bag yields: 34.91, so buy 35 bags

Common questions

Should I order a few spare bags?
A couple, yes. Unopened bags can usually be returned, and running out with three holes left is a genuinely bad afternoon because a partly set batch cannot wait for you. Store the spares dry and off the ground.
How much water will twenty posts take?
Each bag has a stated water requirement on it, so multiply that out and add margin. It is worth working out before you start, because a hose that does not reach the far end of the run is a problem discovered at the worst moment.
Can I do twenty posts in a day?
Digging and setting twenty holes is a hard day for two people with a powered auger and considerably more than a day by hand. Most people split it: dig everything first, then set, so the concrete work happens in one continuous stretch.