Fast-setting or standard mix?

Both work for fence posts. Fast-setting is poured dry into the hole with water added after, which is quicker and suits a long run; standard mix is cheaper and gives you time to plumb the post. Yields differ, so check the bag and enter the figure printed on it.

Do I need gravel at the bottom?

Many builders use four to six inches for drainage, so the post end is not sitting in standing water. It also reduces the concrete needed, because it fills the bottom of the hole — a saving worth taking, and one most calculators do not subtract.

Should I fill the hole to the top?

Most stop an inch or two below grade and crown the surface so water runs away from the post, then backfill with soil. It sheds water better than a flush pour and uses slightly less concrete.

Can I set posts without concrete?

Compacted gravel alone is a real technique and drains better than concrete, which can trap water against the post. It needs more careful compaction and is not right for gate posts or tall fences. This calculator assumes concrete; if you are going gravel-only, the hole volume is still the number you need.

Fence Ledger

Fence Post Concrete Calculator

How much concrete your posts actually need — worked out from the hole, not from a rule of thumb that has no hole in it.

Units

Shape

Fence length

Height

Gates

Hole diameter

Hole depth

Gravel base

Bag yield

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.

Post spacing

Gate width

Picket gap

Waste allowance

Posts

Sections

Pickets

336

Concrete

35 bags

Estimated cost

Materials only (DIY)

$1,400 – $3,600

Professionally installed

$3,950 – $8,850

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
Material	Size	Quantity
Posts	4x4	21
Rails incl. waste	2x4	63
Pickets incl. waste	1x6	336
Concrete mix		35bags
Gravel		8bags
Exterior screws		≈2,200
Walk gate kit		1

Why these numbers

How we got the section count

Run 1: 146 ft ÷ 8 ft = 18.25, rounded up to 19 sections

How we got the post count

3 posts are fixed by the shape itself
2 at the open ends
1 carrying a gate
18 line posts spaced between them
21 posts in total

How we got the picket count

Each picket covers 5.5 in plus a 0.25 in gap, so they repeat every 5.75 in
146 ft of fence needs 305 pickets
Plus 10% for offcuts, rounded up to 336

How we got the concrete

Each 10 in hole is 30 in deep, less 4 in of gravel base, leaving 26 in of concrete
The 4x4 post itself takes up part of that, so each hole needs the remainder
Across 21 posts
Divided by the 0.6 cu ft a bag yields: 34.91, so buy 35 bags

Two bags per post is not an answer

It is the most repeated figure in fence building and it contains no hole diameter, which is the single thing the requirement depends on most. A quantity quoted without its geometry cannot be checked by anyone — including whoever quoted it.

Volume grows with the square of the radius. Widening a 10 inch hole to 12 inches is barely noticeable while you are digging and raises the concrete needed by about 45%. Depth matters too, but proportionally, which makes it the smaller lever of the two.

What actually gets calculated

The bore volume, less the gravel base at the bottom, less the space the post itself occupies. That last term is smaller than people expect: a 4×4 is actually 3.5 inches square, so it displaces about 11% of a 12 inch bore. Using the nominal 4 inches instead overstates it by nearly a third.

The gravel base is the term most calculators forget entirely. Four inches of gravel in a 30 inch hole is 13% of the depth, and it is 13% of concrete you do not need to buy or mix.

Bag yield is a number on the bag

An 80 lb bag of standard concrete mix yields about 0.6 cubic feet, a 60 lb bag about 0.45, a 50 lb bag about 0.375. Those are manufacturer figures and they differ between products.

Check the bag you are actually buying rather than trusting a number from a website — this one included. The field is editable for exactly that reason.

Common questions

Fast-setting or standard mix?: Both work for fence posts. Fast-setting is poured dry into the hole with water added after, which is quicker and suits a long run; standard mix is cheaper and gives you time to plumb the post. Yields differ, so check the bag and enter the figure printed on it.
Do I need gravel at the bottom?: Many builders use four to six inches for drainage, so the post end is not sitting in standing water. It also reduces the concrete needed, because it fills the bottom of the hole — a saving worth taking, and one most calculators do not subtract.
Should I fill the hole to the top?: Most stop an inch or two below grade and crown the surface so water runs away from the post, then backfill with soil. It sheds water better than a flush pour and uses slightly less concrete.
Can I set posts without concrete?: Compacted gravel alone is a real technique and drains better than concrete, which can trap water against the post. It needs more careful compaction and is not right for gate posts or tall fences. This calculator assumes concrete; if you are going gravel-only, the hole volume is still the number you need.