How to use this calculator
- Measure the total run. Add up every straight stretch. If the fence turns a corner, that is still one total length — the corner post is counted along with the rest.
- Pick the post spacing. Eight feet is the default because it matches standard rail lengths. Pre-built panels force six.
- Set the picket width and gap. Zero gap for solid privacy fence; half an inch or more for a picket fence.
- Check the hole dimensions. The concrete figure subtracts the post itself, which most estimates forget.
The formula
sections = total length ÷ post spacing, rounded up
posts = sections + 1
pickets = total length × 12 ÷ (picket width + gap), rounded up
rails = sections × rails per section
concrete per post (cu ft) = π × (hole diameter ÷ 24)² × depth − (post width ÷ 12)² × depth
Worked example — 100 ft of 6 ft privacy fence, posts at 8 ft, solid 5.5 in boards:
- Sections: 100 ÷ 8 = 12.5, so 13 sections
- Posts: 13 + 1 = 14 posts
- Boards: 1,200 ÷ 5.5 = 218.2, so 219 boards
- Rails, two per section: 13 × 2 = 26 rails at 8 ft
- Concrete per post, 10 in hole 2 ft 6 in deep around a 4×4: 1.36 − 0.21 = 1.15 cu ft
- Total concrete: 14 × 1.15 = 16.1 cu ft, which is 27 bags of 80 lb
That last line is 1.9 bags per post. If you are mixing hole by hole rather than in a barrow you cannot use nine tenths of a bag, so round to 2 per post and buy 28. The calculator gives the total volume figure; the difference between the two is one bag, and one spare bag of concrete is never the problem.
The “+1” on the post count is the classic fencepost error, and it is a whole post and two bags of concrete. A run of fence always needs one more post than it has sections.
Post spacing
| Spacing | When to use it |
|---|---|
| 6 ft | Tall privacy fence, or anywhere you are using pre-built panels |
| 8 ft | Most board fence — matches standard rail lengths |
| 10 ft | Rail and wire fence only |
Eight feet is the default because rails are sold in 8 and 16 ft lengths, so the spacing falls out of the material rather than the other way round.
Six feet is worth it for tall privacy fence. A solid 6 ft fence is a sail: it catches the full force of the wind rather than letting it through, and the load lands on the posts. Closer spacing, deeper holes and heavier posts are all cheaper than rebuilding a section that went over in a storm.
Post holes: depth, and why the post volume matters
The rule of thumb is a third of the post length in the ground. A 6 ft fence usually uses 8 ft posts, giving about 2 ft 6 in of depth.
Two things override that rule:
- Frost. Where the ground freezes, every hole must go below the local frost line. Frost heave lifts a shallow post out of the ground a little each winter and it does not go back down. Your municipality publishes the depth.
- Gates. Gate posts take a constantly swinging load and should be larger and deeper than the line posts — often a 6×6 where the rest are 4×4, and a foot deeper.
The concrete figure here subtracts the volume of the post from the hole, which most estimates do not bother with. For a 4×4 in a 10 inch hole that is about 15 percent of the volume — across 14 posts, roughly four bags of concrete you would otherwise have bought and not used.
Does every post need concrete?
Corner posts, end posts and gate posts always do. They take all the lateral load, and a fence fails at its ends first.
Line posts are a genuine choice. Concrete is the conventional answer. Compacted gravel is the alternative, and it has real advantages: it drains, so the buried part of the post stays drier and lasts longer, and a gravel-set post can be pulled and replaced without excavating a concrete plug. Concrete around a wooden post creates a bowl that holds water against it — which is why so many fence posts rot exactly at ground level.
If you are setting line posts in gravel, untick the box above and the concrete estimate drops to the four posts that need it.
Pickets, and how the gap changes the count
The picket count depends on the pitch — width plus gap — not the width alone.
Over 100 feet with 5.5 inch boards:
| Gap | Pitch | Boards |
|---|---|---|
| 0 in (solid) | 5.5 in | 219 |
| ½ in | 6 in | 200 |
| 5.5 in (full gap) | 11 in | 110 |
Nineteen boards is not nothing, and a spaced fence uses close to half the material of a solid one. If the gap is aesthetic rather than structural, it is worth deciding before you order.
Note that a solid privacy fence made of green pressure-treated boards will develop gaps on its own as the wood dries and shrinks. Butting them tight at installation is normal and intentional.
Before you dig
Two things, both of which are cheaper than the alternative.
Locate buried utilities. In the US, calling 811 is free and legally required before you dig. Fence posts are exactly deep enough to find gas, water and electrical service.
Check the boundary and the rules. Height limits, which side the “good” face must show, setback from the property line, and whether a permit is required are all set locally, and in many places by a homeowners’ association as well. A fence built on the wrong side of the line is a legal problem, not a carpentry one.
A material estimate. Depth, height and permitting are governed locally — confirm before you buy.