Fence Post Size Chart: Depth, Length & Concrete Guide

How fence post size is worked out

A fence post is sized from the fence height. The buried depth is one-third of the height plus 6 inches, rounded up to the next 6 inches — the third keeps the post standing against the wind, the extra 6 inches gets it below the frost and the soft top soil. The post size is a 4×4 up to 6 ft and a 6×6 for 7 ft and 8 ft, because a taller fence catches more wind and needs a stiffer post. The post length is the fence height plus the buried depth, rounded up to the next whole foot. The spacing is 8 ft up to 6 ft and 6 ft for the taller fences. The hole is 12 inches across for a 4×4 and 14 inches for a 6×6. The concrete is worked out from the hole volume minus the buried post volume, in 80-lb bags that yield 0.6 cu ft each, rounded up — you cannot buy part of a bag.

At the default — 6 ft fence, 100 ft long, 8 ft spacing — the buried depth is 72 ÷ 3 + 6 = 30 in, the post is a 4×4, the length is 6 + 2.5 = 8.5 ft rounded up to 9 ft, the hole is 12 in across, the net concrete is 1.75 cu ft a post, which is 3 bags a post. The fence needs 100 ÷ 8 = 12.5, rounded up to 13, plus 1 = 14 posts, so 42 bags in total, which is $315.00 at $7.50 a bag.

The worked example, line by line

Setup: fence height 6 ft, fence length 100 ft, post spacing 8 ft, 80-lb bags at $7.50 a bag.

Line 1 — the buried depth. 72 ÷ 3 + 6 = 30 in.

Line 2 — the post size. 6 ft is at most 6 ft, so a 4×4.

Line 3 — the post length. 6 + 30 ÷ 12 = 8.5 ft, rounded up to 9 ft.

Line 4 — the hole. 12 in across for a 4×4, so the hole volume is π × 6² × 30 = 1.96 cu ft.

Line 5 — the buried post. A 4×4 is actually 3.5 × 3.5 in, so the buried post is 3.5 × 3.5 × 30 = 0.21 cu ft.

Line 6 — the net concrete. 1.96 − 0.21 = 1.75 cu ft a post.

Line 7 — the bags per post. 1.75 ÷ 0.6 = 2.92, rounded up to 3 bags.

Line 8 — the posts. 100 ÷ 8 = 12.5, rounded up to 13, plus 1 = 14 posts.

Line 9 — the total. 14 × 3 = 42 bags, which is 42 × $7.50 = $315.00.

Reverse: a $300.00 concrete budget buys 40 bags; at 3 bags a post that is 13 posts, which at 8 ft spacing is 96 ft of 6 ft fence (39 bags, $292.50); at 6 bags a post that is 6 posts, which at 6 ft spacing is 30 ft of 8 ft fence (36 bags, $270.00).

Every branch, checked

4 ft fence, 100 ft: buried 24 in, a 4×4 post 6 ft long, 1.40 cu ft of concrete, 3 bags a post. 14 posts, 42 bags, $315.00 — the same as the 6 ft fence, because the bags per post are the same.

8 ft fence, 100 ft at 6 ft spacing: buried 42 in, a 6×6 post 12 ft long, 3.01 cu ft of concrete, 6 bags a post. 18 posts, 108 bags, $810.00 — the dearest case on the page.

6 ft fence, 50 ft: the same post as the default, 3 bags a post, but only 8 posts: 50 ÷ 8 = 6.25, rounded up to 7, plus 1 = 8. 24 bags, $180.00.

5 ft fence, 100 ft: buried 30 in, a 4×4 post 8 ft long, 1.75 cu ft of concrete, 3 bags a post. 14 posts, 42 bags, $315.00.

7 ft fence, 100 ft at 6 ft spacing: buried 36 in, a 6×6 post 10 ft long, 2.58 cu ft of concrete, 5 bags a post. 18 posts, 90 bags, $675.00.

3 ft fence, 100 ft: buried 18 in, a 4×4 post 5 ft long, 1.05 cu ft of concrete, 2 bags a post. 14 posts, 28 bags, $210.00 — the cheapest case on the page.

6 ft fence, 200 ft: the same post as the default, 3 bags a post, but 26 posts: 200 ÷ 8 = 25, plus 1 = 26. 78 bags, $585.00.

Reverse — the other way round: a $300.00 concrete budget buys 40 bags; at 3 bags a post that is 13 posts, which at 8 ft spacing is 96 ft of 6 ft fence; at 6 bags a post that is 6 posts, which at 6 ft spacing is 30 ft of 8 ft fence.

What the defaults assume

The model assumes the fence height, length and spacing you enter, the chart’s depth and size rules, 80-lb bags yielding 0.6 cu ft, and $7.50 a bag. It assumes the post volume is the actual milled size — a 4×4 is 3.5 × 3.5 in and a 6×6 is 5.5 × 5.5 in — because the concrete fills the hole around the post that is actually there. A fractional height uses the next whole foot up: a 5.5 ft fence uses the 6 ft row. It leaves out gates, corners and end posts that need more concrete, slope, rock, water in the hole, and the fence panels or pickets themselves — the $315.00 at the defaults is the concrete for the posts only, not the fence. Rounding convention: depths and lengths are whole numbers rounded up, concrete is shown to two decimals, bags and posts are whole numbers rounded up, and all figures are computed from unrounded values, rounded only for display.

Output · formula · meaning

Who uses this

• The homeowner pricing a backyard fence: they learn a 6 ft fence needs a 4×4, 9 ft long, buried 30 in, with 3 bags a post — so they know the shopping list, and they check the 100 ft case at $315.00 before buying the concrete.
• The DIYer digging the holes: they learn the hole is 12 in across for a 4×4 — so they know which auger to rent, and they check the 8 ft row at 14 in before digging the corner posts.
• The contractor bidding a privacy fence: they learn an 8 ft fence needs 6 bags a post and 18 posts per 100 ft — so they know the concrete alone is $810.00, and they check the 7 ft case at $675.00 before quoting the job.
• The budget builder stretching $300: they learn $300.00 buys 96 ft of 6 ft fence — so they know the run that fits the budget, and they check the 30 ft case for 8 ft fence before choosing the height.

Accuracy & limitations

The math is exact for the numbers you enter: the depth is a third of the height plus 6 inches rounded up to the next 6 inches, the post length is the height plus the depth rounded up to the next foot, the bags are the net hole volume divided by 0.6 rounded up, and the posts are the length divided by the spacing rounded up plus 1. What the model cannot do is see your soil: check the depth against your local frost line, because a shallow hole in a cold climate heaves. It assumes straight runs and leaves out gates, corners, slope, rock and the panels or pickets. For the cost of the whole fence, see fence cost calculator; for the concrete behind it, see concrete calculator.