BuildWellHow a wood fence goes together
A wood fence is built in sections. Posts are set in the ground, usually in concrete, at an even spacing. Rails run horizontally between the posts, two or three to a section. Pickets are fastened to the rails side by side, tight for a privacy fence or with a gap for a picket fence.
That makes a fence easy to count. The length and the post spacing give the sections; the sections give the posts and rails; the length and the picket width give the pickets. The post holes give the concrete.
This calculator counts a straight run. Every gate, corner and separate run adds a post, so enter those as extra posts. Gates themselves are built and hung as their own line.
The fence formulas
- Sections = fence length ÷ post spacing, rounded up.
- Posts = sections + 1, plus one for each gate, corner or separate run.
- Rails = sections × rails per section.
- Pickets = fence length × 12 ÷ (picket width + gap), rounded up.
- Concrete per post = (π × (hole diameter ÷ 2)² − post width²) × concrete depth, in cubic inches, ÷ 1,728 for cubic feet.
- Bags = concrete per post × posts ÷ the bag’s yield, rounded up.
Use actual sizes. A 4 × 4 post is about 3½ inches square, and a 1 × 6 picket is about 5½ inches wide. Typical bag yields are 0.6 cubic feet for 80 lb, 0.45 for 60 lb and 0.3 for 40 lb.
Posts, rails and pickets by fence length
| Fence | Posts | Rails | Pickets | 80 lb bags |
|---|---|---|---|---|
| 50 ft | 8 | 21 | 110 | 13 |
| 75 ft | 11 | 30 | 164 | 17 |
| 100 ft | 14 | 39 | 219 | 22 |
| 150 ft | 20 | 57 | 328 | 31 |
| 200 ft | 26 | 75 | 437 | 40 |
| 300 ft | 39 | 114 | 655 | 60 |
Concrete per post
| Post | Hole | Concrete per post | 80 lb bags per post | 60 lb bags per post |
|---|---|---|---|---|
| 4 × 4 | 10 in wide, 24 in of concrete | 0.92 cu ft | 1.5 | 2 |
| 4 × 4 | 12 in wide, 30 in of concrete | 1.75 cu ft | 2.9 | 3.9 |
| 6 × 6 | 12 in wide, 30 in of concrete | 1.44 cu ft | 2.4 | 3.2 |
| 6 × 6 | 16 in wide, 36 in of concrete | 3.56 cu ft | 5.9 | 7.9 |
A wider hole holds much more concrete: going from a 10 inch to a 12 inch hole adds about half again to the concrete around a 4 × 4. Size holes to the post and the soil, not bigger than they need to be.
Worked examples
100 feet of 6 foot privacy fence
Posts 8 feet apart: 100 ÷ 8 = 12.5, so 13 sections and 14 posts. Three rails per section is 39 rails. Pickets 5½ inches wide, no gap: 1,200 inches ÷ 5.5 = 218.2, so 219 pickets.
Holes 10 inches across with 24 inches of concrete around a 4 × 4: 78.54 − 12.25 = 66.29 square inches, times 24 = 1,590.96 cubic inches, or 0.92 cubic feet a post. For 14 posts, 12.89 cubic feet: 22 bags of 80 lb mix.
150 feet of picket fence with a gate
Posts 6 feet apart: 150 ÷ 6 = 25 sections, 26 posts, plus one for the gate and one for a corner, 28 posts. Two rails a section is 50 rails. Pickets 3½ inches wide with a 1½ inch gap take 5 inches each: 1,800 ÷ 5 = 360 pickets.
Smaller 8 inch holes with 18 inches of concrete hold 0.40 cubic feet each, 11.09 for all 28: 25 bags of 60 lb mix.
Checking a single post
A 6 × 6 gate post in a 12 inch hole with 30 inches of concrete: π × 6² = 113.1 square inches, less 30.25 for the post, is 82.85 square inches. Times 30 inches, 2,485 cubic inches, or 1.44 cubic feet: about 2.4 bags of 80 lb mix.
Spacing, rails and post depth
Post spacing. Six to eight feet is typical for wood fences. Eight feet makes the most of 8 foot rails; six feet makes a stiffer fence and suits heavier or taller designs. Vinyl and other panel fences are set by the panel width, so follow the maker.
Rails. Many builders use two rails on fences up to about four feet and three on six-foot privacy fences, so the pickets stay flat and don’t cup.
Post depth. A common rule of thumb is to bury a third to a half of the post’s height above ground, and to go below the frost line where the ground freezes. A few inches of gravel in the bottom of the hole helps drainage. Local code and conditions come first.
Hole width. About three times the post’s width is a common rule of thumb: roughly 10 inches for a 4 × 4 and 16 inches for a 6 × 6.
Before you dig. In the US, call 811 a few days ahead so buried utility lines are marked. Confirm the property line, and check any setback, height and permit rules with the city or the homeowners’ association.
Common mistakes
Forgetting gate and corner posts. Each gate needs a post on each side, and each corner is a post. They’re also the posts that work hardest, so many builders use a heavier post there.
Nominal picket widths. A "6 inch" picket is about 5½ inches. Count with the real width.
Short posts. A 6 foot fence on posts set 2 feet deep needs 8 foot posts with nothing to spare. Check the post length against the depth before ordering.
Ignoring the ground. Slopes need racking or stepping, and rock or roots slow digging. Walk the line before you price it.
Too little concrete. Run the numbers per post. A bag short on the last hole means a trip back.
From quantities to an estimate
A fence estimate also covers old fence removal and disposal, layout, digging, setting posts, rails and pickets, gates and hardware, and cleanup. Staining or sealing is its own line.
Vinyl panel and chain link fences go up differently from a wood fence, so price them on their own lines. Removing an old fence is a separate line too.
When you estimate a fence in BuildWell, describe it by voice, text or photos and it plans those lines with labor hours. Lumber, concrete and hardware are priced at stores near the job, checked within 30 days, and labor at your metro’s carpenter wage plus your overhead and markup. You can change any quantity, hour or price before you send it.
