Planning a fence line
A fence is a sequence of sections between posts. Once you know the length of the line and the panel width, the number of posts follows, and each post needs a hole and concrete. This calculator counts posts, panels and concrete bags, and adds up the cost of materials.
The formulas
- Sections = fence length ÷ post spacing, rounded up.
- Posts = sections + 1, plus extra posts for corners and gates.
- Concrete per post = hole volume − post volume. The hole is a cylinder: 3.14 × (diameter ÷ 2)² × depth.
- Bags = posts × concrete per post ÷ bag yield. An 80 lb bag of concrete mix yields about 0.6 cubic feet.
Worked example: a 150 ft backyard fence
The fence uses 8 ft wood panels and 4×4 posts. It has one corner and a gate, so two extra posts are added. Each hole is 10 in wide and 30 in deep.
- Sections: 150 ÷ 8 = 18.75, so 19 panels.
- Posts: 19 + 1 + 2 = 22 posts.
- Concrete per post: (78.5 − 12.25 sq in) × 30 in = 1,989 cu in, or 1.15 cubic feet.
- Bags: 22 × 1.15 ÷ 0.6 = 42.2, so 43 bags of 80 lb.
- At $75 a panel, $20 a post and $7 a bag, materials cost about $2,166, or $14.44 per foot.
Materials by fence length
| Length | Posts | Panels | 80 lb bags | Cost |
|---|---|---|---|---|
| 50 ft | 10 | 7 | 20 | $865 |
| 100 ft | 16 | 13 | 31 | $1,512 |
| 150 ft | 22 | 19 | 43 | $2,166 |
| 200 ft | 28 | 25 | 54 | $2,813 |
Post holes
- Depth: about one third of the post’s total length, so 2 to 3 ft for a 6 ft fence. In cold climates, dig below the frost line, which your building department can tell you.
- Diameter: about three times the post width, so 10 to 12 in for a 4×4.
- Gravel: 3 to 6 in of gravel at the bottom of each hole helps drainage and slows rot.
Before you dig
Call 811 a few days before digging: it is free, and utility companies will mark buried lines. Check property lines, local height limits and any homeowners association rules. Many towns require a permit for fences above a certain height.
Good to know
Gates need sturdier posts, often 6×6 or set deeper, and a gate frame wider than the opening by the hinge and latch clearance. Corners on a sloped site may need stepped panels or racked panels. Add posts as needed and run the calculator again.