How much fence forty acres take
Forty acres are 1,742,400 square feet, a sixteenth of a square mile. Fenced as a square, each side is 1,320 ft, a quarter mile, and the fence around it is 5,280 ft: exactly one mile. As a 2:1 rectangle, 1,866.8 by 933.4 ft, the same forty acres take 5,600 ft, 320 ft or 6% more, 1.06 miles.
Side = √(40 × 43,560) = 1,320 ft; fence = 4 × 1,320 = 5,280 ft
The quarter mile side lines up with how wire is sold. The University of Kentucky notes that barbed wire comes in 80 rod rolls of 1,320 ft, so each side of a square forty takes one roll per strand before waste and splices.
Pull posts every 660 ft
A quarter mile of wire cannot be tensioned from the corners alone. Kentucky braces runs up to 660 ft with double H-braces and adds a braced pull post every 660 ft beyond that, so each full 1,320 ft side gets one pull post at its midpoint. With one 12 ft gate, one side is split into two 654 ft runs that need none, and the square forty needs:
- 3 pull posts
- 26 brace posts and 26 brace rails
- 4 corner and 2 gate posts
- 35 wood posts in total, whatever wire you hang
NRCS California spaces in line pull assemblies every 1,320 ft, the "length of a spool of wire". On a square forty that puts them only at the corners, and saves the 3 pull posts and their 6 brace posts. The Iowa Beef Center's electric fence guide asks for one strainer per quarter mile, which also works out to one per side.
When high tensile saves money
On a mile of fence, line posts are the biggest count, and spacing decides it. For the square forty with one gate:
| Fence | Post spacing | T-posts | Wire rolls | Installed |
|---|---|---|---|---|
| Barbed wire, 5 strand | 12 ft | 476 | 22 of 1,320 ft | $6,740 to $32,210 |
| High tensile, 5 wire | 25 ft | 226 | 8 of 4,000 ft | $6,470 to $13,770 |
| High tensile, 5 wire | 40 ft | 138 | 8 of 4,000 ft | same per foot range |
High tensile needs 250 fewer T-posts at 25 ft, and 338 fewer at 40 ft, and fewer, longer rolls. Kentucky rates 12½ gauge Class III high tensile at about 1,800 lb breaking strength, tensioned to 200 lb, and gives it a post spacing of 16 to 60 ft. The Iowa Beef Center adds that 40 to 60 ft is realistic for high tensile electric.
Iowa State University Extension's 2024 budget, priced for Iowa, puts non-electric high tensile at $0.92 per foot against $1.16 for 5 strand barbed wire. Over the 5,268 ft of fence after the gate, that is about $4,850 against $6,110, a saving near $1,260. Electrified high tensile at $1.22 comes to about $6,430, still close to barbed wire, and a hot fence can use the wider spacing. The national cost guides agree on direction: their top price for high tensile, $2.50 per foot, is well under the $6 top for barbed wire.
Powering a mile of electric fence
NRCS Iowa sizes a charger at one joule per mile of wire, counting every strand. Two hot wires around the square forty are 10,536 ft, 2.0 miles of wire, so about 2 joules. Five electrified high tensile wires are 26,340 ft, just under 5 miles, so about 5 joules. NRCS Iowa calls for 3 ft of ground rod per joule, 15 ft for the five wire fence, with rods at least 10 ft apart.
Compare the 20 acre page, or size an electric fence on its own with the electric fence calculator. The barbed wire calculator and the fence calculator cover the rest.