Fence layout and post spacing fundamentals
Every post-and-rail fence begins with establishing the perimeter run in linear feet. Post spacing is typically 6 feet or 8 feet on-center. Because 2×4 horizontal lumber rails are sold in 8-foot lengths at lumberyards, 8-foot on-center spacing is the most economical and common standard across North America.
To find the number of post sections (bays), divide total fence length by post spacing and round up. Total line and corner posts equal the number of bays plus one (the starting terminal post). If you include gates, add one additional heavy post per gate opening.
How do you calculate square footage from feet and inches?
Tape measures read in feet and inches, but area formulas require decimal feet. Divide any inch measurement by 12 before multiplying length by width, then divide square feet by 9 when ordering carpet or turf sold by the square yard:
where:
- L_ft, W_ft – Whole feet measured along the room or slab length and width;
- L_in, W_in – Extra inches converted to decimal feet (6 in = 0.50 ft, 3 in = 0.25 ft); and
- Area_sq ft – Net surface area in square feet. Convert area into 4×8 panels with the plywood sheet calculator or slab yardage with the concrete calculator.
Rail counts: two rails vs. three rails
The number of horizontal 2×4 rails depends directly on fence height:
Fences 4 feet high or shorter require two rails (top and bottom). However, standard 6-foot privacy fences and 8-foot security fences require three rails (top, center, and bottom). Omitting the center rail allows thin 5/8-inch wood pickets to cup, twist, and pull away from screws under baking summer sun.
| Fence Height | Recommended Rails | Rail Lumber Dimension | Fastener Recommendation |
|---|---|---|---|
| 4 ft picket / garden fence | 2 rails (top & bottom) | 2×4 × 8 ft pressure-treated | 3" exterior polymer screws |
| 6 ft residential privacy | 3 rails (top, mid, bottom) | 2×4 × 8 ft pressure-treated | 3" exterior polymer screws |
| 8 ft tall privacy / estate | 4 rails (every 24" vertical) | 2×4 × 8 ft pressure-treated | 3" exterior polymer screws |
| Split-rail agricultural | 2 to 3 cedar round rails | 10 ft drilled round rails | Mortise-and-tenon post joints |
Worked example: 100 ft residential privacy fence
Material takeoff for a 100-foot long, 6-foot tall privacy fence with 8-foot post spacing and 1 walk gate:
4. Vertical pickets: Using standard 5.5-inch dog-ear cedar pickets with a 1/4-inch expansion gap (5.75 inch on-center pitch): (100 ft × 12) ÷ 5.75 = 208.7 pickets. Adding 5% cutting waste equals 220 pickets.
| Material Item | Calculated Need | Recommended Order |
|---|---|---|
| 4×4 pressure-treated posts | 13 bays + terminal posts | 15 sticks (4×4 × 8 ft or 9 ft) |
| 2×4 horizontal framing rails | 13 bays × 3 rails | 39 sticks (2×4 × 8 ft lumber) |
| 5.5" dog-ear cedar pickets | 209 base pickets | 220 pickets (with 5% waste) |
| Fast-setting post concrete | 2 bags per hole | 30 bags (50-lb dry mix) |
| Picket exterior deck screws | 6 screws per picket (3 rails) | 1,320 screws (1-5/8" ceramic) |
Picket spacing and wood species selection
Western red cedar and pressure-treated southern yellow pine are the two predominant wood fencing materials in North America.
Pressure-treated pine pickets are wet from chemical preservation; install them tightly edge-to-edge because they will shrink 1/8 to 1/4 inch during their first sunny month. Conversely, dry cedar pickets should be installed with an intentional 1/8 to 1/4 inch gap to accommodate wet-weather thermal expansion without buckling.
Post hole and concrete foundation checklist
- Call 811: Always call utility locators before digging; fence lines frequently cross telephone, cable TV, and buried electric lines.
- Depth below frost line: Dig post holes at least 24 to 36 inches deep (below your regional frost line) to prevent winter frost heaving.
- Gravel drainage base: Dump 3 inches of crushed stone at the bottom of each hole before dropping the post to keep end grain out of standing water.
- Plumb and brace: Use a post level and brace each post in two directions with diagonal 1×4 stakes before pouring concrete.
- Slope concrete collar: Trowel the top surface of the concrete footing slightly above ground level with a dome slope so rainwater drains away from the wood post.