How Do I Calculate Fence Posts and Concrete?
Divide your fence length by post spacing and add one — that's your post count. Each hole takes roughly hole-volume minus post-volume worth of concrete. A 100-foot fence at 8-foot spacing needs 14 posts. For 12-inch-diameter holes 30 inches deep with 4×4 posts, you'll need about 27 cubic feet of concrete total — 45 bags of 80-lb mix or 60 bags of 60-lb mix, with 10% waste included.
Estimating fence materials is two independent calculations: post count (purely a function of fence length and spacing) and concrete volume (a function of hole size and post size). Do one then the other. The calculator above automates both and gives you bag counts ready to hand to the supplier.
Post-count formula:Posts = ceil(fence length ÷ post spacing) + 1. The "+1" covers the end post on a straight run. Corners, gate openings, and direction changes each add extra posts you'll factor in by hand.
Concrete-per-hole formula:Hole volume = π × (hole radius)² × hole depth. Subtract the post volume in the hole (cross-section × depth), and you have concrete per hole. Multiply by post count and add a waste factor.
How Deep Should Fence Post Holes Be?
One-third of the above-ground post height, minimum 24 inches, and always below the local frost line. A 6-foot privacy fence needs 24-inch-deep holes in warm climates. In freezing regions, go deeper — 36 inches across most of the US, 48+ inches in the far north.
Depth is the difference between a fence that stands for 30 years and one that leans in three. The pillar of stability is burying enough post to resist lateral wind load and, in cold climates, putting the post bottom below where the ground freezes. Freeze-thaw cycles will lift a too-shallow post a fraction of an inch each winter until the fence goes slack.
What Spacing Should I Use Between Posts?
8 feet is standard for wood privacy fences (matches 8-ft panel spans); 6 feet for chain-link and high-wind areas; 10 feet for split-rail. Never exceed the manufacturer's span rating on your panels or stringers.
The spacing decision is usually made by the fence system you're buying: pre-built 8-foot panels force 8-foot spacing; 6-foot chain-link top rails come in 10.5-foot lengths designed for 10-foot spans. Custom-built fences are more flexible, but closer spacing means stiffer fence and more materials cost.
4×4 vs 6×6 — Which Post Size Should I Use?
4×4 for most residential fences up to 6 feet; 6×6 for 8-foot fences, gate posts, and corner posts. The extra dimension on a 6×6 is worth it anywhere bending load is concentrated — gates, corners, wind-exposed ends.
A 4×4 lumber post is actually 3.5 inches square (US Softwood Lumber Standard PS 20), and a 6×6 is actually 5.5 inches. The stiffness-to-bending ratio roughly triples from 4×4 to 6×6, which is why every gate post and corner post should upgrade regardless of what the line posts are. Gates in particular will sag in a year or two on 4×4 posts carrying real weight.
Concrete, Gravel, or Dry-Set — Which Footing Should I Use?
Concrete for privacy, gate, and corner posts. Tamped gravel is acceptable for split-rail and ranch fencing in well-draining soil. Dry-set (no backfill mix at all) only works for temporary or very light fencing.
Concrete is the long-term choice: it locks the post against lateral load, blocks water migration to the post base, and doesn't shift seasonally. The tradeoff is that concrete around a wood post trap moisture and can accelerate rot — which is why pressure-treated posts (ground-contact rated) or metal standoff brackets are worth the small extra cost.
What Are the Concrete Bag Yields?
An 80-lb bag of concrete mix yields about 0.6 cubic feet; a 60-lb bag yields 0.45 cu ft. These are the same yields used in the Concrete Calculator. Multiply total concrete volume by 1.67 to get 80-lb bags, or 2.22 to get 60-lb bags — but it's easier to just use the calculator.
Do Corner and Gate Posts Need Special Treatment?
Yes. Upsize both to 6×6 minimum, dig the holes deeper and wider, and add diagonal bracing during the concrete cure. The load on a corner post is double a line post (two runs of fence pulling outward); gate posts take dynamic load from every swing.
Related Guides
- How to Install Fence Posts — Eight-step install guide covering 811 calls, frost-depth holes, corner-post bracing, and concrete cure
Cost varies by region. The Estimated Material Cost card pulls from our indicative national-average pricing dataset(refreshed quarterly). Northeast and California metros run 15–40% above the national midpoint while Midwest and Southeast metros run 5–15% below — verify locally for binding quotes.
How Much Does a 4×4×8 PT Pine Post Cost?
4x4x8 PT pine post averages about $16.50 per each nationally as of our April 2026 research. PT southern yellow pine #2 ground contact. Regional variation is significant — Northeast and California metros run 15–40% above the national midpoint while Midwest and Southeast metros run 5–15% below. See our pricing methodology for sources and confidence tiers.
Related Calculators
- Concrete Calculator— Estimate concrete for slabs, footings, and columns
- Gravel Calculator— Calculate tamped-gravel post footings
- French Drain Calculator— Size drainage for fence lines in wet yards
Written by Daniel McCarney — AceCalc