Fence Post Calculator
Post count, real spacing, hole depth with frost line and concrete bags for 4x4 and 6x6 posts — the whole materials list in one pass.
Post count, real spacing, hole depth with frost line and concrete bags for 4x4 and 6x6 posts — the whole materials list in one pass.
Each gate adds 2 heavier gate posts.
How deep the ground freezes in your area. Set 0 if unknown or irrelevant.
Post size
Enter your fence run to get the post count, hole specs and concrete order.
Frequently asked questions
How many fence posts do I need for 100 feet?
Divide the run by your target spacing and round up, then add one: 100 feet at 8 foot spacing needs 13 sections and 14 line posts. If the run includes a gate, add 2 posts per gate because gate posts are separate, heavier members. Spacing at 6 feet instead of 8 raises the count to 17 — the calculator shows the exact count plus the evenly divided spacing you should actually measure to.
How deep should a 6 foot fence post be?
The rule of thirds: bury one third of the total post length, so a 6 foot fence on 9 foot posts means 3 feet in the ground. The calculator adds a 6 inch gravel layer beneath the post for drainage, so it asks you to dig 42 inches when the burial depth is 36. Where the frost line is deeper than the burial depth — much of the northern US freezes 36 to 60 inches down — dig to below the frost line instead, or freeze heave will jack the posts out of the ground within a few winters.
How much concrete per fence post?
For a 4x4 post in a 12 inch diameter hole, plan on about 1.4 cubic feet of concrete — three 80 pound bags or four 50 pound bags per post — when the pour is 24 inches deep. The hole is nominal width times three so wet concrete can flow around the post, and the post itself displaces volume, which is why the per-hole number is smaller than the full hole volume. The calculator totals bags across all posts and rounds up per hole.
Is a 4x4 post enough for a 6 foot fence?
For typical wood privacy and picket fences, yes — a 4x4 set 3 feet deep in concrete handles 6 foot fencing in normal wind. Step up to 6x6 posts when the fence is taller than 6 feet, when panels act like sails (solid privacy in open terrain), for gate posts carrying heavy swing gates, or anywhere high-wind storms are routine. 6x6 posts also resist twist better, which keeps rails and pickets aligned over the years.
What post spacing should I use for wind?
Solid privacy panels in open or coastal wind zones justify tightening spacing from 8 feet to 6 feet: each post then carries roughly 25% less panel area, and the shorter span reduces rail sag between posts. Chain link in the same exposure usually keeps 8 or even 10 foot spacing because the fabric passes air. If your area regularly sees straight-line storms, the cheapest upgrade is deeper holes below the frost line rather than more posts.
How far apart should fence posts be
Six feet or eight feet is the real decision, and it is a trade between lumber and labor. At 8 foot spacing a 100 foot run needs 14 posts; tightening to 6 feet raises that to 17. More posts mean more holes, more concrete and more hours with a post-hole digger — but also shorter rail spans that sag less, a stiffer fence in wind, and smaller loads per post.
The number that matters after rounding is actual spacing. Because sections round up to a whole number, a 100 foot run at a target of 8 feet really divides into 13 spans of 7 feet 8 5/16 inches. Lay it out from both ends toward the middle or mark each post position individually — measuring 8 foot increments down the line and hoping the last bay fits is how fences end up with one awkward 3 foot section at the corner.
Match spacing to what the fence carries. Shadowbox and solid privacy panels push more wind load into each post than spaced pickets; chain link passes air and forgives wide spacing; agricultural fencing often runs 12 to 16 feet with t-posts between wood posts. When in doubt, one extra post per run is the cheapest structural insurance you will ever buy.
Post depth and frost line
Depth is what keeps a fence upright; concrete just keeps the hole from collapsing. The 1/3 rule — one third of the post below grade — is the industry baseline because lateral wind load on the panels turns the post into a lever, and the ground has to resist the pull-out force on the windward side. Undersized depth shows up as rocking posts within a season or two.
Where the ground freezes, depth is dictated by the frost line rather than the 1/3 rule. Water in soil expands about 9% when it freezes, and ice grips a post's concrete collar and lifts it — a process called frost heave that can raise a fence an inch a winter until it looks like a funhouse. Digging the bell of the hole below the local frost line, with a 6 inch gravel base under the concrete for drainage, is the standard defense; the gravel keeps water from pooling and freezing against the post bottom.
A final ordering tip: count bags, not cubic feet, at the store. An 80 pound bag yields about 0.6 cubic feet of mixed concrete and a 50 pound bag about 0.375, so the calculator rounds up per hole — a half bag left over at every post is still better than a short pour at the last one. Mix consistently wet enough to settle around the post with a scrap of rebar, crown the top slightly so water sheds, and let it cure before racking the panels.
Estimates are for planning only. Check local building codes and utility locates before digging — call 811 in the US.
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