Fence Calculator
15 posts and 13 sections.
| Material | Quantity |
|---|---|
| Sections at 8 ft spacing | 13 |
| Posts sections + 1, plus 1 for gates | 15 |
| Post length to buy above grade + buried | 9 ft each |
| Post hole depth a third of the post | 3 ft |
| Rails | 39 (312 linear ft) |
| Pickets 5.5 in wide, 0 in gap | 219 |
| Concrete post volume deducted | 23.2 ft³ — 52 × 60 lb bags |
A run of N sections needs N + 1 posts to bracket it, and every gate adds one more on its far side — the off-by-one that leaves people a post short. Post holes are sized on the standard rule: bury a third of the post's total length, minimum 600 mm, in a hole about three times the post width. Concrete volume deducts the post itself, which a naive cylinder calculation does not. Rail count assumes 2 rails for a fence up to about 4 ft and 3 above that — adjust it if your design differs. Check before you dig: call 811 (or your local utility locate service) before putting a post hole anywhere, confirm the property line with a survey rather than the old fence, and check local rules on height, setback, and which side the finished face must show.
Enter your fence run, post spacing, and height to get posts, rails, pickets, and the concrete to set them — with post hole depth worked out.
Before you dig
Three things to settle before ordering anything, all of which are cheaper to check than to fix:
- Call 811. Free, legally required in most places, and it takes a few days. Putting a post hole through a gas line or a fibre trunk is a genuinely serious outcome.
- Confirm the property line from a survey, not from the existing fence. Old fences are frequently in the wrong place, and a fence built over the line is a fence you may be made to move.
- Check local rules. Height limits differ front and back, setbacks apply, corner lots have sight-line rules, and many jurisdictions require the finished face to point outward.
How to use this calculator
- Pick the fence style — this presets the picket width and gap, which you can then adjust.
- Enter the total run, your post spacing (8 ft is standard), and the height above grade.
- Set rails per section: two up to 4 ft, three for 5–6 ft, four above that.
- Enter the number of gates. Each one adds a post.
The formula
Sections = Length ÷ Spacing · Posts = Sections + 1 + Gates · Pickets = Length ÷ (Picket width + Gap)
Post length = Height × 1.5 · Hole depth = Height × 0.5 · Concrete = Hole volume − Post volume
Worked example — 100 ft of 6 ft privacy fence
Posts at 8 ft, three rails per section, 5.5 in boards butted tight, one gate.
- Sections: 100 ÷ 8 = 12.5 → 13 sections
- Posts: 13 + 1 + 1 gate = 15 posts
- Post length: 6 × 1.5 = 9 ft, with 3 ft buried
- Rails: 13 × 3 = 39, or 312 linear ft
- Pickets: 1,200 in ÷ 5.5 = 219
- Concrete: ≈ 1.55 ft³ per hole × 15 = 23 ft³ → 52 bags of 60 lb
Change the design to a spaced picket fence with a 2 in gap and the picket count drops from 219 to 160 — a third fewer boards for the same run. Everything else stays the same.
The off-by-one that leaves you short
A hundred feet at eight-foot spacing is 12.5 sections, so you round up to 13. But 13 sections need 14 posts, not 13 — there is a post at the start, a post at the end, and one between every pair of sections. Think of a picket fence drawn on paper: the posts are the dividers, and dividers always outnumber the gaps by one.
Gates add another. A gate hangs on one post and latches to another, and the latch post cannot also be a line post carrying a section — it has to be set independently and usually more deeply, because a gate puts a constant twisting load on it. Every gate is one extra post in the order.
Post depth, and why it is not optional
The standard rule is to bury a third of the post's total length. A 6-foot fence therefore needs a 9-foot post, not a 6-foot one — which is why fence posts are sold in 8, 9, and 10 foot lengths while the fences themselves are 4, 6, and 8 feet tall.
A fence is a sail. Wind load on a solid 6-foot privacy fence is substantial, and it all arrives at the posts as a lever trying to rotate them out of the ground. Posts set too shallow do not fail immediately; they lean a little more each year until the fence is visibly crooked and the only fix is starting over.
In cold climates there is a second constraint: the hole must go below the frost line. Water in the soil expands as it freezes and lifts anything embedded in it, and a post that heaves up an inch each winter will not go back down. Frost depth ranges from nothing in the south to 4 feet or more in the northern states — your building department publishes the local figure.
Concrete or gravel?
| Concrete | Compacted gravel | |
|---|---|---|
| Strength | Highest — required for gate posts | Good for line posts in firm soil |
| Drainage | Traps water against the post | Drains freely |
| Post life | Shorter if water pools at the collar | Often longer |
| Removal later | Difficult — you dig out the whole footing | Straightforward |
| Cost and labour | Higher | Lower |
If you use concrete, crown the top so it slopes away from the post and sheds water rather than holding a puddle against the wood. Either way, put a few inches of gravel in the bottom of the hole so the end grain is not sitting in standing water — that is where posts rot first.
What this calculator does not include
- Gate hardware — hinges, latch, and the diagonal bracing or anti-sag kit a gate needs.
- Fasteners. Budget roughly 3 to 4 screws or nails per picket, plus structural screws for the rails, all rated for treated lumber.
- Post caps, which meaningfully extend post life by keeping water out of the end grain.
- Stain or sealer, and the extra material for a fence stepped down a slope, where sections overlap.
Common mistakes to avoid
- Buying one post per section. You need sections + 1, plus one per gate.
- Buying posts the same height as the fence. A 6 ft fence needs 9 ft posts.
- Ignoring the frost line. Heave will lift shallow posts, and they do not settle back.
- Using a line post for a gate. Gate posts carry a constant twisting load and need to be set deeper and in concrete.
- Skipping the picket gap in the count. A 2 in gap changes the picket count by a third.
- Building before locating utilities and the property line. Both are free to check and expensive to get wrong.
How we calculate this
Sections are the fence run divided by the post spacing, rounded up. Posts are sections + 1 to bracket the run, plus one per gate. Post hole depth follows the standard rule — bury a third of the post's total length, so a fence of height H needs a post of 1.5H with 0.5H in the ground — with a 600 mm floor. Concrete volume assumes a hole three times the post width (a 10.5 in hole for a 3.5 in actual 4×4) and deducts the post itself, which a plain cylinder calculation does not, then converts to 60 lb bags at 0.45 ft³ each. Picket counts use the covered width — face plus gap — for the same reason deck boards do.
Sources
Frequently asked questions
How many fence posts do I need for 100 feet?
At 8-foot spacing, 100 feet is 13 sections, which needs 14 posts — sections plus one to close the run. Add one more post for every gate. The off-by-one here is the most common fencing mistake: dividing 100 by 8 gives 12.5 and people buy 13 posts, then find themselves one short at the end.
How deep should fence posts be?
Bury a third of the post's total length, with 24 inches as an absolute minimum. A 6-foot fence needs a 9-foot post with 3 feet in the ground. In cold climates the hole must also go below the frost line or winter heave will push the posts up over a few seasons — check your local frost depth, which can be 4 feet or more in northern states.
How much concrete do I need per fence post?
For a 4x4 post in a 10-inch hole 3 feet deep, about 1.5 cubic feet after deducting the post itself — roughly three to four 60-pound bags. A 100-foot privacy fence with 15 posts needs around 52 bags. The rule for hole diameter is three times the post width, so a 4x4 (actually 3.5 inches) wants a 10 to 12 inch hole.
What is the standard fence post spacing?
6 or 8 feet on centre. Eight feet is the most common because rails and prefab panels are sold in 8-foot lengths, so it wastes nothing. Six feet is stiffer and worth it for tall privacy fences that catch a lot of wind, or on uneven ground. Beyond 8 feet the rails sag and the fence looks wavy within a couple of years.
How many pickets do I need?
Divide the fence length by the covered width of one picket — its face plus the gap. For a solid privacy fence with 5.5-inch boards butted tight, 100 feet needs 219 pickets. For a spaced picket fence with a 2-inch gap, each picket covers 7.5 inches, so the same run needs 160. The gap changes the count by a third, which is why it is a field here rather than an assumption.
Should I set fence posts in concrete or gravel?
Concrete is stronger and standard for gate posts and anything tall, but it traps water against the post and can shorten its life if the top is not crowned to shed water. Compacted gravel drains freely and is easier to remove later, and many fencers prefer it for line posts in well-draining soil. Whichever you use, never set a post in a hole that holds water — add gravel at the bottom for drainage.
How many rails does a fence need?
Two for a fence up to about 4 feet, three for 5 to 6 feet, and four for anything taller or for horizontal-board designs. Rails carry the pickets and stop them twisting, so a tall fence with only two rails will show waves along the top within a year. Space them evenly, with the top rail near the top of the pickets rather than at the very top edge.
Do I need a permit to build a fence?
Often, yes — and even when you do not, height limits, setback rules, and corner-visibility rules usually apply. Many jurisdictions cap front-yard fences at 3 or 4 feet and back-yard fences at 6. Some require the finished side to face outward. Before any of that, call 811 to have utilities marked and confirm your property line from a survey rather than the existing fence, which is frequently in the wrong place.