Never exceed 10 ft between posts.
Run 1
Each corner forces a post.
0 if not used.
Advanced options
About 4 ft each.
About 12 ft each.
Actual width. A 1×6 is 5.5 in.
Three times the post width is the rule.
Drainage under the post. 0 to skip.
This fence calculator works out posts, rails, pickets and concrete — with the post count adjusted for whether your fence is a straight run or a closed perimeter, and the concrete calculated from actual hole geometry rather than a rule of thumb.
The short answer
Sections = length ÷ post spacing, rounded up. On a straight run, posts = sections + 1. On a closed perimeter, posts = sections exactly — the last post is the first one.
A 100 ft privacy fence at 6 ft tall with 8 ft spacing needs 14 posts, 39 rails and about 240 pickets, plus 2 bags of 80 lb concrete per hole.
On this page
How to use this fence calculator
Set the style, height and post spacing, then describe the fence line. Three layouts:
- Straight run — a single length, with corners counted separately
- Closed perimeter — a rectangle enclosing a yard, where the fence meets itself
- Sides I measure — up to four sides of an irregular boundary
The distinction between the first two matters more than it looks, and it’s the subject of the next section.
Under Advanced options: gates, picket width and gap, hole diameter, concrete bag size, gravel base and prices.
Post count — the off-by-one that most calculators get wrong
Every fence calculator uses the same formula: posts = ⌈length ÷ spacing⌉ + 1. And for a straight run that’s correct — you need a post at both ends, so thirteen sections need fourteen posts.
But a great many residential fences aren’t straight runs. They go round a yard and meet themselves. On a closed loop, the last post is the first post — there’s no free end to cap. Thirteen sections need thirteen posts, not fourteen.
Straight run: Posts = Sections + 1
Closed perimeter: Posts = Sections
Plus one extra post per gate, and one per corner on a run
Sections = ⌈Total length ÷ Post spacing⌉
One post is not a catastrophe — but it’s a post, a hole, two bags of concrete and half an hour of work, and it’s a symptom of a calculator applying a formula without asking what shape the fence is. If a tool gets the easy geometry wrong, that’s worth knowing before you trust its picket count.
Worked example — 100 ft privacy fence, 6 ft tall, 8 ft spacing, one walk gate
Post spacing
| Spacing | Use when | Posts per 100 ft |
|---|---|---|
| 8 ft | Standard residential — matches 8 ft rail stock and pre-built panels | 14 |
| 6 ft | High wind, slopes, fences over 6 ft, heavy gates | 18 |
| 10 ft | Chain link and split rail only | 11 |
Eight feet is standard largely because rails come in 8 ft lengths — space posts further apart and you’re cutting and splicing. Never exceed 10 feet between posts on any fence; rails sag and the whole run loses rigidity.
Closer spacing costs more in posts and concrete but buys real stiffness. On a tall fence in an exposed spot, 6 ft spacing is money well spent — a privacy fence is a solid wall to wind, and the leverage on each post is considerable.
Post depth and length
Post length = Fence height × 1.5
Burial depth = One third of total post length
A 6 ft fence needs a 9 ft post set 3 ft deep
An 8 ft fence needs a 12 ft post set 4 ft deep
| Fence height | Post length | Burial |
|---|---|---|
| 3 ft | 4½ ft | 1½ ft |
| 4 ft | 6 ft | 2 ft |
| 5 ft | 7½ ft | 2½ ft |
| 6 ft | 9 ft | 3 ft |
| 8 ft | 12 ft | 4 ft |
In freezing climates, post footings must extend below the local frost line — typically 36 to 48 inches, and deeper in the far north. Water in the soil expands as it freezes and lifts anything shallower, which is why fences in cold regions develop that characteristic drunken lean after a few winters.
The one-third rule and the frost rule are both minimums. Use whichever is deeper. Your local building department publishes the frost depth for your area.
Always put 4 to 6 inches of gravel in the bottom of the hole before the post. It lets water drain away from the end grain instead of pooling against it, which is where posts rot first.
Concrete per post — published figures are badly inconsistent
Search this and you’ll find “one 50 lb bag per post”, “one-third of an 80 lb bag”, “two 80 lb bags”, and “two to three 50 lb bags.” That’s roughly a ninefold spread for the same job, and unlike some estimating disagreements it isn’t a matter of judgement — it’s geometry.
Net concrete = Hole volume − Post volume
Hole = π × (diameter ÷ 2)² × depth · a 4×4 post is really 3.5 × 3.5 in
The post displaces about 16% of a 10 inch hole
For the standard case — a 6 ft fence, 10 inch hole, 3 ft burial with 4 inches of gravel underneath — the concrete depth is 32 inches, giving 1.45 ft³ of hole less 0.23 ft³ of post: 1.23 ft³ of concrete per hole.
| Published claim | Actually needed | Verdict |
|---|---|---|
| One 50 lb bag per post | 3.3 bags | Understated by more than 3× |
| One third of an 80 lb bag | 2.0 bags | Understated by 6× |
| Two 80 lb bags | 2.0 bags | Correct |
| Two to three 50 lb bags | 3.3 bags | Correct at the top of the range |
The two figures that specify a bag size are both right. The ones that don’t are wrong — and the “one bag per post” version, which appears on several fence calculators, would leave you at roughly half the concrete you need.
| Fence height | Concrete depth | ft³ per hole | 80 lb bags | 50 lb bags |
|---|---|---|---|---|
| 4 ft | 20 in | 0.77 | 1.3 | 2.0 |
| 6 ft | 32 in | 1.23 | 2.0 | 3.3 |
| 8 ft | 44 in | 1.69 | 2.8 | 4.5 |
Based on a 10 inch hole with 4 inches of gravel. Gate posts want an extra bag — they carry hinge load and swing weight that line posts don’t.
Fast-setting concrete is generally the better choice for fence posts. It goes in dry, you add water to the hole, and it sets in 20 to 40 minutes — so you can set every post in a day and come back for rails the next morning.
Rails
The rule is one rail per 24 inches of fence height, minimum two.
| Fence height | Rails per section | Rails per 100 ft at 8 ft spacing |
|---|---|---|
| 3–4 ft | 2 | 26 |
| 5–6 ft | 3 | 39 |
| 8 ft | 4 | 52 |
The third rail on a 6 ft fence isn’t optional — it’s what stops pickets cupping and warping over their length. Two rails on a 6 ft picket leaves 6 feet of unsupported board, and it will bow.
Order an extra rail per 50 feet. Some will arrive bowed or with a knot line through them, and you want to reject those rather than use them.
Pickets
Pickets per foot = 12 ÷ (Picket width + Gap)
Use actual lumber width: a 1×6 is 5.5 in, a 1×4 is 3.5 in
| Style | Width + gap | Per ft | Per 100 ft + 10% |
|---|---|---|---|
| Privacy — 1×6 butted | 5.5 + 0 in | 2.18 | 240 |
| Privacy with ⅛ in gap | 5.5 + 0.125 | 2.13 | 235 |
| Semi-privacy — 1×6, 1 in gap | 5.5 + 1 in | 1.85 | 204 |
| Picket — 1×4, 1¾ in gap | 3.5 + 1.75 | 2.29 | 252 |
| Picket — 1×4, 2½ in gap | 3.5 + 2.5 | 2.00 | 221 |
Pressure-treated pickets arrive wet and shrink as they dry. Butt them hard against each other and you’ll get gaps anyway within a season — but unevenly, wherever each board happens to shrink most.
A deliberate ⅛ inch gap costs about 5 pickets per 100 feet and gives a consistent line. Cedar, which is usually drier at purchase, moves less.
Pickets ship in bundles of 8 or 16. Round up to the next full bundle rather than leaving yourself two short on the last section.
Gates
- Walk gate: 3 to 4 ft wide. One per side of the house is the usual arrangement.
- Drive gate: 10 to 12 ft, usually built as a pair of leaves rather than one heavy swing.
- Gate posts must be heavier — 4×6 or 6×6 rather than 4×4. A gate is a lever, and it works on that post every single time it opens.
- Gate openings come off the pickets and rails, but not the posts, since you still need a post either side.
- Set gate posts deeper, with an extra bag of concrete.
Materials by fence length
6 ft privacy fence, 8 ft post spacing, 1×6 butted pickets, straight run, no gates, 10% picket waste.
| Length | Sections | Posts | Rails | Pickets | 80 lb bags |
|---|---|---|---|---|---|
| 50 ft | 7 | 8 | 21 | 120 | 17 |
| 100 ft | 13 | 14 | 39 | 240 | 29 |
| 150 ft | 19 | 20 | 57 | 360 | 41 |
| 200 ft | 25 | 26 | 75 | 480 | 54 |
Before you dig
In the US, calling 811 or filing online has utility owners come out and mark buried gas, electric, water, sewer and communications lines on your property at no charge, usually within a few working days. Post holes go 3 feet down, which is exactly where those lines live.
Hitting a gas line with an auger is life-threatening. Hitting a fibre line is merely a four-figure repair bill. Both are entirely avoidable.
Two other things to settle before the first hole:
- The property line. Have it surveyed if there’s any doubt. A fence built over a boundary is a legal dispute, not a construction problem, and the remedy is usually moving it.
- Permits and rules. Most areas require a permit above 6 ft, many have front-yard height limits, and HOAs frequently dictate style, colour and which side faces out.
Fence estimating mistakes to avoid
- Adding a post for a closed perimeter. The last post is the first one.
- One bag of concrete per post. A standard 6 ft fence hole takes two 80 lb bags.
- Using nominal lumber width. A 1×6 picket is 5.5 inches, not 6.
- Two rails on a 6 ft fence. Pickets bow without the third.
- 4×4 gate posts. A gate is a lever; use 4×6 or 6×6.
- Ignoring the frost line. Shallow posts heave and lean.
- No gravel under the post. Water pools against end grain and rots it.
- Butting wet pressure-treated pickets. They shrink and gap unevenly.
Frequently asked questions
How many fence posts do I need?
Divide fence length by post spacing and round up for the section count. On a straight run add one, because both ends need a post — 100 ft at 8 ft spacing is 13 sections and 14 posts. On a closed perimeter that meets itself, posts equal sections exactly. Add one extra per gate and per corner.
How much concrete per fence post?
For a standard 6 ft fence with a 10 inch hole set 3 ft deep on gravel, about 1.23 cubic feet — two 80 lb bags or three to four 50 lb bags. That’s after subtracting the post itself, which displaces around 16% of the hole. Gate posts want an extra bag.
Why do sources disagree on concrete per post?
Because several quote a bag count without saying which bag. “Two 80 lb bags” and “two to three 50 lb bags” are both correct for a standard hole. “One bag per post” and “one third of an 80 lb bag” are simply wrong — the first is about half what’s needed, the second about a sixth.
How far apart should fence posts be?
8 feet is standard for residential fencing, largely because rails come in 8 ft lengths. Use 6 ft for high wind, slopes, fences over 6 ft, or heavy gates. Chain link and split rail can stretch to 10 ft. Never exceed 10 ft on any fence.
How deep should fence posts be?
One third of the total post length, so a 6 ft fence uses a 9 ft post set 3 ft deep. In freezing climates the footing must go below the frost line — typically 36 to 48 inches — whichever is deeper. Add 4 to 6 inches of gravel underneath for drainage.
How many pickets do I need?
Divide 12 by the picket width plus gap, both in inches, to get pickets per foot. A 1×6 butted with no gap is 5.5 inches, giving 2.18 per foot — so 100 ft needs 218 pickets, or 240 with 10% waste. Use actual lumber width, not nominal.
How many rails does a fence need?
One per 24 inches of height with a minimum of two, so a 4 ft fence takes two rails per section and a 6 ft fence takes three. The third rail on a 6 ft fence prevents pickets cupping and bowing over their unsupported length.
How long should fence posts be?
Fence height times 1.5, so a 6 ft fence needs 9 ft posts and an 8 ft fence needs 12 ft posts. That puts a third of the post in the ground. Buy longer than you think you need — a post can be cut down, but it can’t be extended.
Should I leave a gap between privacy fence pickets?
A small one, about ⅛ inch, if you’re using pressure-treated lumber. It arrives wet and shrinks as it dries, so butting it tight produces uneven gaps within a season anyway. A deliberate gap costs about 5 extra pickets per 100 ft and gives a consistent line.
What size should gate posts be?
4×6 or 6×6 rather than 4×4. A gate is a lever working against that post every time it opens, and hinge load will eventually pull a standard post out of plumb. Set gate posts deeper than line posts and add an extra bag of concrete.
Do I need a permit to build a fence?
Usually above 6 feet, and many areas have separate front-yard height limits. HOAs often add rules on style, colour and which side faces the street. Check with your local building department before ordering materials, and have the property line confirmed if there’s any doubt.
Can I use this fence calculator in metric?
Dimensions are entered in feet and inches, since US lumber and post spacing are dimensioned that way. For metric, multiply feet by 0.3048 for metres and inches by 25.4 for millimetres.
Standards and sources
- Picket, rail and post counts verified against four published fence calculators with exact agreement
- Concrete volumes computed from hole geometry with post displacement subtracted; bag yields per ASTM C387
- Frost depth requirements vary by jurisdiction — confirm with your local building department
- Nominal versus actual lumber dimensions per American Softwood Lumber Standard PS 20
Related calculators
Concrete Bag CalculatorBag counts for any volume
Concrete CalculatorSlabs, footings and posts
Gravel CalculatorDrainage stone by the yard
Square Footage CalculatorMeasure an irregular yard
Paint CalculatorStaining and sealing the finished fence
How this fence calculator works out its numbers
Sections are total length divided by post spacing, rounded up. Post count adds one for a straight run, since both ends terminate, and adds nothing for a closed perimeter, since the run returns to its starting post. Corners and gates each add one post. Rails are sections times one per 24 inches of height, with a minimum of two.
Pickets use the module width — picket plus gap — divided into 12 to give pickets per linear foot, applied to the fence length less gate openings, then multiplied by the waste factor. Actual lumber dimensions are used throughout, so a 1×6 is 5.5 inches.
Concrete is the hole cylinder minus the volume of post inside it, over the burial depth less the gravel base. A 4×4 post is treated as its actual 3.5 inches, which displaces about 16% of a 10 inch hole. Bag counts use ASTM C387 yields of 0.60, 0.45 and 0.375 ft³ for 80, 60 and 50 lb bags.
This calculator produces quantities, not a fence design. Post size, footing depth, wind loading and permit requirements are governed by your local building code. Frost depth in particular varies enormously by region and overrides the general one-third rule. Confirm the property line and call 811 before digging.