Fence Calculator
Posts & panels for a fence run
Building a fence? Enter the total run length to estimate the posts, panels and bags of concrete you need, at standard 8-foot spacing. Below: a materials chart by run length, how deep posts really need to go, what changes at 6-foot spacing, and how much stain the finished fence will drink.
Posts, panels and the extra post
Used as a fence panels calculator, the rule that catches people out is that a run of n sections needs n + 1 posts. Four 8-foot sections span 32 feet but need five posts, because both ends have to be closed. The calculator adds that post for you.
Each section also needs 2 to 3 rails — two for a fence up to about 5 feet, three at 6 feet and above, and three always for anything carrying heavy pickets or facing real wind.
| Run length | Posts | Panels | 50 lb concrete bags | Rails at 3/section |
|---|---|---|---|---|
| 50 ft | 8 | 7 | 16 | 21 |
| 100 ft | 14 | 13 | 28 | 39 |
| 150 ft | 20 | 19 | 40 | 57 |
| 200 ft | 26 | 25 | 52 | 75 |
| 300 ft | 39 | 38 | 78 | 114 |
Tightening the spacing to 6 feet on that same 100 ft run raises the post count from 14 to 18 — about 29% more posts, holes and concrete. It also produces a noticeably stiffer fence, which is worth the money in high wind or for anything over 6 feet tall.
Fence post depth
Every fence post depth calculator comes back to the same two rules, and you follow whichever gives the deeper hole:
One third of the post's above-ground height, or below your local frost line. Whichever is deeper wins.
| Fence height | Hole depth (1/3 rule) | Post length to buy | Hole width |
|---|---|---|---|
| 3 ft | 12 in | 4 ft | 10 in |
| 4 ft | 16 in | 6 ft | 10 in |
| 5 ft | 20 in | 7 ft | 10 to 12 in |
| 6 ft | 24 in | 8 ft | 10 to 12 in |
| 8 ft | 32 in | 12 ft | 12 in |
The frost line is the rule that overrides everything. In northern Minnesota it can exceed 60 inches, in the upper Midwest 42 to 48, across the Mid-Atlantic around 30, and in the deep south it is effectively zero. A 6-foot fence set to the 24-inch rule in a region with a 42-inch frost line will lift and lean within a couple of winters — the ice under the post does not care about the ratio. Check your local building department; many areas also cap fence height without a permit.
Hole width should be about three times the post width, so 10 to 12 inches for a 4×4. Wider and shallower beats narrower and deeper for wind resistance, but neither beats getting below the frost line.
Concrete per post
The calculator allows two 50 lb bags of fast-setting mix per post, which suits a 10-inch hole at 24 inches deep. Bigger holes need more:
| Hole | Volume | 50 lb bags | 80 lb bags |
|---|---|---|---|
| 8 in × 24 in | 0.70 ft³ | 2 | 2 |
| 10 in × 24 in | 1.09 ft³ | 3 | 2 |
| 10 in × 30 in | 1.36 ft³ | 4 | 3 |
| 12 in × 30 in | 1.96 ft³ | 6 | 4 |
| 12 in × 36 in | 2.36 ft³ | 7 | 4 |
A 4×4 post is actually 3.5 inches square and displaces about 0.21 ft³ over a 30-inch depth, so you can knock roughly half a bag off each hole. For a fence post concrete calculator that handles your exact hole size and count, including the circular geometry, use the concrete calculator with the shape set to Circle.
Fast-setting mix goes in dry: pour it into the hole around the plumbed post, add water on top, and leave it. It sets in 20 to 40 minutes, so plumb and brace each post before the water goes in.
Gates, corners and sloped ground
- Gate posts carry the load and sag first. Use 6×6 rather than 4×4, set them deeper, and brace the gate diagonally from the top hinge side down to the bottom latch side.
- Corner and end posts take tension from two directions. Set these first, string a line between them, and every post in between will line up.
- On a slope you either rack or step. Racking lets the panel follow the grade and keeps the bottom gap even. Stepping keeps each section level and looks cleaner, but leaves triangular gaps underneath that pets and small children find immediately.
- Set gate width from the post faces, not the centres, and allow for hinge and latch hardware.
Staining and sealing the finished fence
A fence has two sides, and people budget for one. A 100 ft run at 6 ft tall is 600 ft² per face, so 1,200 ft² in total, plus posts and rails. Stain coverage on rough sawn timber runs about 150 to 250 ft² per gallon — far less than on smooth wood, because rough grain drinks it. That is 5 to 8 gallons for both sides of a 100 ft fence.
New pressure-treated timber usually needs to weather for several months before it will accept stain; test by sprinkling water on it, and if the water beads rather than soaking in, it is not ready. The deck stain calculator handles coverage by product type.
Fence mistakes that cost the most
- Pricing panels without posts. A fence material and post calculator has to count posts, rails, concrete and hardware too.
- Forgetting the extra post. Every run needs one more post than it has sections.
- Ignoring the frost line. The most expensive mistake here, and invisible until the second winter.
- Not checking the property line. A fence built even a few inches over is a legal problem, not a carpentry one. Many areas also have height limits and setback rules.
- Missing utility locates. Call before you dig. It is free and hitting a buried line is not.
- Setting posts without bracing. Fast-setting concrete grabs in half an hour and a post that was not plumb stays that way.
- Concrete filled flush or dished. Crown it above grade so water runs away from the post rather than pooling around it.
- Buying 6 ft posts for a 6 ft fence. You need 8 ft posts, because two of those feet go in the ground.
How this fence calculator works out its numbers
Posts are ceil(run ÷ spacing) + 1 and panels are ceil(run ÷ panel width), both at 8 ft by default. The +1 closes the end of the run.
- Concrete allows 2 bags of 50 lb fast-setting mix per post, sized for a 10 in hole at 24 in deep. Larger holes need more; the table above gives figures by hole size, and the concrete calculator handles exact geometry.
- One straight run is assumed. Corners, gates and separate sections each need their own end post, so count runs separately or add posts manually.
- Post depth guidance is the standard one-third rule, overridden by local frost depth. Frost lines are set by local code and range from zero in the deep south to over 60 in in northern Minnesota.
- Stain coverage of 150 to 250 ft² per gallon reflects rough sawn timber, which absorbs far more than the 350 to 400 ft² typical of smooth surfaces.
- Not included: gate hardware, fasteners, post caps, and the extra heavy posts a gate needs.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Fence Calculator: frequently asked questions
How far apart should fence posts be?
6 to 8 feet on centre. 8 ft matches standard panel widths and is the calculator's default; 6 ft uses about 29% more posts but gives a noticeably stiffer fence, which is worth it in high wind or above 6 feet tall.
How many fence posts do I need for 100 feet?
14 posts at 8 ft spacing, with 13 panels. A run always needs one more post than it has sections, because both ends must be closed. At 6 ft spacing the same run takes 18 posts.
How much concrete per fence post?
Two to three 50 lb bags of fast-setting mix for a typical 10-inch hole at 24 inches deep. A 12-inch hole at 30 inches needs six. Subtract about half a bag for the volume the post itself takes up.
How deep should fence post holes be?
One third of the post's above-ground height, or below your local frost line, whichever is deeper. A 6 ft fence needs 24 inches by the first rule, but 42 inches or more in the upper Midwest where the frost line governs.
What length posts do I need for a 6 foot fence?
8 foot posts. Two feet go into the ground, leaving six above. Buying 6 ft posts for a 6 ft fence is a common and expensive ordering mistake.
How wide should a fence post hole be?
About three times the post width, so 10 to 12 inches for a 4×4. Wider and shallower resists wind better than narrow and deep, though neither substitutes for getting below the frost line.
How much stain does a fence need?
About 5 to 8 gallons for both sides of a 100 ft, 6 ft tall fence. That is 1,200 ft² of surface at roughly 150 to 250 ft² per gallon on rough sawn timber, which absorbs far more than smooth wood.
Should fence panels be racked or stepped on a slope?
Racking follows the grade and keeps the bottom gap even, which suits gentle slopes and pet containment. Stepping keeps each section level and looks tidier, but leaves triangular gaps at the bottom of each step.