To estimate a wood fence the way our fence calculator does, divide the fence length by the post spacing and round up to get sections; a straight run needs one more post than it has sections, and each section needs its own set of rails. For pickets, divide the fence length in inches by the picket width plus any gap. A 100 ft straight fence with posts 8 ft apart needs 14 posts, 26 rails (two per section), and 230 pickets 5.5 inches wide with 5 percent extra.
Enter your run below. Add posts for every corner, end, and gate, as explained in the steps.
How to measure for a fence
- Call 811 first. 811 is the free national call-before-you-dig service. Contact it a few business days before you dig so buried utility lines can be marked with paint or flags.
- Confirm the line and the rules. Know where your property line is, and check local zoning or HOA rules for fence height, setbacks, and permits before you buy materials.
- Measure each straight run separately. Stake the corners and run a string line. Measure each straight side in feet. The calculator handles one straight run at a time.
- Mark corners, ends, and gates. Each corner, each end, and each side of a gate opening needs a post. When two runs meet at a corner, they share that post, so subtract one post per shared corner when you add runs together.
- Choose post spacing. Lowe's fence estimating guide uses posts every 8 ft on center, and also lists 6 ft sections. Your rails must be at least as long as the spacing.
- Measure the actual picket width. Lumber is sold by nominal size. A 1x6 board actually measures 3/4 × 5-1/2 inches, according to the Southern Pine Inspection Bureau, which is why the calculator defaults to 5.5 inches.
The fence formula
The calculator uses:
- Sections = fence length ÷ post spacing, rounded up
- Posts = sections + 1
- Rails = sections × rails per section
- Pickets = (fence length × 12 ÷ (picket width + gap), rounded up) × (1 + extra % ÷ 100), rounded up again
- Concrete bags = posts × bags per post, rounded up
This matches the method in Lowe's fence estimating guide, which divides the fence length in inches by the actual picket width plus spacing.
Worked example (the calculator's default values)
A straight 100 ft fence with posts every 8 ft, 2 rails per section, 5.5 in pickets with no gap, 2 bags of concrete per post, and 5 percent extra pickets:
- Sections: 100 ÷ 8 = 12.5, rounded up to 13
- Posts: 13 + 1 = 14
- Rails: 13 × 2 = 26
- Pickets: 1,200 in ÷ 5.5 = 218.2, rounded up to 219; plus 5 percent = 229.95, rounded up to 230
- Concrete: 14 × 2 = 28 bags
The calculator shows: "You need 14 posts, 26 rails, and 230 pickets. 13 sections of up to 8 ft for a 100 ft straight run, and about 28 bags of concrete to set the posts."
Because 100 isn't evenly divisible by 8, one section ends up shorter, or you can space all 13 sections evenly at about 7.7 ft.
Example: a fence on three sides of a yard
Say you fence a backyard with runs of 50, 100, and 50 ft, and the two side runs meet the long run at corners. Running each side through the calculator gives 8 + 14 + 8 = 30 posts. The two corners are shared, so the real total is 28 posts. Pickets for the three runs are 116 + 230 + 116 = 462.
Pickets per 8 ft section
| Picket width | Gap | Pickets per 8 ft section | Pickets per 100 ft (no extra) |
|---|---|---|---|
| 5.5 in | None (privacy) | 18 | 219 |
| 5.5 in | 1/2 in | 16 | 200 |
| 5.5 in | 2 in | 13 | 160 |
| 3.5 in | None (privacy) | 28 | 343 |
| 3.5 in | 1.5 in | 20 | 240 |
| 3.5 in | 3.5 in | 14 | 172 |
Counts are rounded up per section. The 5.5 in pickets with a 2 in gap match the spacing example in Lowe's guide (7.5 inches per picket plus space).
Rails, post depth, and holes
| Item | Guideline | Source |
|---|---|---|
| Rails, 4 ft tall fence | 2 per section | Lowe's fence guide |
| Rails, 6 ft tall fence | 3 per section | Lowe's fence guide |
| Post hole diameter | About 3 times the post width (about 10.5 in for a 4x4, which measures 3.5 in) | Quikrete |
| Post hole depth | 1/3 to 1/2 of the post height above ground (at least 2 ft for a 6 ft fence) | Quikrete |
| Gravel at hole bottom | About 6 in, compacted | Quikrete |
Frost depth and local code can require deeper holes, so check with your building department. For tall fences or windy, exposed sites, a fence contractor can confirm post size and depth.
How much concrete per fence post?
The calculator defaults to 2 bags per post, but the right number depends on your hole. Quikrete's data sheet for 50 lb Fast-Setting Concrete Mix gives a yield of about 0.375 cu ft per bag and says two bags will set a 3 in square post in a 9 in diameter hole 30 inches deep.
Hole width matters more than most people expect, because the volume grows with the square of the diameter. A 4x4 post actually measures 3.5 × 3.5 in, so Quikrete's "3 times the post width" rule gives a hole about 10.5 in across. Scaling from Quikrete's two-bag figure, that hole takes roughly 3 bags at 30 inches deep. A 12 in hole holds nearly twice as much concrete as the 9 in hole, or about 4 bags. Set the bags-per-post field to 3 and the example fence needs 42 bags instead of 28; at 4 bags it needs 56.
Quikrete's post-setting guide, which pours the dry mix into the hole and then adds water, calls for about a gallon of water per 50 lb bag, filling the hole to 3 to 4 inches below ground, and waiting about 4 hours before putting heavy loads on the posts. The mix sets in 20 to 40 minutes. For the gravel layer, Quikrete's shopping list for the job includes 50 lb bags of All-Purpose Gravel.
Tips for buying fence materials
- Order extra pickets. Lowe's recommends increasing the picket count by 10 percent. The calculator defaults to 5 percent; raise it if you expect to cull warped or split boards.
- Buy rails at least as long as your spacing. With 8 ft on center, 8 ft rails span post to post.
- Plan the gates first. If you buy a prebuilt gate, subtract the opening from the length you enter for pickets. If you build the gate from the same pickets, leave it in.
- Count hardware. Lowe's guide lists one post cap per post. Gates need their own hardware, and fasteners depend on your rail and picket count.
- Use the actual picket width. Measure a board at the store. A small difference adds up over 100 feet.
Common mistakes to avoid
- Forgetting extra posts. Each corner, end, and gate side needs one.
- Double-counting shared corners when you add up several runs.
- Using nominal picket width. A "1x6" is 5.5 inches wide, not 6.
- Undersizing concrete. Wider holes take far more concrete than the 2 bag rule of thumb.
- Digging before utilities are marked. Call 811 a few business days ahead.
Frequently asked questions
How many fence posts do I need for 100 feet?
For a straight run, 14 posts at 8 ft spacing or 18 posts at 6 ft spacing. Add posts for gates, and remember that corners are shared between runs.
How far apart should fence posts be?
Lowe's fence estimating guide uses 8 ft on center, and 6 ft sections are also common. Closer spacing means more posts and concrete but shorter spans for the rails.
How many pickets are in an 8 ft fence section?
With 5.5 in pickets and no gap, about 18. With a 1/2 in gap, 16. With 3.5 in pickets and no gap, 28.
How many bags of concrete do I need per fence post?
Quikrete says two 50 lb bags of Fast-Setting Concrete Mix set a 3 in post in a 9 in hole 30 inches deep. For a 4x4 in a hole three times its actual width (about 10.5 in), plan on roughly 3 bags at that depth, and about 4 bags in a 12 in hole.
How deep should fence posts be?
Quikrete's post-setting guide says 1/3 to 1/2 of the post's height above ground, which is at least 2 ft for a 6 ft fence. Check local frost depth and code requirements too.
How many rails does a fence need?
Lowe's guide uses 2 rails per section for a 4 ft fence and 3 rails per section for a 6 ft fence.