How to calculate fence materials
A standard wood fence is a row of posts, two to four horizontal rails between each pair of posts, and pickets nailed or screwed to the rails. Once you know the length of the fence and how far apart the posts are, every other number follows. The calculator takes the gate openings out of the length, divides the rest into sections, then counts posts, rails, pickets and the concrete to set each post.
Sections = fence run ÷ post spacing, rounded up
Posts = sections + 1 + number of gates
Rails = sections × rails per section
Pickets = fence run (in) ÷ (picket width + gap), rounded up
Concrete per hole (cu ft) = [π × (hole diameter in ÷ 24)² − (3.5 ÷ 12)²] × depth (ft)
The last line is the volume of a round hole minus the space the 4×4 post takes up (a 4×4 actually measures 3-1/2 × 3-1/2 in.).
Worked example: 100 ft privacy fence with one gate
A 6 ft tall privacy fence, 100 ft long, with one 4 ft walk gate, posts 8 ft apart, 3 rails, 1×6 pickets (5-1/2 in. wide) with no gap, and 10 in. holes 24 in. deep:
- Fence run: 100 − 4 = 96 ft.
- Sections: 96 ÷ 8 = 12 sections.
- Posts: 12 + 1 + 1 (gate) = 14 posts.
- Rails: 12 × 3 = 36 rails (8 ft 2×4s).
- Pickets: 96 × 12 = 1,152 in. ÷ 5.5 = 209.5. Add 10% waste: 209.5 × 1.10 = 230.4, so buy 231 pickets.
- Concrete per hole: π × (10 ÷ 24)² × 2 = 1.09 cu ft, minus the post (0.29 × 0.29 × 2 = 0.17 cu ft) = 0.92 cu ft.
- All holes: 0.92 × 14 = 12.9 cu ft, plus 10% = 14.2 cu ft. In 50 lb fast-setting bags (0.375 cu ft each): 14.2 ÷ 0.375 = 37.8, so 38 bags.
- Post length: 6 ft above ground + 2 ft in the ground = 8 ft, so buy 8 ft posts.
If you build the gate yourself from the same pickets, add 4 × 12 ÷ 5.5 = 8.7, or 9 more pickets, plus lumber for the gate frame.
Post spacing, depth and length
Eight feet on center is the standard spacing for wood fences because 2×4 rails are sold in 8 ft lengths. Closer 6 ft spacing makes a stiffer fence and is worth considering on windy sites or for heavy board-on-board designs. Vinyl and other pre-built panels set the spacing for you, so follow the panel width.
A common rule is to bury each post about one-third to one-half of its above-ground height. In cold climates, posts set above the frost line can heave, so many areas call for 36–48 in. or deeper. Check with your local building department, and don't go shallower than the table below.
| Fence height | Typical depth in ground | Post length needed | Post to buy |
|---|---|---|---|
| 4 ft | 2 ft | 6 ft | 8 ft (or a 12 ft post cut in half) |
| 5 ft | 2 ft | 7 ft | 8 ft |
| 6 ft | 2–3 ft | 8–9 ft | 8 or 10 ft |
| 8 ft | 2-1/2–3 ft | 10-1/2–11 ft | 12 ft |
Posts that go in the ground should be pressure-treated lumber rated for ground contact (the tag will say "ground contact" or UC4A or higher). Lumber rated only for above-ground use rots much faster at the soil line.
Rails and pickets
Most fences up to about 4–5 ft tall use 2 rails per section. A 6 ft privacy fence needs 3 rails (top, middle and bottom) to keep the pickets flat, and some 8 ft fences use 4. Rails are almost always 2×4s. With 8 ft post spacing, one 8 ft 2×4 spans each section. With 6 ft spacing you can cut 8 ft boards and waste 2 ft from each, or buy 12 ft 2×4s and cut each one into two rails.
Pickets are usually sold as 1×4 or 1×6 boards in 4, 5, 6 and 8 ft lengths, often with a dog-ear top. Measure one before you buy: a "1×6" picket is about 5-1/2 in. wide, but some are slightly narrower, and a small difference adds up over a long fence.
| Picket and gap | Pickets per 8 ft section | Pickets per 6 ft section |
|---|---|---|
| 5-1/2 in., no gap (privacy) | 18 | 14 |
| 5-1/2 in., 1-1/2 in. gap | 14 | 11 |
| 3-1/2 in., no gap | 28 | 21 |
| 3-1/2 in., 2-1/2 in. gap (picket fence) | 16 | 12 |
The table rounds up per section. The calculator works from the total fence length, so its count can be a few pickets lower than sections × the table value. For board-on-board or shadowbox fences, run the calculator once for each side, using the gap you'll leave between pickets on that side, and add the two counts.
How much concrete per fence post
The table shows concrete for one 4×4 post with the post volume already subtracted. Bag counts are decimals because you can split bags between holes; the calculator totals all holes before rounding up.
| Hole size | Concrete per post | 50 lb fast-setting bags | 80 lb bags |
|---|---|---|---|
| 8 in. × 2 ft | 0.53 cu ft | 1.4 | 0.9 |
| 10 in. × 2 ft | 0.92 cu ft | 2.5 | 1.5 |
| 12 in. × 2 ft | 1.40 cu ft | 3.7 | 2.3 |
| 10 in. × 3 ft | 1.38 cu ft | 3.7 | 2.3 |
| 12 in. × 3 ft | 2.10 cu ft | 5.6 | 3.5 |
Fast-setting post mix is popular because you pour it dry into the hole and add water per the bag directions, with no mixing. Standard mix is a little cheaper per cubic foot but needs to be mixed in a wheelbarrow first. Many builders put 4–6 in. of gravel in the bottom of each hole for drainage; the gravel calculator can size that, and the concrete calculator handles single holes or other footings.
Before you dig
- Call 811 a few days before digging. The utility-locating service is free, and hitting a buried line is dangerous and expensive.
- Confirm your property line with a survey or plat, and check zoning, HOA rules and permit requirements. Height limits are common, especially in front yards.
- Run a string line between the end posts and mark each hole along it so the fence stays straight.
- Set end, corner and gate posts first, let them cure, then fill in the line posts.
- Buy a few extra pickets. Wood pickets often arrive split, cupped or badly knotted, which is why the calculator includes a waste allowance.
Building a deck at the same time? The deck calculator counts boards, joists and screws.
Frequently asked questions
How many posts do I need for a 100 ft fence?
With posts 8 ft apart, 100 ft is 12.5 sections, which rounds up to 13 sections and 14 posts. At 6 ft spacing it is 17 sections and 18 posts. Add one post for each gate and for any corner that doesn't land on a regular post.
How many pickets are in an 8 ft fence section?
About 18 standard 1×6 pickets (5-1/2 in. wide) with no gaps, or 28 pickets if you use 1×4s. With gaps, divide 96 in. by the picket width plus the gap and round up.
How deep should fence posts be?
A common rule is one-third to one-half of the above-ground height, so 2 to 3 ft for a 6 ft fence. In cold climates, go below the frost line. Local code and HOA rules can require more, so check before you dig.
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 in. hole 2 ft deep, about 0.92 cubic feet: roughly 2.5 bags of 50 lb fast-setting mix or 1.5 bags of 80 lb mix. A 3 ft deep hole takes about 3.7 bags of 50 lb mix. See the concrete calculator for other hole sizes.
Should fence posts be 6 or 8 feet apart?
Eight feet is standard for wood fences and matches 8 ft rail lengths. Use 6 ft spacing for extra strength in high wind or for heavy board-on-board designs. Pre-built vinyl or metal panels set their own spacing.