Take the gates out of the length, divide by the post spacing and round up for the sections; you need one more post than sections, plus one for each gate. A 100 ft privacy fence 6 ft high, posts 8 ft apart, with one 4 ft gate: 12 sections, 14 posts, 36 rails and 210 1 × 6 boards. Each post is 9 ft long with 3 ft in the ground, in a 10½ in hole: about 4 bags of 50 lb fast-setting concrete a post.
How to measure for a fence
- Length: along the line the fence will follow, post to post at the ends. Stay inside your property line; the fence doesn’t have to be exactly on it.
- Each side on its own: a fence around a yard is several straight runs. Run each side through the calculator, then take one post off for each corner the runs share.
- Height: the height of the boards above the ground. Posts and the hole depth follow from it.
- Gates: the opening each gate needs, post to post.
The math, with your numbers
These lines follow whatever you typed into the calculator above.
Concrete fills the hole from the gravel to 3 in below the ground, around the post; QUIKRETE tops the hole with soil. Bags are counted for each post, because you mix them hole by hole.
How far apart should fence posts be?
For a wood, vinyl or composite fence, 8 ft is the standard spacing, and Family Handyman says any farther and the posts won’t be strong enough in the wind. It also lets you use 8 ft rails without waste, so Family Handyman sets posts just under 8 ft apart. Chain-link posts can go up to 10 ft. This Old House spaces residential fence posts 6 to 8 ft apart, and 4 to 6 ft at corners and other spots that take more load.
Ready-made panels decide the spacing for you: they are usually 6 or 8 ft wide, and the posts have to match them exactly. The calculator splits your run into equal sections no longer than the spacing you enter. More in fence post spacing and depth.
How deep to set fence posts, and how much concrete
QUIKRETE digs the hole about 3 times as wide as the post and a third of the post’s length deep. With a third of the post in the ground, the post is half again as long as the fence is high. This Old House adds 6 in of gravel under the post for drainage, and says a 6 ft fence needs posts 36 in deep to resist frost heave; in cold climates, dig to the frost line your building department gives you.
| Fence height | Post length | In the ground | Hole depth | Concrete | 50 lb Fast-Setting | 80 lb Concrete Mix |
|---|---|---|---|---|---|---|
| 4 ft | 6 ft | 24 in | 30 in | 0.90 cu ft | 3 bags | 2 bags |
| 5 ft | 7½ ft | 30 in | 36 in | 1.16 cu ft | 4 bags | 2 bags |
| 6 ft | 9 ft | 36 in | 42 in | 1.42 cu ft | 4 bags | 3 bags |
| 8 ft | 12 ft | 48 in | 54 in | 1.94 cu ft | 6 bags | 4 bags |
Our arithmetic for a 4 × 4 post (3½ in square, NIST PS 20) in a 10½ in hole with 6 in of gravel and the top 3 in of soil. QUIKRETE’s yields: 0.375 cu ft for a 50 lb bag of Fast-Setting Concrete Mix, 0.45 cu ft for 60 lb, 0.6 cu ft for an 80 lb bag of Concrete Mix.
The concrete calculator does the same for any hole or footing size. This Old House’s contractor sets most posts in tamped soil and gravel, and keeps concrete for gate posts or where rock stops him digging deep enough; pick No concrete if you set posts that way.
Rails, boards and pickets
Each section between posts gets its rails, and the boards are nailed or screwed to them. Family Handyman uses at least three 2 × 4s, or two 2 × 6s, per section of a privacy fence; with fewer, panels sag even on short spans.
A board’s real width is less than its name: a 1 × 6 is 5½ in wide and a 1 × 4 is 3½ in (NIST PS 20). Boards = the fence run ÷ (board width + gap). For a privacy fence the boards touch; Family Handyman’s gate leaves about ¼ in between them; a picket fence leaves whatever gap you choose.
Under the model residential code, a fence up to 7 ft high doesn’t need a building permit (IRC R105.2), but towns often limit fence height in their zoning rules, especially in front yards. Check before you order.
Questions people ask
How many fence posts do I need?
Fence length minus the gates, divided by the post spacing and rounded up, plus one, plus one for each gate. A 100 ft fence with posts 8 ft apart and one 4 ft gate needs 14.
How far apart should fence posts be?
8 ft for wood, vinyl and composite fences, and up to 10 ft for chain link (Family Handyman). Closer at corners and gates.
How deep should fence posts be?
A third of the post’s length, which is half the fence height: 3 ft for a 6 ft fence, plus 6 in of gravel. Go to the frost line in cold climates.
How many bags of concrete per fence post?
About 4 bags of 50 lb fast-setting concrete, or 3 of 80 lb concrete mix, for a 4 × 4 post on a 6 ft fence in a 10½ in hole. Smaller holes take less: QUIKRETE sets a 3 in post in a 9 in hole 30 in deep with two 50 lb bags.
How many pickets do I need?
The fence run in inches ÷ (picket width + gap). 96 ft of privacy fence in 1 × 6 boards is 1,152 ÷ 5½ = 210 boards.
Sources
- Family Handyman, How far apart should fence posts be? and How to build a fence for privacy
- This Old House, How to install a fence post
- QUIKRETE, Fast-Setting Concrete Mix data sheet (revised November 2025) and Concrete Mix data sheet
- NIST, PS 20-20 American Softwood Lumber Standard (dressed lumber sizes)
- International Code Council, 2021 IRC chapter 1 (R105.2, work exempt from permit)
Checked October 12, 2026. Metric values are converted from feet and inches.