Fence Calculator

What it calculates and who it’s for

This calculator is for anyone planning a wood picket fence along one line: a privacy fence, a picket fence, or a board-on-board fence. From the fence length, post spacing, and picket size, it counts the posts, rails, and pickets to buy. It also sizes the hole for each post and counts the bags of concrete to set them. Fence-post concrete is part of this page, so you don’t need a separate concrete calculator for your posts.

All sizes are in US units: feet for the fence, inches for pickets and holes, cubic feet for concrete and gravel, and pounds for bags.

How to use

1. Fence line. Enter the total fence length in feet, gates included, and the fence height. Enter how many gates the line has.
2. Posts. Set the post spacing (center to center), the post size (4x4 or 6x6), and the full length of the posts you will buy. 9 ft isn’t a standard lumber length: if you want a 9 ft post, buy a 10 ft one and cut it down.
3. Pickets. Enter the actual face width of one picket and the gap between pickets. For board-on-board, enter the overlap on each edge as a negative gap.
4. Post concrete. Choose the concrete bag you plan to use.
5. Advanced. Set the gate width, rails per section, your local frost depth (if you know it), the soil cap over the concrete (4 in to start), and the picket waste allowance.
6. Read the results. The four shaded numbers at the top are your shopping list. Below them are the fence layout, the size of each post hole, and the concrete and gravel totals.

Results update as you type. To start over, reload the page.

The formula

Fence line

• Fence run (ft) = fence length − (number of gates × gate width)
• Sections = fence run ÷ post spacing, rounded up to a whole section
• Posts = sections + 1 + number of gates
• Actual post spacing (ft) = fence run ÷ sections

One straight run with n sections needs n + 1 posts. Each gate opening splits the run, which adds one more post.

Rails and pickets

• Rails = sections × rails per section (each rail spans one section, post to post)
• Pickets before waste = (fence run × 12) ÷ (picket width + picket gap), rounded up
• Pickets to buy = (fence run × 12) ÷ (picket width + picket gap) × (1 + waste % ÷ 100), rounded up

With a negative gap (board-on-board overlap), each picket covers its width minus the overlap.

Post holes (Quikrete Setting Posts method)

• Hole diameter (in) = 3 × actual post width (3.5 in for a 4x4, 5.5 in for a 6x6)
• Depth of post in the ground (in) = the larger of (post length in inches ÷ 3) and your frost depth
• Hole depth (in) = depth of post in the ground + 6 in of gravel
• Concrete height (in) = depth of post in the ground − soil cap (4 in unless you change it)
• Concrete per post (cu ft) = (π × (hole diameter ÷ 2)² − post width²) × concrete height ÷ 1,728
• Gravel per post (cu ft) = π × (hole diameter ÷ 2)² × 6 ÷ 1,728

The concrete stops below ground: Quikrete’s directions fill the hole with concrete to 3 to 4 in below grade, then fill the top with soil or sod. That top layer is the soil cap. The square of the post is subtracted because the post fills that space, not concrete. 1,728 is the number of cubic inches in a cubic foot.

Bags

• Bags per post = concrete per post ÷ bag yield
• Bags to buy = (concrete per post × posts) ÷ bag yield, rounded up
• Bag yields: 50 lb Fast-Setting = 0.375 cu ft, 60 lb Fast-Setting = 0.45 cu ft, 80 lb Concrete Mix = 0.60 cu ft (Quikrete data sheets)

Post length check

• Post height above ground (ft) = post length − (depth of post in the ground ÷ 12)
• Shortest post for your fence height (ft) = the larger of (fence height × 1.5) and (fence height + frost depth ÷ 12)

Worked example

A 100 ft privacy fence, 6 ft tall, with one 4 ft gate. Posts are 4x4 by 10 ft at 8 ft spacing. Pickets are 1x6 (5.5 in actual) with a 0.5 in gap, 3 rails per section, 5% picket waste, no frost depth entered, a 4 in soil cap, and 50 lb Quikrete Fast-Setting bags. These are the calculator’s starting values.

1. Fence run = 100 − (1 × 4) = 96 ft
2. Sections = 96 ÷ 8 = 12 (already whole)
3. Posts = 12 + 1 + 1 = 14 posts
4. Rails = 12 × 3 = 36 rails
5. Pickets before waste = (96 × 12) ÷ (5.5 + 0.5) = 1,152 ÷ 6 = 192
6. Pickets to buy = 192 × 1.05 = 201.6, rounded up = 202 pickets
7. Hole diameter = 3 × 3.5 = 10.5 in
8. Depth of post in the ground = (10 × 12) ÷ 3 = 40 in. Hole depth = 40 + 6 = 46 in
9. Concrete height = 40 − 4 in soil cap = 36 in
10. Concrete per post = (3.1416 × 5.25² − 3.5²) × 36 ÷ 1,728 = (86.59 − 12.25) × 36 ÷ 1,728 = 1.55 cu ft
11. Bags per post = 1.5488 ÷ 0.375 = 4.13 bags
12. Concrete for all posts = 1.5488 × 14 = 21.68 cu ft. Bags = 21.68 ÷ 0.375 = 57.8, rounded up = 58 bags
13. Gravel = 86.59 × 6 ÷ 1,728 = 0.30 cu ft per post × 14 = 4.21 cu ft
14. Post height above ground = 10 − (40 ÷ 12) = 6.67 ft (about 6 ft 8 in). Shortest post for a 6 ft fence = 6 × 1.5 = 9 ft

The last step is why the calculator starts at 10 ft posts. With Quikrete’s one-third rule, a 6 ft fence needs a post at least 9 ft long, but 9 ft isn’t a standard lumber length. A 10 ft post stands about 8 in taller than the fence; trim the top, or cut the post to 9 ft before you set it (a 9 ft post goes 36 in into the ground with 32 in of concrete, 1.38 cu ft and 52 bags for this fence). A more common 8 ft post would be buried 32 in and stand only 5 ft 4 in out of the ground, shorter than a 6 ft picket.

How to read your results

• Posts, rails, pickets, and bags are already rounded up to whole pieces. Buy at least those numbers.
• Concrete height is the post’s depth in the ground minus the soil cap. Stop pouring there and fill the rest of the hole with soil or sod.
• Bags per post is a decimal on purpose. With 4.13 bags per post you can’t put exactly one bag count in every hole. Open bags and share them between holes, or round up to 5 per hole if you’d rather not split bags (that’s 70 bags for the example, not 58).
• Actual post spacing can be shorter than what you typed. The run is split into equal sections, so a 46 ft run at 8 ft spacing becomes 6 sections of about 7.67 ft each.
• Rails are counted one per section per rail row. Buy rails at least as long as your actual post spacing. Rails often come in 8 ft lengths, so 8 ft spacing is a common match.
• Gravel for post bases is in cubic feet. Small jobs are usually bought in bags, so read the bag’s cubic-foot label. For bigger orders, see the Gravel Calculator.
• Shortest post for your fence height is the post length that keeps the post top level with the top of the fence. If your post length is shorter, the result for post height above ground shows how far short it is. If it comes out at a length you can’t buy, like 9 ft, buy the next standard length (10 ft) and cut it down.

Assumptions and limits

These are the assumptions behind every number, and why each one is there:

• One continuous fence line. If the line turns corners, a post must land on every corner, so each side gets split into its own sections. That can add a post or two; running the calculator once per side and subtracting the shared corner posts gives the exact count. Separate runs that don’t connect each need one extra end post. Add those by hand.
• Each gate adds one post. A gate splits the run, and the post on the far side of the opening is the extra one. Gates are not counted for pickets or rails. Many gates are bought pre-built or framed separately.
• Post spacing of 8 ft is a rule of thumb. We did not find a manufacturer or code figure for it, so it is an input you can change.
• Hole size follows Quikrete’s Setting Posts guide: a hole 3 times the post width, 1/3 of the post buried, and 6 in of gravel underneath. Quikrete’s Fast-Setting Concrete data sheet also puts the hole depth at 1/3 of the overall post height. This is a manufacturer’s project method, not a building code.
• Frost depth. If you enter a frost depth deeper than 1/3 of the post, the post is buried to the frost depth instead, so the bottom of the concrete reaches below the frost line. There is no national frost depth. Your local building department can tell you yours.
• Actual post widths are 3.5 in (4x4) and 5.5 in (6x6). These are standard dressed lumber sizes, but we have not checked them against the lumber standard for this page.
• The concrete stops 4 in below ground, under a soil cap. Quikrete’s Fast-Setting Concrete data sheet says to fill the hole to 3 to 4 in from the top and fill the rest with sod or the soil you dug out, and Quikrete’s Setting Posts in Concrete guide says to fill to 3 to 4 in below ground level and backfill with soil and/or sod. The calculator starts at 4 in. You can set the soil cap from 0 in (concrete to ground level) to 6 in in the Advanced step; it must be less than the post’s depth in the ground. The cap changes only the concrete, not the hole size or how deep the post goes.
• The post is subtracted from the concrete; the gravel is not reduced for the post. The post sits on the gravel.
• Bag yields come from Quikrete’s data sheets. Other brands print their own yields.
• Picket waste of 5% is our estimate, not a manufacturer figure. Raise it for rough-sawn or lower-grade boards.
• No hardware, stain, or cost. Nails, screws, brackets, gate hardware, and post caps are not counted, and there are no prices.

Before you dig: call 811 a few days ahead so underground utility lines can be marked. Check your local rules on fence height, setbacks from the property line, and post depth, and any HOA rules. Follow the instructions on your concrete bag. Confirm quantities with your supplier before you order.

FAQ

How many fence posts do I need for a 100 ft fence?

At 8 ft spacing with no gates, a straight 100 ft line needs 13 sections and 14 posts (100 ÷ 8 = 12.5, rounded up to 13, plus 1). With one 4 ft gate, the run drops to 96 ft (12 sections), and the gate adds a post, so it is still 14. At 6 ft spacing with no gates, it’s 17 sections and 18 posts.

How much concrete do I need per 4x4 fence post?

Using Quikrete’s method with a 10 ft post, the hole is 10.5 in wide and the post is buried 40 in. The concrete stops 4 in below ground, so it is 36 in tall and holds about 1.55 cu ft once the post is subtracted: about 4.1 bags of 50 lb Fast-Setting, 3.4 bags of 60 lb Fast-Setting, or 2.6 bags of 80 lb Concrete Mix. The table below covers other post lengths.

How much concrete per fence post 2 feet deep?

For a 4x4 in a 10.5 in hole with 2 ft (24 in) of concrete around the post: (86.59 − 12.25) × 24 ÷ 1,728 = about 1.03 cu ft. That’s about 2.75 bags of 50 lb Fast-Setting, 2.3 bags of 60 lb, or 1.7 bags of 80 lb Concrete Mix. If 2 ft is how deep the post goes in the ground, take off the 4 in soil cap: 20 in of concrete is about 0.86 cu ft (2.29 bags of 50 lb Fast-Setting). Two feet in the ground is what Quikrete’s one-third rule gives a 6 ft post.

How deep should a fence post be for a 6 ft fence?

Quikrete’s guide buries 1/3 of the post’s length, plus 6 in of gravel below. For the post to stand 6 ft tall, it needs to be 9 ft long, with 3 ft in the ground and a hole 42 in deep. Because 9 ft isn’t a standard lumber length, buy a 10 ft post and cut it to 9 ft, or set the full 10 ft post 40 in deep (a 46 in hole) and trim the top. Either way the concrete stops about 4 in below ground and the top is filled with soil. In cold areas the post may need to go deeper to clear the frost line. Your local building code has the final say.

What is the recommended depth for fence posts?

There is no single national figure. Quikrete’s project guide uses 1/3 of the post length in the ground plus 6 in of gravel, with concrete up to 3 to 4 in below ground. Local codes and frost depth can require more. Enter your frost depth in the Advanced step and the calculator deepens the hole when it governs.

How many pickets are in an 8 foot section?

An 8 ft section is 96 in. With 5.5 in pickets and a 0.5 in gap, that’s 96 ÷ 6 = 16 pickets. Butted tight with no gap, it’s 96 ÷ 5.5 = 17.5, so 18 pickets. Board-on-board with 1 in overlap on each edge takes 96 ÷ 4.5 = 21.3, so 22 pickets. Pre-built panels vary by product, so count the boards on the panel you’re buying.

Should I use Fast-Setting concrete or regular Concrete Mix for posts?

Both are listed. Fast-Setting comes in 50 and 60 lb bags. Concrete Mix 80 lb bags hold more per bag (0.60 cu ft), so you buy fewer bags, but each one is heavier. Follow the mixing and setting directions on the bag you choose.

Concrete bags per post, by post size and length

These rows use the same Quikrete hole rule as the calculator, with no frost depth and the 4 in soil cap (concrete height = depth in the ground − 4 in). Each figure is for one post, with the post’s own volume taken out. Round up to whole bags only once you’ve added up all your posts. The 9 ft rows are for a 10 ft post cut down to 9 ft.

Post	Post length	Hole (diameter × depth)	Concrete height	Concrete per post	50 lb Fast-Setting	60 lb Fast-Setting	80 lb Concrete Mix
4x4	6 ft	10.5 in × 30 in	20 in	0.86 cu ft	2.29 bags	1.91 bags	1.43 bags
4x4	8 ft	10.5 in × 38 in	28 in	1.20 cu ft	3.21 bags	2.68 bags	2.01 bags
4x4	9 ft	10.5 in × 42 in	32 in	1.38 cu ft	3.67 bags	3.06 bags	2.29 bags
4x4	10 ft	10.5 in × 46 in	36 in	1.55 cu ft	4.13 bags	3.44 bags	2.58 bags
4x4	12 ft	10.5 in × 54 in	44 in	1.89 cu ft	5.05 bags	4.21 bags	3.15 bags
6x6	6 ft	16.5 in × 30 in	20 in	2.12 cu ft	5.67 bags	4.72 bags	3.54 bags
6x6	8 ft	16.5 in × 38 in	28 in	2.97 cu ft	7.93 bags	6.61 bags	4.96 bags
6x6	9 ft	16.5 in × 42 in	32 in	3.40 cu ft	9.07 bags	7.55 bags	5.67 bags
6x6	10 ft	16.5 in × 46 in	36 in	3.82 cu ft	10.20 bags	8.50 bags	6.37 bags
6x6	12 ft	16.5 in × 54 in	44 in	4.67 cu ft	12.46 bags	10.39 bags	7.79 bags

A 6x6 post takes about 2.5 times the concrete of a 4x4 at the same length, because its hole is about 1.6 times as wide (16.5 in vs 10.5 in), which makes the hole’s area about 2.5 times as large.

Posts, rails, and pickets by fence length. The same settings as the worked example (8 ft spacing, one 4 ft gate, 3 rails, 5.5 in pickets with a 0.5 in gap, 5% waste, 4x4 × 10 ft posts, 4 in soil cap, 50 lb Fast-Setting):

Fence length	Posts	Rails	Pickets	50 lb bags
50 ft	8	18	97	34
100 ft	14	36	202	58
150 ft	21	57	307	87
200 ft	27	75	412	112

Change any setting above the page to see your own numbers. The donut chart under your results shows how one post hole splits into concrete, gravel, buried post, and the soil cap on top.