Fence Calculator

What Is the Fence Calculator?

The Fence Calculator produces a material list for a wood picket or privacy fence. From the total length of the fence and a few choices about spacing, it counts the posts, horizontal rails, pickets and bags of concrete needed.

Fences are built from repeating sections. Each section spans the distance between two posts, carries two or three rails, and is covered by a row of pickets. Because of that repetition, a small error in the count for one section multiplies along the whole run. Working out the quantities properly before buying avoids a second trip and a mismatched batch of lumber.

How the Calculator Works

The calculator divides the fence length by the post spacing and rounds up to get the number of sections. A straight run needs one more post than it has sections, because there is a post at each end.

Rails are counted per section. Two rails are standard for fences up to about 5 feet tall, and three for 6 foot fences.

Pickets are counted by dividing the fence length in inches by the width of one picket plus the gap between pickets. For a solid privacy fence, the gap is zero or very small. For a traditional picket fence, the gap is often 2 to 3 inches.

Concrete is estimated per post. Two 50 pound bags of fast-setting mix is a common allowance for a 4×4 post in a hole about 10 inches across and 2 feet deep.

The Fence Formula

Sections = Fence length ÷ Post spacing, rounded up

Posts = Sections + 1

Rails = Sections × Rails per section

Pickets = Fence length (in) ÷ (Picket width + Gap), rounded up

Concrete bags = Posts × Bags per post

Equalising section widths

If the length does not divide evenly, do not leave one short section at the end. Divide the length by the number of sections to get an equal width for all of them. The calculator shows this as the actual section width.

Step-by-Step Explanation

1. Measure the full length of the fence line in feet.
2. Divide by your post spacing and round up to get sections.
3. Add one to get the number of posts.
4. Multiply sections by the number of rails per section.
5. Convert the fence length to inches and divide by picket width plus gap.
6. Multiply posts by the number of concrete bags per post.

Worked Example

A backyard needs 100 feet of 6 foot privacy fence. Posts will be 8 feet apart, pickets are 5.5 inches wide with a 1/4 inch gap, and there are three rails per section.

• Sections: 100 ÷ 8 = 12.5, rounded up to 13.
• Posts: 13 + 1 = 14.
• Actual section width: 100 ÷ 13 = 7.69 feet.
• Rails: 13 × 3 = 39.
• Pickets: 1,200 ÷ 5.75 = 208.7, so 209.
• Concrete: 14 × 2 = 28 bags.

Adding 5 percent to the pickets for splits gives 220. If the fence turns two corners and has one gate, add posts for those as described below.

How to Use the Calculator

1. Enter the total fence length.
2. Enter the post spacing you plan to use.
3. Enter the actual picket width and the gap between pickets.
4. Choose two or three rails per section.
5. Set the number of concrete bags per post.
6. Press Calculate and note the material list.

For large footings or a concrete mowing strip under the fence, the Concrete Calculator gives volume and bags. Drainage stone for the bottom of each hole can be estimated with the Gravel Calculator. To price posts and rails by the board foot, use the Lumber Calculator. The same board-and-gap arithmetic applies to deck surfaces in the Decking Calculator, and the Square Footage Calculator gives the fence face area if you plan to stain or paint it.

Practical Tips

• Call your utility locating service before digging. It is free in most areas and required by law in many.
• Check the property line and local rules on fence height and setback before buying materials.
• Set corner and end posts first, run a string between them, and set the line posts to the string.
• Dig holes to about one third of the post’s total length, and below the frost line in cold climates.
• Put a few inches of gravel in each hole for drainage before setting the post.
• Use pressure-treated posts rated for ground contact.
• Use a spacer block for consistent picket gaps, and check for plumb every few pickets.
• Keep the bottom of the pickets 2 inches above the soil to slow rot.

Common Mistakes

Forgetting the extra post. A straight run always has one more post than sections.

Not allowing for gates. Each gate needs a post on both sides, and gate posts should be larger or set deeper to carry the weight.

Spacing posts too far apart. Beyond 8 feet, rails sag and the fence weakens in wind.

Using nominal lumber sizes. A 1×6 picket is 5.5 inches wide, not 6. Using 6 inches undercounts by about 8 percent.

Shallow post holes. Posts set less than 2 feet deep lean within a few seasons.

Ignoring slope. On sloping ground, decide whether the fence will step down or follow the grade. Stepped fences need longer posts on the downhill side.

Corners, Ends and Gates

The formula assumes one straight run. For a fence with corners, the simplest method is to calculate each straight side separately, then subtract one post for each corner where two sides share a post. For a gate, subtract the gate width from the fence length and add one post. A 4 foot walk gate in a 100 foot fence leaves 96 feet of fencing and adds a second gate post.

Post Size and Depth

Line posts for a 6 foot fence are usually 4×4 lumber, 8 feet long, with 2 feet in the ground. In windy areas or for fences over 6 feet, 6×6 posts or 4×6 posts are stronger. Gate posts and corner posts benefit from the larger size. In regions with frost, the bottom of the hole should be below the local frost depth to prevent heave.

Choosing Fence Lumber

Pressure-treated pine is the most economical and takes stain well once dry. Cedar and redwood resist rot naturally and stay straighter, at a higher price. Whichever you choose, use hot-dipped galvanised or stainless fasteners. Standard screws corrode quickly in treated lumber and leave black streaks on cedar.

Fence Styles and Their Picket Counts

A solid privacy fence uses pickets butted together or with a small gap, as in the example above. A shadowbox fence alternates pickets on both sides of the rails with a gap about equal to the picket width, so it uses roughly the same number as a solid fence. A board-on-board fence overlaps pickets and can use 30 to 50 percent more. A spaced picket fence with a 2.5 inch gap uses about 30 percent fewer than a solid one. Enter the gap that matches your style, and for overlapping designs use a negative overlap by reducing the picket width to the exposed width.

Estimating Cost and Time

To price the fence, multiply each quantity from the calculator by its unit price: posts, rails, pickets, concrete and fasteners. Add post caps, a gate kit and hinges if you need them. Screws are worth counting too. Each picket takes two screws per rail, so 209 pickets on three rails need about 1,250 screws. As a guide to time, two people can usually set posts for a 100 foot fence in a day, let the concrete cure overnight, and hang rails and pickets over the next one to two days. Renting a powered auger makes the digging far quicker in most soils, though rocky ground and tree roots still call for hand digging.