Fence Calculator

Tip: call 811 a few days before you dig. Utility lines are marked for free, and hitting one with a post-hole digger is dangerous and expensive.

Worked example

A backyard fence runs 100 ft in total with posts every 8 ft, one gate and two corners. It is a 6 ft privacy fence with three rails per section and 5.5 inch pickets spaced 1/4 inch apart.

100 / 8 = 12.5, which rounds up to 13 sections. 13 sections need 14 line posts, and the gate and two corners add one post each, so you need 17 posts. Three rails per section is 39 rails.

Each picket plus its gap takes 5.75 inches. The fence is 1,200 inches long, so 1,200 / 5.75 = 208.7, which rounds up to 209 pickets. Setting 17 posts takes about 34 bags of 60 lb concrete.

How to calculate fence posts

Divide the total fence length by the post spacing and round up. That is the number of sections. A straight run always needs one more post than it has sections, because there is a post at each end.

Gates and corners usually add posts. A gate needs a sturdy post on each side, and the opening rarely lands exactly on your regular spacing. A corner changes direction, and unless the corner falls exactly on a post position, it adds one. The calculator adds one post for each gate and each corner, which is a safe estimate for planning. Once you have staked out the layout, count the actual post positions and adjust.

Choosing post spacing

Eight feet is the most common spacing for wood fences because rails come in 8 ft lengths and a section that long stays stiff. Six feet is better for heavy privacy fences in windy areas, for board-on-board styles and for fences on slopes. Vinyl and pre-built panel fences use the panel width, often 6 or 8 ft, so enter that as the spacing.

Closer spacing means more posts and more concrete, but each post carries less wind load. If your area gets strong storms, the extra posts are cheap insurance.

Rails and pickets

Rails are the horizontal boards between posts that the pickets attach to. Fences up to 4 ft tall usually have two rails per section. Six-foot privacy fences usually have three, top, middle and bottom, so the pickets do not bow.

For pickets, divide the fence length in inches by the picket width plus gap. Privacy fences use little or no gap. Many builders leave about 1/4 inch so wet boards have room to swell. Picket fences and spaced styles use gaps of 2 inches or more. Board-on-board fences overlap pickets, so they need roughly 40% to 50% more pickets than a single row. The calculator does not subtract gate openings, which gives you a few spare pickets to build the gates themselves.

Setting posts in concrete

A common rule is to bury one third of the post's length, with a minimum of 2 ft and below your local frost line. A 6 ft fence typically uses 8 ft posts set 2 ft deep. Dig the hole about three times the post width, so 10 to 12 inches for a 4x4.

A 4x4 in a 10 inch hole 2 ft deep takes a little under 1 cubic foot of concrete, about two 60 lb bags. Larger holes, 6x6 posts or deeper frost lines take more. For an exact figure for round holes, the volume of a cylinder is 3.14 x radius squared x depth, minus the post. Gravel in the bottom of each hole helps drainage and keeps the post end out of standing water.

Other things to buy

• Gate hardware: hinges, a latch and a drop rod for double gates.
• Fasteners: galvanized or coated nails or screws, two per picket per rail.
• Post caps and stain or sealer if you are finishing the fence.

Check property lines, HOA rules and permit requirements before you buy. Many towns limit fence height and require the finished side to face the neighbors.

More calculators

Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes.