Fence Calculator

How Many Fence Posts Do I Need?

Posts = fence length ÷ post spacing, rounded up, plus 1. The rounded-up part is the number of sections between posts, and a straight run always has one more post than sections. Millard Lumber puts posts about 6 to 8 ft apart, and the USDA NRCS Vermont wooden board fence specification, the City of Mansfield, Texas privacy fence standards and the Heber City, Utah wood fence specification all cap the spacing at 8 ft. The table is for a straight run with no gates or corners.

Gates and corners change the count. A gate opening comes out of the run and needs its own pair of posts, a hinge post and a latch post. A corner post is shared by the two runs that meet, but the spacing restarts there, so a corner can add a section. The tool splits the run at every gate and corner, assumes the pieces are equal, and counts each one, so the result is a good order quantity, not a survey. Set Closed perimeter when the fence goes all the way round, because a closed loop has no end posts.

How to Calculate a Fence

1. Measure the run. Add up every straight side the fence will cover, in feet, along the line the posts will take. Subtract each gate opening. A 100 ft run with one 4 ft gate leaves 96 ft of fence.
2. Divide by the spacing. 96 ÷ 8 = 12 sections. If the answer has a fraction, round up, because a short last section still needs a post.
3. Add the posts. A straight run has sections + 1 posts. Each gate adds two, and each corner shares one.
4. Count the rails. Multiply the sections by the rails per section. The next sections give the rule.
5. Count the boards. Divide the fenced length by the board width plus the gap, and add a few percent for cuts.
6. Size the holes. Dig half the fence height deep and three post widths across, and divide the volume by the bag yield for the concrete.

The calculator above does every step and shows the sections, so you can check the count against a sketch of your own yard.

Fence Post Depth

Bury about a third of each post. Millard Lumber says to set wood fence posts with about 1/3 of their total length in the ground, and the QUIKRETE Fast-Setting Concrete Mix data sheet says the hole should be one-half of the above ground post height deep. Those are the same rule: a 6 ft fence needs a hole 36 inches deep, and the post is 9 ft long, with 3 ft in the ground and 6 ft showing. The table applies it to common heights, with a 4x4 post (3 1/2 inches square) in a hole three times as wide, as the QUIKRETE sheet advises.

Local rules can ask for more or less. The City of Mansfield, Texas standard sets 24 inches for a 6 ft fence and 36 inches for an 8 ft fence, but for steel pipe posts, and the USDA NRCS Vermont specification sets at least 36 inches for line posts and sets gate, end and brace posts 46 inches on a board fence. Where the ground freezes, a post set above the frost line can heave, so confirm the frost depth and any permit with your building department. Type your own depth under Optional to override the rule.

Concrete for Fence Posts

The QUIKRETE data sheet gives three numbers for setting a post: a hole three times the post width, dry mix poured to about 3 to 4 inches below the top, and a yield of about 0.375 cubic foot from each 50 lb bag. The tool fills the round hole to 3.5 inches below the top, subtracts the square post, and divides by the yield. For a 4x4 in a 36 inch hole that is about 1.4 cubic feet, or 3.7 bags per post. The sheet’s own example, two 50 lb bags for a 4 inch post in a 10 inch hole 2 ft deep, comes out at about 2.1 bags by the same method.

The bag count for the whole fence is rounded up once, not per hole, so leftover mix from one hole counts toward the next. Heber City, Utah specifies a 12 inch diameter concrete base and at least 6 inches of concrete under the post for its cedar posts. Millard Lumber puts a large stone or two shovels of gravel in the bottom of each hole; the Gravel Calculator sizes gravel if you want to buy it for a long run.

Fence Rails

One rail for every 2 ft of height, rounded up, and at least two. The City of Mansfield, Texas standard asks for one stringer for every two feet of height, which makes three for a 6 ft fence and four for an 8 ft fence, and sets a minimum size of 1 1/2 by 3 1/2 inches, the dressed size of a 2x4. The Heber City, Utah specification calls for a top rail and a bottom rail on a cedar fence. The table gives the rail count per section and for 100 ft at 8 ft spacing. Split rail fences use the same math with no boards; choose the Split rail style, and type the rails per section if your design uses two or three.

Fence Board Calculator: How Many Boards

Boards = (length + gap) ÷ (board width + gap), rounded up. Count the boards and the gaps between them: n boards have n minus 1 gaps, so the length is n times the width plus n minus 1 times the gap, which rearranges to the formula. Use the dressed width, not the nominal one: a 1x6 is 5 1/2 inches wide. For a privacy fence, set the gap to 0 for boards butted tight, or to a small gap if you want room for the wood to swell. For a picket fence, set the gap you want between pickets. The table shows the boards in 100 ft of fence before any extra.

Order a few percent over. The tool adds 5 percent by default for cuts, splits and warped stock, and you can change it under Optional. The heights, board widths and gaps that each style starts with are only starting values, so change them to match your design.

Picket Fence Calculator

Pickets = (length + gap) ÷ (picket width + gap), rounded up. Choose the Picket style in the tool and it starts from a 3 ft fence of 1x4 pickets, 3 1/2 inches wide, with a 3 inch gap, which are starting values you can change. Each picket and its gap use 6.5 inches, so a foot of fence takes about 1.85 pickets. The rest of the count works as it does for a privacy fence: posts from the spacing, rails from the height, and concrete from the hole. The table shows how the gap changes the picket count for 100 ft of fence with 1x4 pickets.

For example, a 40 ft front-yard picket fence with one 3 ft gate, 1x4 pickets, a 3 inch gap and posts 8 ft apart: the fenced length is 37 ft, which splits into two pieces of 18.5 ft and takes 6 sections and 8 posts. The pickets come to 444 in ÷ 6.5 in, rounded up, which is 69 pickets, or 73 with 5 percent extra. A 3 ft fence takes 2 rails per section, so 12 rails. Even spacing of the pickets is a layout job: divide the leftover space so the first and last pickets sit evenly, and the gap you land on may be a little off the one you typed.

Fence Cost Calculator: How Much Does a Fence Cost?

Fence prices swing with the region, the wood or vinyl, the height and the labor, so this page does not quote a price per foot. The calculator works out the quantities, and you supply the prices from your lumber yard, your store or a written quote. Open Optional: fence cost from your prices under the calculator and type a price for a post, a rail, a board and a bag of concrete. It adds up the materials, and nothing is assumed, so any price you leave blank counts as nothing.

Materials cost = posts × post price + rails × rail price + boards × board price + bags × bag price. If a contractor quotes by the foot, type the quote per foot (or per metre) and the tool multiplies it by the fenced length, which leaves out the gate openings because gates are often priced separately. Ask what your quote includes, since installation, hardware, gates, old fence removal, permits and delivery can each be priced on their own line. The table lists the quantities for the worked example further down, a 6 ft privacy fence with 96 ft of fence after the gate, ready for your prices.

Add the line totals for the materials cost, then add the gate hardware, screws or nails, post caps and stain or sealer that the counts do not include. To compare the do-it-yourself figure with a contractor, divide your materials cost by the 96 ft of fence to get a cost per foot of materials, and set it beside the quoted installed price per foot. Height, style and material move the answer most: a taller fence needs longer posts and more rails, and a privacy fence needs many more boards than a picket fence of the same length.

Lumber Sizes: Nominal vs Actual

Lumber is sold by a nominal name that is bigger than the board. Engineering Toolbox lists the dressed sizes below, and the Southern Pine Inspection Bureau confirms the 4x4, the 2x4 and the 1x6. Posts, rails and boards all use these, so the calculator’s size buttons use the actual width.

Worked Example

Say you are building a 6 ft privacy fence of 1x6 boards, 100 ft long with one 4 ft gate, posts 8 ft apart, 4x4 posts, boards butted tight.

1. Fenced length: 100 − 4 = 96 ft.
2. Sections: the gate splits the run into two pieces of 48 ft, and 48 ÷ 8 = 6, so 12 sections.
3. Posts: each piece has 6 + 1 = 7 posts, so 14 posts, and two of them are the gate posts.
4. Rails: 6 ft ÷ 2 = 3 per section, and 3 × 12 = 36 rails.
5. Boards: 96 ft × 12 = 1,152 in, and 1,152 ÷ 5.5 = 209.5, so 210 boards, or 221 with 5 percent extra.
6. Posts and holes: a 6 ft fence needs 9 ft posts in holes 36 in deep and 10.5 in wide.
7. Concrete: 1.40 cu ft per hole ÷ 0.375 = 3.7 bags each, and 14 holes need 53 bags of 50 lb mix, about 2,650 lb.

That is more concrete than many people expect, which is why the tool shows the hole depth and width next to the bag count. If you plan to plant against the new fence, the Mulch Calculator works out the bed.

Common Mistakes

• Forgetting the extra post. A run of 12 sections has 13 posts, not 12. Counting sections as posts leaves the last section hanging.
• Leaving the gate in the board count. A 4 ft gate takes about 9 boards out of a privacy fence. The tool subtracts the opening before it counts boards and rails.
• Spacing posts more than 8 ft apart. Three public specifications stop at 8 ft for a wood fence. Longer spans sag and need heavier rails.
• Using nominal widths. A 1x6 board is 5 1/2 inches wide, not 6, so counting with 6 inches under-orders by about 8 percent.
• Setting posts too shallow. A post buried less than a third of its length leans and works loose. In freezing climates, check the frost depth too.
• Ordering exactly what the math says. Boards split and warp, so order a few percent extra, and buy posts a little long so you can trim the tops level.
• Mixing units. Spacing and height are in feet, board widths and gaps are in inches. Entering a 5.5 inch board as 5.5 ft is the classic slip, so check the unit labels beside each field.

Sources & further reading

The concrete figures, a hole three times the post width, a hole depth of one-half of the above ground post height, dry mix poured to 3 to 4 inches from the top and a yield of 0.375 cubic foot per 50 lb bag, are from the QUIKRETE Fast-Setting Concrete Mix data sheet. Burying a third of the post and spacing posts 6 to 8 ft apart come from Millard Lumber’s Building Wood Fences. The 8 ft spacing limit, the post depths and the rail rules come from the USDA NRCS Vermont Construction Specification 050WB, Fence, Wooden Board, the City of Mansfield, Texas wood privacy fence construction standards and the Heber City, Utah Section 02832, Fences. Dressed lumber sizes are from Engineering Toolbox’s softwood lumber dimensions and the Southern Pine Inspection Bureau’s nominal vs. actual lumber sizes. Building codes, height limits, setbacks and frost depth differ by place, so check with your local building department. For how we source and check the numbers on every calculator, see our methodology page, and for the full topic see the complete guide to lawn and landscaping.