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Fence Post Spacing Calculator

Each run in even bays, none wider than you allow — the spacing to mark out and the posts it takes.

An estimate — mark the first and last posts, then measure the bays between them.

40' · 480 in · 40 ft · 12.192 m

40 ft

5% covers pickets split by a screw or culled for warp and knots. It is added once, to the pickets — the posts, rails and concrete are counted from the fence itself.

This page recalculates as you type. There is no Calculate button.

Answer

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Widest post spacing

8' on center

40 ft of fence · 5 bays · 6 posts in holes 42" deep

Posts 6 posts
Pickets, with waste 93 pickets
Rails 15 2×4s · 3 a bay
Fence, gates left out 40 ft
Concrete 20 × 50 lb fast-setting
Gravel under the posts 4 × 50 lb bags (0.5 ft³)
Ticked items cost $648.87
More numbers
Post holes 42" deep, 10" across
DrawingThe project drawn to scale. Left: 40' × 6'.40'6'Left

Buying noteCall 811 a few business days before you dig. It is the free national before-you-dig service: the utilities mark their lines, and every post hole needs it.
Buying noteYour frost depth is not set. Where the ground freezes, a post set above the frost line heaves: ask your building department for the depth (IRC Table R301.2(1)) and type it under Assumptions — every hole follows it.

Breakdown

Run or gate Measurements Qty Length Share
Left 40' Add ×1 40 ft

What to buy

Buying noteCall 811 a few business days before you dig. It is the free national before-you-dig service: the utilities mark their lines, and every post hole needs it.
Buying noteYour frost depth is not set. Where the ground freezes, a post set above the frost line heaves: ask your building department for the depth (IRC Table R301.2(1)) and type it under Assumptions — every hole follows it.

Ways to buy the concrete

The bags for every post hole together, by the shape of the holes. Every way is worked out; pick the one you will buy.

20 × 50 lb fast-setting Bags 20 Left over 0.02 ft³ Weight 1,000 lb Cost $159.60 Reference price $7.98. Type your store's.

Shopping list

6 6 × 10 ft $101.40 Reference price $1.69. Type your store's.
93 93 × 6 ft $212.04 Reference price $0.38. Type your store's.
15 15 × 8 ft $72.00 Reference price $0.60. Type your store's.
2 1 × 5 lb box (453) + 1 × 1 lb box (126) $48.94 Reference price $10.97. Type your store's. Reference price $37.97. Type your store's.
1 1 × 5 lb box (309) $32.97 Reference price $32.97. Type your store's.
6 6 × 4×4 caps $47.88 Reference price $7.98. Type your store's. Reference price $12.98. Type your store's.
20 20 × 50 lb fast-setting $159.60
4 4 × 50 lb bags (0.5 ft³) $21.92 Reference price $5.48. Type your store's.
Ticked items $648.87

Untick what you already have. Lumber is priced by the foot, beside its row; the concrete's bags are priced in the table above.

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How this was worked out

Every number below is yours. Each constant links to the row where you can change it.

Left 40' 40 ft
Posts 5 bays in 1 runs: 4 line + 2 end + 0 corner + 0 gate 6 posts

Change:Widest post spacing, wood

How deep a post goes 0.5 × 72" out of the ground, or the frost depth 0" where deeper 36" + 6" = 42"

Change:Post in the ground, share of its height aboveYour frost depthGravel in the bottom of a hole

Posts, length 72" + 36" = 108" 6 × 10 ft
Concrete, each post (π ÷ 4 × 10" × 10" − 3 1/2" × 3 1/2") × 32 1/2" 1.247 ft³ × 6 = 7.481 ft³

Change:Hole across, 4×4 postConcrete stops under the ground

Bags of concrete 7.481 ft³ ÷ 0.375 ft³ 19.95 → 20 × 50 lb fast-setting

Change:Yield, 50 lb fast-setting bag

Gravel under the posts π ÷ 4 × 10" × 10" × 6" × 6 1.636 ft³ → 4 × 50 lb bags (0.5 ft³)

Change:Gravel in the bottom of a hole

Rails 5 × 3 = 15 15 × 8 ft

Change:A rail every

Pickets, edge to edge 480" ÷ 5 1/2" 88 × 6 ft

Change:Gap under the pickets

With the waste for culls 88 × (1 + 5%) 92.4 → 93
Screws 88 pickets × 3 rails × 2 + 15 boards × 2 ends × 2 528 + 60

Change:Screws where a picket crosses a railScrews at each end of a rail

What it comes to 6 × 10 ft × $1.69/ft + 93 × 6 ft × $0.38/ft + 15 × 8 ft × $0.60/ft + 1 × $37.97 + 1 × $10.97 + 1 × $32.97 + 20 × $7.98 + 4 × $5.48 $648.87

Assumptions

Every one of these is editable. Change one and the answer changes with it.

Assumption Value Confidence
Widest post spacing, woodPosts of a wood fence no more than 8 ft apart, center to center: the spacing Inch Calculator and Omni both work at, and the length of the 2×4 rail that spans a bay. ≈ in Trade convention
A rail everyOne rail for every 24 in of fence height, never fewer than a top and a bottom one: Inch Calculator's rule - three rails on a 6 ft fence, two up to 4 ft. ~ in Derived or single-supplier
Gap under the picketsPickets stop about 2 in above the ground so they do not wick water; Freedom's vinyl kit sets its panels about 2 in up, and 2 in is the most ISPSC 305.2.1 allows over grass or gravel. ≈ in Trade convention
Screws where a picket crosses a railTwo screws where each picket crosses a rail, the usual nailing of a privacy fence. ~ screws Derived or single-supplier
Screws at each end of a railTwo screws at each end of a rail or a gate board, into the post or the frame. ~ screws Derived or single-supplier
Post in the ground, share of its height aboveQuikrete Fast-Setting Concrete Mix spec sheet, section 5: a post hole one half as deep as the post stands above ground. Quikrete's Setting Posts sheet says the same as one third of the post's length, and so does Home Depot's chain link guide for its posts. ratio From a data sheet or an agency document
Your frost depthYour frost depth, from your building department (IRC Table R301.2(1) is filled in locally). 0 means not given: every hole is then as deep as the post's own rule. Typed in, no hole stops above it. ≈ in Trade convention
Gravel in the bottom of a holeQuikrete Setting Posts, step 2: 6 in of gravel or crushed stone in the bottom of the hole, for drainage. in From a data sheet or an agency document
Concrete stops under the groundQuikrete Setting Posts, steps 4 and 6: fill the hole to 3-4 in below the top, then finish with soil. The middle of the range. in From a data sheet or an agency document
Hole across, 4×4 postQuikrete Fast-Setting spec sheet, section 3: a 4 in post set in a 10 in hole. Its rule of three times the post would make 10½ in for a 4×4. in From a data sheet or an agency document
Hole across, 6×6 postA 6×6 gate post. Quikrete's three times the post would be 16½ in, a hole few dig for a fence; 12 in is an auger's size. ~ in Derived or single-supplier
Yield, 50 lb fast-setting bagQuikrete Fast-Setting Concrete Mix spec sheet. ft³/bag From a data sheet or an agency document

How far apart to set fence posts

Divide each run into equal bays no wider than the spacing you allow, and the bays round up. 100 feet at no more than 8 feet is 12½ bays, so 13 bays of 7' 8 5/16" on center, and 14 posts. Thirteen bays of 8 feet would overshoot by 4 feet; twelve would leave one bay 4 feet wider than the rest. Even bays put every rail on the same cut and every post the same distance from the next.

RunBays at 8 ft or lessOn centerPosts
50 ft77' 1 11/16"8
60 ft87' 6"9
75 ft107' 6"11
100 ft137' 8 5/16"14
150 ft197' 10 3/4"20

Which spacing

A wood fence on 2×4 rails is set no more than 8 feet apart, the length of the rail that spans a bay; wider than that a rail sags under the pickets, which is what builders say and what Inch Calculator and Omni both use — no standard gives the figure. Chain link line posts may be 10 feet apart. A panel fence has no choice: its posts stand a panel apart, 96 inches for an 8 foot vinyl or wood panel and 72½ for a 6 foot aluminum one. Split rail stands a rail apart, about 10 feet 3 inches for an 11 foot rail. Each is a figure under Assumptions you can change.

Several runs

Every run is divided on its own, from corner to corner or from end to gate, because a corner post is where both runs stop. A back yard of 50, 100 and 50 feet is 7, 13 and 7 bays — 27 — and 28 posts, the corners counted once.

Marking it out

Set the end and corner posts first and pull a string between them at post height. Measure the on-center spacing along the string from one end, mark each post, and dig. Measuring each bay from the last post adds up its errors; measure every mark from the same end. For the gaps between pickets — the even spacing a spacer block is cut to — use the picket fence calculator. We are not connected with any maker, store or supplier named here.