MathBench

Fence Calculator

Enter your fence line's length and get the full shopping list: posts at your chosen spacing, rails, pickets at your width and gap, and concrete bags to set every post.

Your shopping list

—posts
—rails
—pickets
—concrete bags (50 lb)

How this calculator works

The calculator divides your fence line into sections at the post spacing you choose, rounds partial sections up (a 100-foot run at 8-foot spacing is 13 sections, not 12½), and counts one more post than sections to close the run. Rails multiply by section count, pickets divide the run by the width of one picket plus its gap, and concrete scales with the post count.

Fences with corners are just several straight runs: run the calculator once per side and add the results. The shared corner post means you can subtract one post per corner from the total, or keep the extra as your spare.

The formula

Sections = ceil(run ÷ post spacing)
Posts    = sections + 1        (straight run)
Rails    = sections × rails per section
Pickets  = ceil(run × 12 ÷ (picket width + gap))
Concrete = posts × bags per post

The default two 50-lb bags per post fits a 4×4 post in an 8-inch-diameter hole about 24 inches deep, following fast-setting concrete manufacturer guidance (e.g., QUIKRETE's post-setting instructions). Deeper frost-line holes and 6×6 gate posts need more.

Worked example

Say you're building a 100-foot, 6-foot-tall privacy fence, with 8-foot post spacing, 3 rails per section, 5½″ pickets butted tight:

  1. Sections: 100 ÷ 8 = 12.5 → 13 sections
  2. Posts: 13 + 1 = 14
  3. Rails: 13 × 3 = 39
  4. Pickets: (100 × 12) ÷ 5.5 = 218.2 → 219 (order ~230 with spares)
  5. Concrete: 14 × 2 = 28 bags (50 lb fast-setting)

That's roughly a full pallet of pickets, worth a delivery rather than twenty pickup-truck trips.

Assumptions & tips

  • Call 811 before digging. Post holes and buried utilities occupy the same 24 inches. The locate service is free and legally required in most of the US. Schedule it a few days ahead.
  • Set corner and gate posts first. Stretch a string line between them and set line posts to the string, not to a tape measure: fences are judged by eye down the line, and the string is what makes it straight.
  • Slopes: rack or step. Racked (parallelogram) sections follow gentle grades with the same material counts. Stepped fences on steeper grades create triangular gaps and need longer posts. Either way the picket count barely changes. It's the posts that get longer.
  • Gates are their own project. Each gate wants 6×6 posts (or steel), set deeper with more concrete, plus hinge and latch hardware. Frame the opening at the hardware's specified width, not the gate's.
  • Know the line and the law. Confirm the property line before setting posts an inch past it, and check local rules. Many municipalities cap height at 6 feet and require the "good side" facing out.

Frequently asked questions

Should fence posts be 6 or 8 feet apart?

Eight feet is the standard maximum, because rails come in 8-foot lengths and spanning further makes them sag. Go down to 6 feet for tall solid-privacy fences in windy areas. A 6-foot privacy fence is effectively a sail, and the shorter span stiffens the whole run. Chain link and wire fences follow their own spacing rules.

How deep should fence posts be set?

Rule of thumb: one-third of the post height above ground, with 24 inches as a practical minimum, and always below the local frost line so heave cannot lift the footing. A 6-foot privacy fence therefore wants posts about 2 feet in the ground, so it uses 8-foot posts.

How many bags of concrete does each post need?

For a 4×4 post in an 8-inch-diameter hole about 24 inches deep, plan on roughly two 50-pound bags of fast-setting concrete per post. Bigger holes, deeper frost lines, and 6×6 gate posts push that toward three or four bags. Fast-setting mix is poured dry into the hole and watered, no mixing tub involved.

Sources

  1. Fast-Setting Concrete Mix, Product No. 1004-50, 60, 66, 25 — Product Data Sheet. The QUIKRETE Companies. quikrete.comThe two-bags-per-post default: the sheet states that two 50 lb bags will set a 3-inch square post in a 9-inch diameter hole 30 inches deep, and it describes the dry-pour-then-water method the FAQ refers to.
  2. Setting Posts in Concrete — project instructions. The QUIKRETE Companies. quikrete.comThe hole-sizing rule behind the post-depth answer: a hole three times the width of the post, dug to one-third to one-half of the above-ground post length plus 6 inches.
  3. 2021 International Residential Code, Section R403.1.4.1 — Frost protection. International Code Council. Requires permanent supports to extend below the frost line specified for the location, so post depth is set by the local frost depth rather than by a rule of thumb.
  4. 811 Before You Dig. Common Ground Alliance. 811beforeyoudig.comThe free national utility-locate service behind the call-811 tip, including its instruction to place the request a few business days before any digging begins.

  • Concrete Calculator

    Slabs, footings, and round columns: cubic yards plus 60 lb and 80 lb bag counts.

  • Gravel Calculator

    Area and depth to cubic yards and tons, with density presets for crushed stone, pea gravel, and sand.

  • Mulch Calculator

    Rectangular and circular beds to cubic yards and bag counts at your chosen depth.

  • Paint Calculator

    Room size to gallons: wall area minus doors and windows, per coat, at your paint’s coverage rate.