How this instrument works
Building a fence takes more than one material, and each quantity depends on the others in a specific order. Posts come first, set at a chosen spacing along the fence length. Rails run horizontally between those posts, so the rail count depends directly on how many post-to-post sections exist. Pickets fasten to the rails at their own separate pitch (picket width plus gap), independent of post spacing. And each post needs a hole, typically filled with concrete, whose volume depends on the hole's own diameter and depth.
This instrument works through all four in sequence. It first finds the post count from fence length and post spacing (the same formula this site's dedicated fence-post calculator uses), then derives the number of sections and multiplies by rails-per-section for total rail count. Separately, it divides fence length by the picket pitch (picket width plus gap) for the picket count. Finally, it computes the volume of a single cylindrical post hole and multiplies by the post count, then divides by a concrete bag's yield to get the total bags needed.
Several of the inputs here are typical, adjustable figures rather than fixed constants: picket width and gap vary by fencing style, post-hole diameter and depth vary by post size and local frost depth, and concrete bag yield depends on which bag size you're buying (commonly 0.30 cu ft for a 40lb bag, 0.45 for 60lb, or 0.60 for 80lb — check your own bag's label). Adjust each to match your actual materials rather than relying on the defaults for a final order.
- Enter Total fence length (ft) and Post spacing (ft) — drives both the post count and the section count that rails are based on.
- Enter Rails per section — how many horizontal rails run between each pair of posts, commonly 2 or 3.
- Enter Picket width (in) and Gap between pickets (in) — sets the picket pitch used to find total picket count.
- Enter Post hole diameter (in) and Post hole depth (ft) — sizes the concrete volume needed per hole.
- Enter Concrete bag yield (ft³/bag) — check your bag's label; typical values are 0.30 (40lb), 0.45 (60lb), or 0.60 (80lb) cubic feet per bag.
- Read Posts needed, Rails needed, Pickets needed, and Concrete bags needed — a full materials list for one straight fence run.
Worked example — 100 ft of fence at 8 ft post spacing
A 100 ft fence at 8 ft post spacing, 3 rails per section, 5.5in pickets with a 0.25in gap, 10in-diameter post holes dug 2 ft deep, filled with 80lb bags (0.60 cu ft yield). Enter these into their matching fields. Posts needed = ceil(100÷8)+1 = 13+1 = 14, the same count this site's plain fence-post calculator gives. Sections = 13, so rails needed = 13 × 3 = 39.
Picket pitch is 5.5 + 0.25 = 5.75in; fence length in inches is 1,200, so pickets needed = ceil(1200 ÷ 5.75) = ceil(208.696) = 209. Each post hole holds π × (10/24)² × 2 = 1.0908 cu ft; 14 holes need 14 × 1.0908 = 15.2716 cu ft total, and Concrete bags needed = ceil(15.2716 ÷ 0.60) = ceil(25.45) = 26 bags.
Questions
Why does the picket count use a different distance unit than the post count?
Because picket pitch is measured in inches (picket width plus gap, both typically a few inches) while post spacing is measured in feet (typically several feet), so this calculator converts fence length to inches specifically for the picket calculation while keeping it in feet for the post calculation. Both ultimately measure the same fence length, just in the unit that keeps each formula's numbers in a sensible range.
How is this different from the plain fence post calculator?
This is the comprehensive version — it reuses the exact same posts-needed formula as the dedicated fence-post calculator, then adds rails, pickets, and post-hole concrete on top, giving a full shopping list from one set of inputs. Use the plain post calculator if all you need is the post count; use this one when you're ready to price out the whole job.
What concrete bag yield should I use?
It depends on the bag size you're buying, not a universal constant — a typical 40lb premixed concrete bag yields around 0.30 cubic feet, a 60lb bag around 0.45, and an 80lb bag around 0.60, following the standard yield chart most premixed-concrete brands publish. Always check your specific product's label, since exact yields can vary slightly by brand and formulation.
Should I buy extra pickets and rails beyond the calculated counts?
Yes — a small buffer beyond the raw calculated counts is standard practice, since some boards will split, warp, or get cut wrong during installation. Many installers add roughly 5-10% extra material on top of a calculator's exact count, and having a couple of spare matching pickets on hand also makes future repairs easier if one gets damaged.
Does the post-hole depth need to account for frost line in my area?
Yes, if you're in a climate with ground freeze — posts set above the local frost line risk being pushed up by frost heave over repeated freeze-thaw cycles, so many regions require holes dug deeper than a simple one-third-of-post-height rule of thumb would suggest. Check your local building department's frost line depth requirement, and use this site's dedicated fence-post-depth calculator if you want that specific comparison worked out.