Use this fence post spacing calculator to figure out how many posts you need, the exact spacing between them, and how many bags of concrete to buy for setting each post hole.
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Enter your total fence length, the maximum distance you want between posts, your post width, and the size of the hole you plan to dig. The calculator instantly returns the number of posts required, the actual (evenly divided) spacing between them, and how many bags of concrete you need to fill every hole around each post.
Getting fence post spacing wrong wastes money and creates a wobbly, uneven fence line. Most residential wood and vinyl fences use 6-8 ft spacing between posts, while chain link often runs 10 ft on center. If you guess at spacing instead of dividing your total run evenly, you end up with one short, awkward section at the end that looks unbalanced and weakens the last panel. Concrete is the other half of the equation — under-ordering means a mid-project trip to the hardware store, while over-ordering wastes cash on unused bags. A typical 4x4 post set 24 in deep in a 10 in hole needs roughly two 60 lb bags of concrete, but that adds up fast across 15-20 posts on a 100 ft fence line. Contractors, DIY homeowners, and rental yards all use this kind of calculation to quote jobs accurately and buy exactly what's needed before digging a single hole.
The tool first determines the number of equal sections along your fence: sections = ROUNDUP(length ÷ max spacing). Posts = sections + 1, and actual spacing = length ÷ sections (always ≤ your maximum). For concrete, it treats the hole as a cylinder and the post as a square column running the full depth of the hole, subtracting post volume from hole volume:
Hole volume (cu ft) = π × (diameter/2)² × depth ÷ 1728
Post volume (cu ft) = width² × depth ÷ 1728
Concrete per hole = Hole volume − Post volume
Bags needed = ROUNDUP(total concrete ÷ bag yield)
Note: this assumes the post fills the hole's full cross-section for the entire depth, a standard simplification used for material estimating. If your post sits on a footing pad or doesn't reach the hole's full depth, actual concrete needed will be slightly higher than this estimate.
Say you're fencing a 100 ft backyard run with a max spacing of 8 ft, using 4x4 posts in a 10 in diameter hole dug 24 in deep, filled with 60 lb bags (0.45 cu ft yield each).
Sections = ROUNDUP(100 ÷ 8) = 13, so posts = 14 and actual spacing = 100 ÷ 13 ≈ 7.69 ft. Hole volume = π × 5² × 24 ÷ 1728 ≈ 1.09 cu ft. Post volume = 4² × 24 ÷ 1728 ≈ 0.22 cu ft. Net concrete per hole ≈ 0.87 cu ft. Total for 14 posts ≈ 12.16 cu ft, which needs ROUNDUP(12.16 ÷ 0.45) = 28 bags of 60 lb concrete.
• Always dig holes 1/3 the post's above-ground height, and never less than local frost depth in your region.
• Round hole diameter up to the nearest trowel or auger size you actually own — most post-hole augers come in 6, 8, 10, and 12 in sizes.
• Don't forget gate openings: subtract the gate width from your total fence length before calculating post spacing for the remaining runs.
• Corner and end posts often need larger holes and more concrete than line posts because they bear more lateral load.
• Order 5-10% extra bags to cover spillage, uneven ground, and holes that turn out deeper than planned.
Most wood and vinyl privacy fences use 6 to 8 ft spacing between posts, while chain link fences often use 10 ft spacing since the fabric itself provides tension support. Taller or heavier fence styles usually need tighter spacing for stability.
A common rule of thumb is one-third of the post's total length below ground, with a minimum of 24 in for most residential fences. Always check your local frost line depth, since posts set above frost depth can heave in freeze-thaw cycles.
It depends on hole size, but a typical 10 in diameter hole dug 24 in deep for a 4x4 post needs roughly 0.85-0.9 cu ft of net concrete, which works out to about two 60 lb bags since each bag only yields 0.45 cu ft. Use the calculator above with your exact hole and post dimensions for a precise number.
It's best to divide your total fence length evenly so every section is the same width — this looks better and keeps consistent load distribution. This calculator automatically evens out the spacing so no section exceeds your maximum.
Yes, fast-setting concrete mixes are popular because you just pour dry mix around the post and add water — no premixing required. The volume calculation is the same regardless of which type of concrete mix you choose.
Corner, end, and gate posts typically bear more lateral stress and are often set in larger diameter holes with more concrete than mid-line posts. Many installers use a hole diameter one size larger for these key posts.