Fence Post Concrete Calculator
Bags per post and for the whole fence line — hole sizing included.
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Materials list
Generated by AllUnits.io — allunits.io/calculators/fence-post-concrete
How this calculator figures it
Hole sizing follows the Quikrete setting-posts guide: hole diameter three times the post width, depth one-third to one-half of the above-ground post height, plus a 6-inch gravel base for drainage. Concrete per hole is the cylinder volume minus the post itself, using the 0.375 cu ft yield of a 50 lb fast-setting bag from the manufacturer data sheet.
How it works
The hole is a cylinder; the post displaces part of it, so the concrete is the ring between them. Post cross-sections use actual dressed dimensions — a 4×4 measures 3.5 × 3.5 inches. Total concrete is per-hole × posts plus the waste factor, divided by the bag yield and rounded up to whole bags. Worked example — 10 posts, 4×4, 6 ft above ground: 10.5" hole × 24" deep holds 1.20 cu ft; minus the post leaves 1.03 cu ft. Times 10 posts plus 10% waste is 11.4 cu ft — 31 bags of 50 lb fast-setting mix, about 3.1 per post.
Fence post FAQs
How deep should a fence post go?
One-third to one-half of the above-ground height — a 6 ft fence needs at least 2 ft in the ground — and below the frost line. Per Quikrete's setting-posts guide.
Can I pour fast-setting concrete dry?
Yes — Quikrete Fast-Setting is designed to be poured dry into the hole, then watered.
How long before the post takes load?
Fast-setting concrete hardens in 20–40 minutes; wait about 4 hours before heavy loading — confirm the figure on your bag's data sheet.
Why is the hole 3 times the post width?
That is Quikrete's guidance — it leaves room for a proper concrete collar around the post.
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Related calculators & sources
Counting the whole fence? The Wood Fence Calculator figures posts, rails, and pickets — and links back here with the post count filled in. Pouring a pad too? Use the Concrete Slab Calculator. Deck footings and form tubes: Sonotube Calculator. Hole sizing and bag yields come from Quikrete product data; convert measurements with the volume converter.
Estimates for planning and purchasing. Not engineering or code-compliance advice — confirm structural work with your local building department.