Fence Post Concrete Calculator

How much concrete goes around a fence post?

A fence post only needs concrete in the gap between the post and the sides of the hole, not the hole’s full volume — the post itself displaces part of that space. Getting this right matters more in Canada than in milder climates, because colder-region holes are often dug deeper to reach below the local frost line, which increases the concrete volume substantially for the same hole diameter.

How the calculator accounts for post displacement

Because Canadian holes are often dug deeper to clear the frost line, post displacement matters more here than in milder climates: the calculator subtracts the buried post’s volume (cylinder or prism, matching your hole shape) from the gross hole volume before pricing bags — and the deeper the hole, the bigger that subtraction gets relative to a shallow one.

How bag yield affects the answer

Bag yield varies by product and by weight — a 30 kg bag and a 60 lb bag won’t yield identical volumes, and different manufacturers’ mixes cure to slightly different volumes even at the same weight. Pick a common size as a starting point, then use your product’s stated yield for a tighter number.

Bags are rounded up per hole rather than once for the whole project, so a deep, frost-resistant hole doesn’t accidentally come up short partway through mixing.

Fence run length to post count

If you know your total run but not your post count, enter the fence length and target post spacing (commonly 6–8 ft). The calculator places a post at each end and at every full span, rounding up so the last gap is never longer than your spacing.

Worked example

An 80 ft fence run with posts every 8 ft needs 11 posts. This example intentionally uses a deeper, frost-appropriate hole — 12 in. diameter, 42 in. deep, with a 4 in. gravel base and a 3.5 in. square post:

• Concrete depth per hole: 38 in. (42 in. total depth minus the 4 in. gravel base)
• Concrete needed per hole, with 10% waste: about 2.4 ft³
• Bags needed per hole (60 lb bag, ~0.45 ft³ yield): 6
• Total for the project: about 26.8 ft³, or 66 bags

Compare that to the shallower US worked example on this site (26 in. concrete depth, 2 bags per hole) — the extra 12 in. of depth alone roughly triples the bags needed per post. That gap is exactly why local frost-depth guidance matters more here than the hole diameter does.

Why a gravel base

A gravel layer at the bottom of the hole helps drain water away from the post base, reducing rot and — in freeze-thaw climates — reducing the amount of water sitting directly under the post that can contribute to frost heave. It also slightly reduces the concrete volume needed, since it displaces part of the hole depth.

Common bag sizes in Canada

Ready-mix concrete is commonly sold in both 30 kg and 60 lb bags depending on the retailer, yielding roughly similar volumes for their respective weights. Always check the yield printed on your specific product rather than assuming a fixed figure across brands.

Methodology

This tool calculates material quantity — the volume of concrete implied by your hole and post dimensions, converted to bags using the yield you enter, plus a waste allowance. It does not calculate structural adequacy or frost-safe embedment depth: those depend on your province, municipality, soil, fence height, and wind exposure. Confirm suitable hole dimensions with your local building department first, then use this calculator to plan materials.

View Buildmator’s full methodology