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Post Footing Depth Calculator

Find the footing depth a post needs from frost line, post height, and a practical minimum.

  • footing
  • post
  • frost
  • frost line
  • depth
  • deck
  • fence
  • pier
  • canada

About Post Footing Depth Calculator

In most of Canada the depth of a post footing isn't really about the post — it's about the frost. Water in the soil expands as it freezes, and a footing that sits above the frost line gets heaved up every winter, taking your deck, fence, or pergola with it. The fix is to bear the footing below the depth that frost reaches, which varies dramatically across the country: well under half a metre on the mild coast, a metre or more across southern Ontario, and approaching two on the Prairies.

This calculator takes a regional frost depth, the post's above-ground height, and a footing diameter, and returns the depth to dig — the deepest of the frost line, a lateral-stability rule for freestanding posts, and a practical minimum — along with the concrete each footing will take. It's a planning aid, not a substitute for your municipal building department, which sets the figure that actually governs.

How to use

Pick the region closest to your site to load a typical frost depth, or choose "Custom depth" and enter the figure your building department quotes — that's always the most reliable number. Set the post's height above grade and the diameter of the hole or footing form you're using.

The result shows the recommended depth and which requirement set it: the frost line (usual in cold regions), the embedment rule (one third of the post height, which governs tall freestanding posts in mild areas), or the 600 mm practical minimum. The cards break out each contributing figure and the concrete volume per footing with a bag count. Always confirm the depth locally — frost penetration changes street to street, and any footing carrying real load also needs its bearing area checked, not just its depth.

Frequently asked questions

  • How deep should a post footing be in Canada?

    Deep enough to bear at or below the local frost line, which is the depth winter frost reaches in the soil. That ranges from roughly 0.45 m on coastal BC to about 1.2 m across southern Ontario and the Atlantic provinces, 1.5 m around Ottawa, Montreal, and Alberta, and up to 1.8 m or more on the Prairies. Your municipal building department publishes the figure that applies to your address — always confirm it there.

  • Why does frost depth matter for footings?

    When soil moisture freezes it expands, and if it freezes around or under a shallow footing the ground lifts it — called frost heave. Over a few winters that pushes posts up unevenly, racking decks and fences and cracking anything rigid attached to them. Placing the bottom of the footing below the frost line keeps it in ground that never freezes, so it stays put. It's the single most important number for footings in a cold climate.

  • Does the post height change the footing depth?

    For a freestanding post — a fence, pergola, or sign with nothing bracing the top — yes. A common rule embeds at least one third of the above-ground height for lateral stability, so a 3 m post wants about 1 m in the ground regardless of frost. For a braced or load-bearing post (a deck tied into a structure), frost and bearing area usually govern instead. This tool takes the deeper of the frost line and the embedment rule.

  • How much concrete does a post footing need?

    For a straight cylindrical hole it's the circle area times the depth: a 250 mm-diameter hole 1.2 m deep holds about 0.06 m³, or roughly six 20 kg bags. The calculator works this out for your diameter and recommended depth. If your design uses a belled base or a wider pad footing for bearing, add that extra volume — the figure here is for a plain cylinder.

  • Do I still need a permit and an inspection?

    Often, yes — many decks, large fences, and any structure carrying load require a permit, and the footing depth is exactly what an inspector checks. Treat this calculator as a planning aid: it gets you a realistic depth to design and budget around, but the building department's published frost depth and your site's soil bearing are what the work must actually meet. Northern and permafrost sites need site-specific engineering.

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