Footing Calculator

A common residential rule is a footing twice the width of the wall it carries and at least as deep as the wall is thick — but the governing numbers are your local frost depth and the soil's bearing capacity, both of which are set by code. Check with your building department before you dig.

Measure along the centreline of each trench run and add them together. For a rectangular building, use the perimeter measured at the footing centreline rather than the outside face, or you will over-order by roughly the footing width at each corner.

Yes — that is a monolithic or thickened-edge slab, and it is common for sheds, garages and additions in mild climates. It saves a pour and eliminates a cold joint. In deep-frost regions, code usually requires separate frost-depth footings.

Forty-five. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet. That is around 3,600 pounds of bags to mix by hand — past roughly 1 cubic yard, ready-mix or a rented mixer stops being optional.

Bags make sense up to about half a cubic yard — post holes, small pads, repairs. Beyond that, ready-mix is cheaper per yard, far faster, and gives a consistent mix with no cold joints. Most suppliers have a 1 yard minimum plus a short-load fee.

Ten percent is the usual allowance. Subgrade is never perfectly flat, forms bow slightly, and running short mid-pour forces a cold joint you cannot undo. If your slab is deep or the subgrade is rough, go to 15%.