Concrete footing calculator

Last updated: August 20, 2026By ConcreteFinder Research Team

Footing size in Ontario is not a rule of thumb, it is a table. How many floors sit on it sets the base width, what the wall above is built of adds to it, and the thickness follows from how far the footing sticks out past the wall. Set the four things below and you get the code minimum, in millimetres and inches, plus the concrete it takes.

What size does the footing have to be?

What is the footing under?Floors the footing carries

Which wall?What the wall above is built of

Foundation wall thickness (mm)Total footing run (feet)

Is the water table high?

Minimum strip footing, Ontario Building Code Table 9.15.3.4

480 mm wide (18.9 in) by 140 mm thick

Table 9.15.3.4 base width, 2 floors on an exterior wall: 350 mm
Article 9.15.3.5 addition, 65 mm per storey x 2: 130 mm
Minimum width: 480 mm / 18.9 in
Projection past each face of the wall: 140 mm
Minimum thickness, Article 9.15.3.8 (the greater of 100 mm and the projection): 140 mm / 5.5 in
Concrete for 120 ft of footing: 2.46 m³ / 3.21 yd³
Minimum depth below finished grade for frost: 48 in / 1.2 m

These are code minimums for ordinary Part 9 housing, not a design. Poor soil, a deeper joist span, a point load or anything outside Part 9 all take precedence, and that is an engineer's call rather than a table's.

Table 9.15.3.4, in full

The City of North Bay's Building Services department publishes this table in a span-table booklet dated February 2026, stating its tables are copies of the 2024 Ontario Building Code. It is reproduced here as it appears there.

Ontario Building Code Table 9.15.3.4, minimum footing sizes
Floors supported	Exterior wall	Interior wall	Column at 3 m o.c.
1	250 mm (9 7/8 in)	200 mm (7 7/8 in)	0.4 m² (4.3 ft²)
2	350 mm (13 3/4 in)	350 mm (13 3/4 in)	0.75 m² (8.1 ft²)
3	450 mm (17 3/4 in)	500 mm (19 3/4 in)	1 m² (10.9 ft²)

The table applies where the walls above are masonry, flat insulating concrete form, or light wood frame, where supported joist spans do not exceed 4.9 m, and where the specified live load on any floor does not exceed 2.4 kPa. Outside those limits the code sends you to a formula or to Section 4, which means a designer.

The three rules the calculator applies

The base width comes straight out of the table above, chosen by how many floors bear on the footing and whether the wall is exterior or interior. Note that at three floors the interior figure is larger than the exterior one, 500 mm against 450, which surprises people: an interior bearing wall picks up floor load from both sides.

The wall addition is Article 9.15.3.5, and it is per storey, not per house. Masonry veneer over wood frame adds 65 mm for each storey of veneer, masonry carried by the foundation wall adds 130 mm per storey, and a flat insulating concrete form wall adds 150 mm per storey. Those additions are written for exterior walls; interior walls have their own article, 9.15.3.6, which this calculator does not reproduce, so it applies no addition to an interior wall and says so on the result.

The thickness is Article 9.15.3.8: not less than the greater of 100 mm, or the width of the projection of the footing beyond the supported element. The projection is half the difference between the footing width and the wall thickness, so a wide footing on a thin wall gets thicker, which is the code stopping the footing from snapping off at the edge.

One rule sits outside all three: Sentence 9.15.3.4.(3) doubles the widths and areas where a foundation rests on gravel, sand or silt in which the water table is less than the footing's own width below the bearing surface. The calculator has a switch for it because it changes the answer by a factor of two, and nothing else on a footing does.

Frequently asked questions

What is the minimum footing width in Ontario?

Ontario Building Code Table 9.15.3.4 sets it by how many floors the footing carries. For an exterior wall it is 250 mm for one floor, 350 mm for two and 450 mm for three. For an interior wall it is 200 mm, 350 mm and 500 mm. Column pads at 3 m on centre need 0.40, 0.75 and 1.0 square metres.

Does brick veneer change the footing width?

Yes. Article 9.15.3.5 adds 65 mm per storey for masonry veneer over wood frame, 130 mm per storey for masonry construction carried by the foundation wall, and 150 mm per storey for a flat insulating concrete form wall. On a two-storey brick-veneer house that is 350 plus 130, so 480 mm.

How thick does a footing have to be?

Article 9.15.3.8 puts it as the greater of 100 mm or the width of the projection of the footing beyond the supported element. A 480 mm footing under a 200 mm wall projects 140 mm each side, so the footing has to be 140 mm thick, not 100.

How deep does a footing have to be in Toronto?

Below frost. Builders Ontario states the minimum is the greater of 1.2 m or the local frost depth and puts the GTA and southern Ontario at about 1.2 m, which is 48 inches. Depth is separate from width and thickness, and it is the one most likely to be cut short on a small job.

When do footings have to be twice as wide?

Sentence 9.15.3.4.(3): where a foundation rests on gravel, sand or silt in which the water table is less than the width of the footings below the bearing surface, the widths and areas must be not less than twice what is otherwise required.

Is this a substitute for engineered drawings?

No. These are the minimums for ordinary Part 9 housing where joist spans do not exceed 4.9 m and floor live load does not exceed 2.4 kPa. Poor soil, a longer span, a point load or any Part 4 structure needs a designer. Underpinning in Toronto requires engineer-stamped drawings before a permit is issued at all.

Sources

Every figure above is from the Ontario Building Code, read in two places on 2026-08-25 and cross-checked between them. Dataset July 2026.