Free Tool
Post and Footing Size Calculator
Work out the footing area and thickness under a post or column, the minimum post size, and the foundation depth rules, from the Ontario Building Code.
Post and Footing Size Calculator
Result
We form and place footings, piers and posts for decks, porches, additions and post and beam frames across Ontario.
Get a quote for footingsPreliminary planning only. This tool does not replace the drawings. The designer or engineer of record and the building permit set govern every size, span and detail.
We form and place footings, piers and posts for decks, porches, additions and post and beam frames across Ontario. Footing framing, Deck and porch framing. Request a free quote.
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Where the numbers come from
- Footing area, house. Article 9.15.3.3. (application), Table 9.15.3.4., Minimum Footing Sizes, and Articles 9.15.3.7. and 9.15.3.8. (column area adjustment and footing thickness: the greater of 100 mm and the projection beyond the supported element). (source)
- Soil. Article 9.15.1.1. (soils of 75 kPa or more), Table 9.4.4.1. (allowable bearing pressures) and Article 9.15.3.4.(3) (doubling with a high water table). (source)
- Foundation depth. Table 9.12.2.2., Minimum Depths of Foundations, and Sentence 9.12.2.2.(7) for decks. (source)
- Posts. Articles 9.17.2.1., 9.17.3.1., 9.17.3.2., 9.17.4.1. and 9.17.4.3. (source)
- Deck footing basis. Article 9.4.2.3. for the 1.9 kPa occupancy load. The 0.5 kPa dead load and the 75 kPa soil pressure follow the figures in the Town of Carleton Place deck guide (Figure 2). (source)
Worked example
A deck post carrying a beam span of 10 ft (3.05 m) and a joist length of 10 ft (3.05 m), on firm clay, deck attached to the house. Area = 3.05 x 3.05 x 2.4 / 75 = 0.30 m2, about 548 mm square, at least 204 mm thick. The deck is attached, so the 1.2 m minimum depth of Table 9.12.2.2. applies and not less than the frost depth the municipality sets. The post is wood, 140 x 140 mm at the least.
Table 9.15.3.4. at a glance
| Floors supported | Column footing area at 3 m spacing |
|---|---|
| 1 | 0.40 m2 |
| 2 | 0.75 m2 |
| 3 | 1.0 m2 (column designed under Part 4, Article 9.17.1.1.) |
A foundation that is insulated to reduce heat flow to the soil beneath the footings is treated as containing no heated space for depth (Note to Table 9.12.2.2.).
Last checked 2026-11-09 against the 2024 Ontario Building Code (O. Reg. 163/24) compendium and SB-1 as published on ontario.ca.
This tool is for preliminary planning. The designer or engineer of record and the building permit set govern.
Frequently asked questions
How is a house column footing sized?
Table 9.15.3.4. gives a minimum footing area for columns spaced 3 m apart: 0.40 m2 for 1 floor, 0.75 m2 for 2 floors and 1.0 m2 for 3 floors. For other spacings the area is adjusted in proportion (Article 9.15.3.7.). The table applies where the supported joists span 4.9 m or less.
How is a deck post footing sized?
The tool multiplies the supported beam length by the supported joist length by 2.4 kPa (the 1.9 kPa occupancy load of Article 9.4.2.3. plus 0.5 kPa dead load) and divides by 75 kPa, the lowest soil pressure Section 9.15. covers. The municipal deck guide of the Town of Carleton Place prints the same figures.
Why does the soil change the answer?
Section 9.15. applies only to stable soils with an allowable bearing pressure of 75 kPa or more (Article 9.15.1.1.). Table 9.4.4.1. lists loose sand or gravel at 50 kPa and soft clay at 40 kPa, so those, and peat, fill and sensitive clay, go to an engineer. On gravel, sand or silt with a high water table the footing area is doubled (Article 9.15.3.4.(3)).
How deep must the footing be?
Table 9.12.2.2. sets minimum depths by soil, heat and drainage. In clay or soils not clearly defined that is 1.2 m, and not less than the depth of frost penetration for unheated space. The code does not print a frost depth for each city, and this tool does not invent one. Ask your building department for the frost depth at your site.
Does a deck always need footings below frost?
Not always. Sentence 9.12.2.2.(7) exempts a deck of 1 storey, 55 m2 or less, 600 mm or less above ground, with no roof and not attached to another structure. A deck attached to a house is not exempt.
Is this a quote?
No. It is a planning tool. Contact us with your plans and we will arrange a free site visit.
To size the joists and beam that sit on the posts, use the Deck Joist and Beam Span Calculator.
Related: Footing framing, Deck and porch framing, Post and beam framing.
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We form and place footings, piers and posts for decks, porches, additions and post and beam frames across Ontario.
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