How is a septic tank sized in Ontario?
A septic tank's working capacity must be at least twice the daily design sanitary sewage flow, and for a residential occupancy not less than 3,600 litres. Design flow is calculated from bedroom count and finished floor area.
Daily design sewage flow for a dwelling is taken from Table 8.2.1.3. and grows with the number of bedrooms and the total finished area, with plumbing fixture units as an additional check — a 3-bedroom house of typical size lands at 1,600 L/day, so its tank must hold at least 3,600 L (the residential floor governs until the calculated flow exceeds 1,800 L/day). The two-times-daily-flow rule gives sewage roughly 48 hours of quiet retention so solids settle and scum floats before clarified effluent moves to the leaching bed, whose size is in turn driven by the same design flow and the soil's percolation rate. Undersized tanks send solids to the bed and destroy it — replacement beds cost tens of thousands.
The tank must hold two full days of the household's wastewater — and never less than 3,600 litres for a house. More bedrooms means more flow means a bigger tank and a bigger bed. Undersizing is not a shortcut; it is a slow-motion demolition of your leaching bed.
Get answers like this for any code question
BUILDAiD Assist searches the building code and answers in plain English with exact citations — in seconds.
This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.