How Ontario Sizes a Septic System
Short answer
An Ontario septic tank needs a working capacity of at least 3 600 L or twice the daily design flow, whichever is more (OBC 8.2.2.3.). The daily design flow comes from the number of bedrooms, with more added for a large finished area or many fixtures (Table 8.2.1.3.-A). Most 1 to 3-bedroom houses under 200 m² land on the 3 600 L minimum.
Step 1: the daily design flow (Q)
Every number in a Part 8 design starts with Q, the total daily design sanitary sewage flow in litres. For a house, Table 8.2.1.3.-A gives 750 L for 1 bedroom, 1 100 L for 2, 1 600 L for 3, 2 000 L for 4 and 2 500 L for 5. To that you add the largest of three extras: 500 L for each bedroom over 5; 100 L for each 10 m² of finished area between 200 and 400 m², 75 L per 10 m² between 400 and 600 m² and 50 L per 10 m² beyond that; or 50 L for each plumbing fixture unit over 20. The area is the finished area above a basement: a finished basement is not counted (Note 3).
Step 2: the septic tank
Article 8.2.2.3. sets the tank's working capacity at the greater of 3 600 L and 2 × Q for a residence. Sewage must pass through at least two compartments. The first holds at least 1.3 × Q and never less than 2 400 L, and each later compartment holds at least half of the first. If you use two tanks in series instead of one divided tank, no tank in the series may be under 3 600 L.
Step 3: the leaching bed
The bed is sized from Q and the soil's percolation time T, the minutes it takes water to drop 1 cm. For absorption trenches with distribution pipe, the total pipe length is L = QT/200 metres, or QT/300 when the effluent comes from a Level II, III or IV treatment unit, and never less than 40 m (8.7.3.1.). Type I leaching chambers use QT/200 and Type II chambers QT/300 (8.7.3.2.). Trenches are about the same length and no longer than 30 m each, so a long bed becomes several runs side by side (8.7.3.3.).
When the soil does not allow a trench bed
A leaching bed may not be built in soil with a percolation time under 1 min, or over 50 min for ordinary absorption trenches (8.7.2.1.). Slow clay soils usually lead to a fill-based bed, a filter bed or a shallow buried trench, which is allowed up to 125 min but needs a Level IV treatment unit and pressurized distribution (8.7.6.1.). Those designs follow other rules and are not sized by this calculator.
Once you know the size, check where it can go with the septic setback tool: clearances to wells, the house, water and lot lines come from Tables 8.2.1.6.-A and -B. Every new or replacement system needs a site evaluation and a permit.
This calculator is a sizing check, not a septic design or permit document. Values from the Ontario Building Code 2024 (O. Reg. 163/24, January 16, 2025 update), Division B Table 8.2.1.3.-A, Articles 8.2.2.3., 8.7.2.1., 8.7.3.1. to 8.7.3.3. and 8.7.6.1., checked against the printed tables on October 27, 2026. © King's Printer for Ontario, 2024. Reproduced with permission.
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About Septic Tank Size Calculator Ontario (OBC)
At least 3 600 L of working capacity, or twice the daily design flow if that is more (OBC 8.2.2.3.(1)). A 3-bedroom house under 200 m² has a design flow of 1 600 L, so the 3 600 L minimum governs.
By the ToolFluency team · Updated October 2026
What size septic tank does an Ontario house need? 3 600 L or 2 × daily flow by bedrooms and floor area, plus leaching bed length from perc time. OBC Part 8.
How to use
- Enter the number of bedrooms. Table 8.2.1.3.-A gives a design flow for 1 to 5 bedrooms and adds 500 L for each bedroom over 5.
- Enter the finished floor area in m² or ft², not counting a finished basement. Area over 200 m² adds flow.
- If you know the house's plumbing fixture units, enter them. Each fixture unit over 20 adds 50 L per day.
- Enter the soil percolation time (T) from your site evaluation: percolation tests at 3 or more spots (use the highest) or a soil classification per SB-6.
- Pick the leaching bed type, and tick the box if the effluent comes from a Level II, III or IV treatment unit.
- Read the minimum tank working capacity, the compartment sizes and the total length of distribution pipe or leaching chamber, with the number of trench runs and their spacing.
Examples
3-bedroom bungalow, 140 m², sandy loam (T = 10)
Design flow 1 600 L/day. Tank: 2 × 1 600 = 3 200 L, so the 3 600 L minimum governs; first compartment 2 400 L (minimum), second at least 1 200 L. Bed: QT/200 = 1 600 × 10 / 200 = 80 m of distribution pipe, 3 runs of about 26.7 m at 1.6 m centres.
4-bedroom two-storey, 320 m² above grade, silt (T = 30)
Design flow 2 000 + 12 × 100 = 3 200 L/day (320 m² is 12 steps of 10 m² over 200). Tank: 2 × 3 200 = 6 400 L; first compartment 1.3 × 3 200 = 4 160 L, second at least 2 080 L. Bed: QT/200 = 3 200 × 30 / 200 = 480 m of pipe, or 320 m with a Level II to IV treatment unit.
Frequently asked questions
What size septic tank do I need in Ontario?
At least 3 600 L of working capacity, or twice the daily design flow if that is more (OBC 8.2.2.3.(1)). A 3-bedroom house under 200 m² has a design flow of 1 600 L, so the 3 600 L minimum governs. A 5-bedroom house (2 500 L) needs 5 000 L, and a 4-bedroom house with 300 m² of finished area (2 000 + 1 000 = 3 000 L) needs 6 000 L.
How is the daily design flow worked out for a house?
Table 8.2.1.3.-A gives 750 L for 1 bedroom, 1 100 L for 2, 1 600 L for 3, 2 000 L for 4 and 2 500 L for 5. On top of that you add the largest of: 500 L for each bedroom over 5; 100 L for each 10 m² (or part) of finished area from 200 to 400 m², 75 L per 10 m² from 400 to 600 m² and 50 L per 10 m² over 600 m²; or 50 L for each fixture unit over 20. Note 2 to the table says to use the calculation that gives the highest flow.
Does a finished basement count toward the floor area?
No. Note 3 to Table 8.2.1.3.-A says the area is the total finished area excluding the area of a finished basement.
How long does the leaching bed need to be for a 3-bedroom house?
It depends on the soil. With a design flow of 1 600 L and a percolation time of 15 min, distribution pipe in absorption trenches needs QT/200 = 1 600 × 15 / 200 = 120 m, laid as 4 runs of 30 m at 1.6 m centres. The code minimum is 40 m of pipe however fast the soil (8.7.3.1.).
What percolation time is too slow or too fast for a septic bed?
A leaching bed may not go in soil with a percolation time under 1 min, or over 50 min for absorption trenches. A shallow buried trench, which needs a Level IV treatment unit and pressurized distribution, is allowed up to 125 min (8.7.2.1.(1)(b) and 8.7.6.1.).
How big is each septic tank compartment?
Sewage must pass through at least 2 compartments. The first must hold at least 1.3 times the daily design flow and never less than 2 400 L, and each later compartment at least half of the first (8.2.2.3.(2) and (3)). The whole tank must still meet the working capacity, the greater of 3 600 L and twice the daily flow.
Does a treatment unit make the leaching bed smaller?
Yes, for absorption trenches. With effluent from a Level II, III or IV treatment unit (Table 8.6.2.2.), distribution pipe is sized with QT/300 instead of QT/200 (8.7.3.1.(3)), which cuts the length by a third. The 40 m minimum still applies.
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