What Size Septic Tank Do You Need?

Standard Septic Tank Sizes (Gallons & Litres Explained) 

The OBC uses two main factors for sizing: daily wastewater flow and soil conditions. Also, in Ontario, septic tank sizes are typically measured in litres, although imperial gallons are still commonly used throughout the industry.

Gallons vs. litres Most septic tank manufacturers list capacities in US gallons (1 US gallon ≈ 3.78 litres). For example, a 1,000-gallon septic tank holds about 3,785 litres. However, Ontario regulations use metric measurements, so always confirm the tank’s litre capacity to ensure it meets local code requirements.

The septic tank must meet the Ontario Building Code minimum capacity requirements, which are based on the calculated Daily Design Flow and are never less than 3,600 litres for a residential septic tank.When choosing between two tank sizes, it’s usually better to size up than risk installing one that’s too small.

How To Size A Septic Tank Properly For Your Home

When people ask, “What are septic system sizes?” they often conflate the tank size with the system’s overall capacity. The size and design of a septic system depend on two primary factors: the Daily Design Flow (DDF) and your property’s specific soil and site conditions.

Step 1: Calculate the maximum daily flow

In Ontario, septic system sizing starts with the number of bedrooms. This is because the OBC uses bedroom counts as the standard metric for a home’s maximum potential wastewater load, rather than current occupancy or the number of bathrooms.The Ontario Building Code assigns a specific ‘Daily Design Flow’ (Q) to residential systems based on the number of bedrooms.

• 1 Bedroom: 750 L/day
• 2 Bedrooms: 1,100 L/day
• 3 Bedrooms: 1,600 L/day
• 4 Bedrooms: 2,000 L/day
• 5 Bedrooms: 2,500 L/day

If you are building a home with more than 5 bedrooms, consult a specialist.These values are used to ensure the system is sized to handle the home’s peak potential capacity.Here are other factors to consider:

Note: Your designer uses the greater of these factors to determine your final “Q.” Your septic tank must meet the minimum Ontario Building Code capacity requirement, which is at least 3,600 litres or the calculated design flow, whichever is greater.

Step 2: Figure out soil/site conditions

The second key factor in septic system design is how quickly treated wastewater can be absorbed into the soil. This is known as the “T-time” (percolation time), and it measures how many minutes it takes for the water level to drop by 1 cm in a test hole at the proposed drain field location.Soil type and T-timeSoil absorption rates are measured by “T-time,” which is the time in minutes it takes for the water level to drop 1 cm in a test hole.

Sandy Loam: Typically ranges from 10 to 20 min/cm. This is often considered an ideal range, offering a perfect balance of absorption and biological treatment.

Clay Soil: Often exceeds 50 min/cm. Because of extremely fine, tightly packed particles, water movement is severely restricted. Soils in this range typically cannot support a conventional trench system and require specialized, engineered designs like raised filter beds.

How T-time is measured
The most accurate way to determine soil absorption is through a professional percolation test conducted by a licensed septic designer or a laboratory soil analysis. A field percolation test is generally preferred for new builds as it provides immediate, site-specific data that accounts for the actual conditions of your drain field location.

Once both the Daily Design Flow (Q) and the T-time are known, engineers can properly size the septic system. These two values determine how much wastewater must be processed and how quickly the surrounding soil can safely absorb it.

Septic System Sizing Requirements: What Size Septic System Do You Need?

In most residential systems, the tank is also built as a dual-chamber unit, with approximately two-thirds of the volume in the first chamber. This design improves sedimentation and helps protect the drain field from solids carryover.

• If a garbage disposal is installed, it increases the amount of solids entering the system and may require a larger tank or a different system design, depending on local requirements and the Ontario Building Code.
• If soil conditions indicate very slow absorption, a conventional trench system may not be suitable, and alternative designs, such as raised-bed systems, may be required.

Types of septic tanks:

Always ensure your site plan accounts for the excavation hole, which will be significantly larger than the tank itself to accommodate the necessary bedding material and drainage layers.

Sizing & Compliance: What the Ontario Building Code Actually Requires

In Ontario, you cannot simply pick a tank size from a catalogue. The Ontario Building Code (Part 8) requires that your system be sized based on a calculated Daily Design Flow (DDF).

1. Regardless of the size of your home, the code requires the septic tank to meet the minimum residential capacity of 3,600 litres and to be sized according to the calculated Daily Design Flow.
   

   Even for a small cottage, you cannot legally install a tank smaller than this threshold. This ensures your tank has enough “retention time” for solids to settle effectively before the liquid reaches your drain field.

2. The OBC requires every septic tank to have at least two compartments. The first compartment acts as the primary settling zone where the bulk of the solids drop out (sludge) and grease floats (scum). The second compartment provides a final “polishing” of the effluent.
   

   This multi-stage process significantly reduces the risk of solids entering your drain field, which is the #1 cause of premature, expensive system failure.

3. One of the most practical requirements in the code is accessibility. Access openings must be located no more than 300 mm below the ground surface. If the access openings are deeper than 300 mm below grade, risers or other approved extensions are typically required to provide proper access for inspection and maintenance.
   

   Never bury your access ports or cover them with permanent landscaping (like concrete patios or heavy sheds). If you have to dig up your yard every time you need an inspection, you are significantly more likely to skip essential maintenance.

4. Your tank must be designed to hold a liquid depth of at least 1,000 mm (with 900 mm allowed in specific rock-excavation scenarios). This depth is necessary to ensure that the settling process works correctly and that the tank can withstand ground pressure without shifting or cracking.

Septic regulations can be complex and vary by specific site conditions. Always partner with a licensed septic designer or qualified installer who can ensure your site-specific system is fully compliant with local municipal requirements.

Disclaimer: This information is for educational purposes. In Ontario, all septic systems must be designed by a professional holding a valid BCIN (Building Code Identification Number) or by a professional engineer, and you must obtain a permit from your local building department before starting any work