Septic Tank Size Calculator
Calculate the right septic tank size for your household. Get capacity in gallons and litres with formula, size table, and Ontario/Canada code notes.
Sizing a septic tank correctly is not optional — undersize it and you risk system failure, environmental contamination, and expensive repairs. Our Septic Tank Size Calculator uses daily wastewater flow and retention time to recommend the right tank capacity for your household. Results are given in both US gallons and litres. Whether you're building a new home, replacing an old system, or checking compliance, this tool gives you a reliable starting point. Always confirm with a licensed septic engineer or your local authority — local regulations set hard minimums that override any general calculation.
Formula
Tank Size = Daily Wastewater Flow (gallons/day) × Retention Time (days)
- Daily Wastewater Flow — standard estimate is 50–75 gallons per person per day (US EPA uses 70 gpd per person)
- Retention Time — standard is 2 days for residential systems
Worked Example:
- Household: 4 people
- Daily Flow: 4 × 70 = 280 gallons/day
- Retention Time: 2 days
- Tank Size = 280 × 2 = 560 gallons
Most codes require a minimum of 1,000 gallons regardless of calculated size — always check local minimums.
Size Reference Table by Household Size
| Bedrooms | Estimated Occupants | Daily Flow (70 gpd/person) | Calculated Min Size | Typical Code Minimum |
|---|---|---|---|---|
| 1 Bedroom | 2 people | 140 gpd | 280 gal / 1,060 L | 750–1,000 gal |
| 2 Bedrooms | 3 people | 210 gpd | 420 gal / 1,590 L | 1,000 gal |
| 3 Bedrooms | 4 people | 280 gpd | 560 gal / 2,120 L | 1,000–1,250 gal |
| 4 Bedrooms | 6 people | 420 gpd | 840 gal / 3,180 L | 1,250–1,500 gal |
| 5 Bedrooms | 8 people | 560 gpd | 1,120 gal / 4,240 L | 1,500–2,000 gal |
High-water-use appliances (jacuzzis, garbage disposals, water softeners) increase required capacity.
Ontario / Canada Note
Ontario's Building Code (O. Reg. 332/12) sets a minimum residential septic tank capacity of 3,600 litres (approximately 950 US gallons). Other Canadian provinces have their own minimums — British Columbia requires 2,700 litres for 1–4 bedrooms. In Canada, tank sizing must be certified by a qualified designer or engineer registered with the relevant provincial authority. Never rely solely on an online calculator for permit applications.
FAQs
How do I calculate the right septic tank size for my home?
The standard formula is: Tank Size (gallons) = Daily Wastewater Flow (gallons/day) × Retention Time (days). For most homes, retention time is 2 days. Daily flow is estimated at 50–75 gallons per person per day. A 3-bedroom home with 4 occupants typically needs a 750–1,000 gallon tank minimum.
What size septic tank do I need for a 3-bedroom house?
Most building codes require a minimum 1,000-gallon septic tank for a 3-bedroom house. This accounts for an assumed occupancy of 4–6 people. Local regulations may require larger tanks — always verify with your county or municipal authority before installation.
How do I calculate septic field size?
Septic (drain) field size depends on daily effluent flow and your soil's percolation rate. The formula is: Drain Field Area (sq ft) = Daily Flow (gallons/day) / Soil Application Rate (gallons/sq ft/day). A soil perc test determines the application rate. Sandy soils absorb faster (1.2 gal/sq ft/day); clay soils need larger fields.
What is the minimum septic tank size in Ontario, Canada?
Ontario's Building Code requires a minimum 3,600-litre (approximately 800 US gallon) septic tank for residential properties. For homes with more than 3 bedrooms or high water usage, larger tanks are mandated. Always consult your local municipality or the Ontario Ministry of the Environment guidelines.
How often should a septic tank be pumped based on its size?
As a rule of thumb, divide tank capacity (gallons) by annual sludge accumulation (~150 gallons/year for a typical household). A 1,000-gallon tank for a 4-person household typically needs pumping every 3–5 years. Larger tanks may go longer; high water usage shortens the interval.