Updated 24 March 2026
What Size Septic Tank Do You Need?
Tank size is determined by bedrooms, not bathrooms. Here is the sizing guide with costs.
Septic Tank Size by Bedrooms
| Bedrooms | Min. Tank Size | Daily Flow (gal) | Tank Cost | Total Install Cost |
|---|---|---|---|---|
| 1-2 | 750 gallons | 300 | $600-$1,000 | $3,000-$6,000 |
| 3 (most common) | 1,000 gallons | 450 | $800-$1,200 | $3,500-$7,000 |
| 4 | 1,250 gallons | 600 | $1,000-$1,500 | $4,000-$8,000 |
| 5-6 | 1,500 gallons | 750 | $1,200-$1,800 | $5,000-$10,000 |
| 7+ | 2,000+ gallons | 900+ | $1,800-$3,000 | $7,000-$15,000 |
Why Bedrooms, Not Bathrooms?
Building codes use bedroom count because it represents the maximum number of occupants. A 4-bedroom home could house 8 people. Each person uses 60-70 gallons/day. The septic system must handle peak capacity even if you live alone.
Extra bathrooms do not change the tank size requirement because they do not increase the number of people using the system.
Tank Cost by Material
| Material | 1,000 gal Cost | Lifespan | Pros |
|---|---|---|---|
| Concrete | $800-$1,200 | 40+ years | Most durable, most common |
| Fiberglass | $1,500-$2,500 | 30-40 years | Lightweight, no cracking |
| Polyethylene (plastic) | $500-$900 | 20-30 years | Cheapest, easy to install |
| Steel | $600-$1,000 | 15-20 years | Rusts - not recommended |
Should You Go Bigger?
Yes, if possible. A larger tank costs only $200-$400 more but gives you:
- • Less frequent pumping (every 5 years instead of 3)
- • Buffer for extra guests, parties, holiday visitors
- • Room to add a bedroom in the future without replacing the system
- • Better treatment (more settling time = cleaner effluent)
Calculate your total septic system cost
Including tank, drain field, permits, and installation labour.
Use the Calculator →