Updated 14 November 2026
What Size Septic Tank Do You Need?
Tank size is set by bedrooms, not bathrooms. In most state codes a 3-bedroom home needs a 1,000-gallon tank, a 4-bedroom home 1,000 to 1,250 gallons, and a 5-6 bedroom home 1,250 to 1,500 gallons. Massachusetts requires 1,500 gallons for every home.
This is the quick answer. The full septic tank size by bedroom guide has the state-by-state minimums, the sizing maths and the cost of each size.
Septic Tank Size by Bedrooms
| Bedrooms | Typical minimum tank | Concrete tank list price |
|---|---|---|
| 1-2 | 750-1,000 gallons | 750 gal: $1,337.50 (NH) |
| 3 (most common) | 1,000 gallons | $1,412.50 (NH), $1,987 (MT) |
| 4 | 1,000-1,250 gallons | 1,250 gal: $1,712.50 (NH) |
| 5-6 | 1,250-1,500 gallons | 1,500 gal: $2,112.50 (NH), $2,950 (MT) |
| 7+ | 1,500-2,000+ gallons | 2,000 gal: $2,812.50 (NH) |
Minimums vary by state and county. Tank-only retail list prices from AJ Foss (New Hampshire) and Lockwood Precast (Montana), checked 14 November 2026; each is one regional data point. Delivery, setting and the drain field are extra.
Why Bedrooms, Not Bathrooms?
Codes use bedroom count because it caps how many people a home can house, and the system has to handle that even if you live alone. New Jersey, for example, sizes at 250 gallons per bedroom with a 1,000-gallon minimum.
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 (1,000 gallons)
| Material | Published price |
|---|---|
| Concrete | $1,412.50 (AJ Foss, NH); $1,987 one-piece (Lockwood Precast, MT) |
| Polyethylene (plastic) | Norwesco: $2,287 regular, $2,100 sale (Lockwood Precast) |
| Fiberglass | No public price found; quote |
Prices checked 14 November 2026. Compare sizes: 750 · 1,000 · 1,250 · 1,500 · 2,000 gallons.
Should You Go Bigger?
Often, yes. On the AJ Foss list a 1,250-gallon tank is $300 more than a 1,000-gallon one. A larger tank gives you:
- • Longer between pump-outs at the same household size
- • Buffer for extra guests and holiday visitors
- • Room to add a bedroom later without replacing the tank
- • More settling time, so cleaner effluent reaches the drain field
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