Septic tank sizing
What size septic tank do I need?
The short version: about 1,000 gallons for up to 3 bedrooms, 1,250 for 4, 1,500 for 5, and 2,000 for 6. Set your bedrooms below for the exact figure and the math behind it.
Estimate only. Your local health authority sets the legal minimum — always confirm before buying or installing a tank.
The standard
How the size is determined
Septic tank size is set by your home's design flow, and design flow is set by bedrooms. Most onsite codes estimate 150 gallons per bedroom per day. The tank holds that flow for a 48-hour retention so solids can settle, and the result is then held to your local code minimum (usually 1,000 gallons) and rounded up to a standard tank size.
Quick reference
| 1–3 bedrooms | 1,000 gal |
| 4 bedrooms | 1,250 gal |
| 5 bedrooms | 1,500 gal |
| 6 bedrooms | 2,000 gal |
A garbage disposal adds a sludge allowance that can bump you to the next size; high-efficiency fixtures lower the design flow a little. Toggle both in the calculator. For a full chart, see the septic tank size chart.
Frequently asked questions
What size septic tank do I need?
For most homes: 1,000 gallons up to 3 bedrooms, 1,250 for 4, 1,500 for 5, and 2,000 for 6. The figure comes from 150 gallons per bedroom per day held for a 48-hour retention, floored at the 1,000-gallon code minimum.
What size septic tank for a 3-bedroom house?
A 3-bedroom home needs about 1,000 gallons — 450 GPD of design flow, which floors to the 1,000-gallon code minimum in most jurisdictions.
What size septic tank for a 4-bedroom house?
1,250 gallons is typical for a 4-bedroom home. A garbage disposal can push it to 1,500.
Should I size by bedrooms or by how many people live here?
By bedrooms. Codes size to design occupancy (two people per bedroom), which is a fixed property of the house and does not change as occupants come and go.
Can I install a bigger tank than required?
You can, and a modest step up is fine, but a vastly oversized tank can under-flow and not scour properly. Size to the standard and confirm with local code.