What size septic tank do I need?
Pre-filled for a 3-bedroom home with four occupants and no disposal. Change the inputs below; the recommendation rebuilds instantly.
Recommended tank size
1,000 gallons
(the minimum in most jurisdictions InspectAPedia surveyed)
planning estimate from our sizing table — not a county sizing decisionWhy: design flow is set at 150 gallons per day per bedroom in Connecticut's rules (state codes vary), so 3 bedrooms ≈ 450 gal/day. Tank size comes from the bedroom table below.
Change the inputs below — your recommendation rebuilds instantly
When more people live in the house than the bedroom count suggests, this tool adds one size step (our assumption).
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The sizing table, and where it comes from
| Bedrooms | Tank size (gal) | Design flow (gal/day) |
|---|---|---|
| 2 | 1,000 (minimum in most jurisdictions InspectAPedia surveyed) | 300 |
| 3 | 1,000 | 450 |
| 4 | 1,250 (our assumption; InspectAPedia's table: 1,200) | 600 |
| 5–6 | 1,500 (InspectAPedia's table: 1,500 for five, 1,750 for six) | 750–900 |
The bedroom table above adapts InspectAPedia's table[1], and the differences are labeled: for four bedrooms this site uses 1,250 gallons where InspectAPedia lists 1,200 (our assumption[2]), and for five or six bedrooms it uses 1,500 gallons[1], which is 250 gallons below that table at six bedrooms and above. Design flow uses the 150 gal/day/bedroom figure Connecticut sets[3]; other states set their own. Your county's code table is the final word — take this as the planning answer, not the permit answer.
Code design flow is set per bedroom. When more people live in the house than the bedroom count suggests, this tool adds one size step (our assumption, listed on the methodology page), and the pumping cadence on this site shortens for larger households. When the recommendation exceeds the install calculator's pricing basis, the tool says so and links back with your bedroom count pre-set.
The disposal question, and why it is a sizing question
Where the bedroom rule stops working
Bedroom count is a proxy for occupancy, and it fails wherever the two come apart. A converted attic or garage that a county has not recorded as a bedroom still sends its occupants' flow to the same tank. A home office with a full bathroom, a rental suite, or a long-term guest all add load the bedroom count cannot see. In the other direction, two people in a five-bedroom house are sized generously and will find their real pumping interval at the long end of the cadence table rather than the short end.
Two more cases sit outside this tool entirely. Anything non-residential — a shop, a tasting room, a licensed daycare — is sized from a commercial flow schedule your health department publishes, not from bedrooms, and the numbers are not comparable. And a water softener that discharges its backwash to the septic tank adds volume no bedroom count models; ask your installer or health department how yours is handled. In every one of these cases the county's own table is the answer, and this page is only the sanity check you take into that conversation.
Frequently asked questions
What size septic tank does a 4-bedroom house need?
InspectAPedia's sizing table lists 1,200 gallons for four bedrooms. This site uses 1,250 (our 250-gallon step over the 1,000-gallon basis, an assumption shown on the methodology page). Your county's table decides. In this tool, a household larger than typical for the bedroom count, or a garbage disposal, adds a size step.
What is the minimum septic tank size?
1,000 gallons is the minimum in most US jurisdictions InspectAPedia surveyed — even for one- and two-bedroom homes. Your county's table is the final word.
Does a garbage disposal change the tank size?
In this tool, yes: a garbage disposal adds 250 gallons. That is our assumption, not a code citation. New York's standard, as quoted by InspectAPedia, counts a garbage grinder as an extra bedroom when sizing the tank, and EPA advises limiting disposal use because it adds fats, grease and solids. Ask your county whether it requires a larger or two-compartment tank.
How is septic design flow calculated?
150 gallons per day per bedroom is the figure Connecticut DPH sets; other states set their own figure. This tool multiplies it by the bedroom count. The tank size itself comes from the bedroom table, not from that product.
Sources for the figures on this page
- InspectAPedia septic tank size tables — https://inspectapedia.com/septic/Septic_Tank_Size_Tables.php (retrieved 2026-07-30; that copy was not retained — checked against our saved copy of 2026-10-05)
- our assumption — https://septictankcostcalculator.com/methodology/ (labeled 2026-10-05) [our assumption — no direct source]
- Connecticut Department of Public Health — Determining Design Sewage Flow — https://portal.ct.gov/dph/environmental-health/environmental-engineering/determining-design-sewage-flow (retrieved 2026-07-30; that copy was not retained — checked against our saved copy of 2026-10-05)