How to use this calculator
- Pick your state. The calculator loads that state’s published bedroom-count minimums. Use Typical when your state is not listed, then verify the local table.
- Count bedrooms honestly. Codes size by bedrooms as an occupancy proxy — rooms that could be bedrooms (dens, offices with closets) often count. Add the garbage-disposal step if one is installed or likely.
- Read both methods. The state table minimum and the 2x daily-flow estimate are shown side by side; the larger value is the answer and the table is always a floor.
- Verify with the local health department. The locally adopted code governs tank size, compartments, filters and setbacks — and the drainfield is sized separately from soil testing.
How it works
US onsite-wastewater codes size residential septic tanks from the bedroom count, not the people currently living there — bedrooms are the occupancy the house can hold. Each state publishes a minimum-capacity table; this calculator carries twelve cited rows plus a typical default and reads the minimum at your effective bedroom count, where a garbage disposal steps the count up by one (New York’s rule verbatim; other states vary, see the notes).
The second method estimates the tank from flow:
Q = occupants × q · V = 2 × Q
with 2 occupants per bedroom and q ≈ 60–75 gal/person/day
(70 default) — roughly 48 hours of detention so solids can settle. The
answer is the larger of the two, and the published table
minimum is always a floor: at normal occupancy the table controls, and
only unusual occupancy pushes the flow method past it.
The tank is only half the system — the drainfield is sized separately from soil and percolation testing. For the rest of the run, the pipe slope calculator lays out the building sewer fall, the trench backfill calculator takes off the tank and lateral excavation, and the French drain calculator covers the curtain-drain work that keeps groundwater away from the field.
Worked example
Verified against the live calculator
A 3-bedroom house, no garbage disposal, state not listed
(typical minimums): the table gives 1,000 gallons. The
flow cross-check assumes 6 occupants at
70 gal/day → 420 gal/day design flow, and
2 × 420 = 840 gal — smaller, so the
table controls at 1,000 gallons (about 3,785 L).
Same house in Pennsylvania: 25 Pa. Code 73.31 publishes 900 gallons through 3 bedrooms — still above the 840-gal flow figure. Add a bedroom and a garbage disposal (typical row, 4 BR + disposal = 5 effective bedrooms) and the minimum steps to 1,500 gallons.
Reference data
Published minimum tank capacities by bedrooms. The calculator reads this table directly; the same CSV renders it here so you can audit the source for your state.
| State | ≤2 BR (gal) | 3 BR (gal) | 4 BR (gal) | 5 BR (gal) | +gal/BR >5 | Code basis |
|---|---|---|---|---|---|---|
| Typical (state not listed) | 750 | 1000 | 1250 | 1500 | 250 | Typical adopted minimum pattern across US onsite codes; your state or county table governs |
| California | 750 | 1000 | 1200 | 1500 | 150 | UPC Appendix H Table H 201.1(1) as adopted by the CA Plumbing Code (1-2 BR 750; 3 BR 1000; 4 BR 1200; 5-6 BR 1500; +150/extra BR); county LAMP/local agency governs |
| Florida | 900 | 1050 | 1200 | 1500 | 250 | FAC 64E-6 Table I (<=2 BR & <=1200 sqft 900; 3 BR <=2250 sqft 1050; 4 BR <=3300 sqft 1200); larger homes sized by estimated flow - 5+ BR shown as typical adopted minimum |
| Massachusetts | 1500 | 1500 | 1500 | 1500 | 250 | 310 CMR 15.223 Title 5 (>=200% of design flow at 110 gal/day/BR; 1500 gal floor governs through 6 BR); garbage grinder requires a two-compartment tank |
| Minnesota | 1000 | 1000 | 1500 | 1500 | 500 | Minn. R. 7080.1930 Table V banded (<=3 BR 1000; 4-5 BR 1500; 6-7 BR 2000; 8-9 BR 2500); with a garbage disposal MN requires >=1.5x these values |
| New Jersey | 1000 | 1000 | 1000 | 1250 | 250 | N.J.A.C. 7:9A-8.2 (250 gal per bedroom; 1000 gal minimum; expansion attics count as bedrooms) |
| New York | 1000 | 1000 | 1250 | 1500 | 250 | 10 NYCRR Appendix 75-A.6 Table 3 (1-3 BR 1000; 4 BR 1250; 5 BR 1500; 6 BR 1750; +250/BR beyond); garbage grinder counted as one additional bedroom |
| North Carolina | 1000 | 1000 | 1000 | 1250 | 250 | 15A NCAC 18E .0801 Table XIV (<=4 BR 1000; 5 BR 1250); >5 BR sized by Table XV design flow - extension shown is the typical adopted pattern |
| Ohio | 1000 | 1500 | 2000 | 2000 | 500 | OAC 3701-29-12(C)(1) (1-2 BR 1000; 3 BR 1500; 4-5 BR 2000; 6+ BR 2500 + 250/BR); 1500 gal and larger requires two tanks or compartments |
| Pennsylvania | 900 | 900 | 1000 | 1100 | 100 | 25 Pa. Code 73.31(b) (900 gal through 3 BR; +100 gal per bedroom over 3); base design flow already includes garbage grinders |
| Texas | 750 | 1000 | 1250 | 1250 | 250 | 30 TAC 285.91 Tables II+III (Q 225/300/375/450 gal/day for 1-2/3/4/5 BR -> V 750/1000/1250/1250); 6+ BR sized as V = 2.5xQ - extension shown is conservative |
| Washington | 1000 | 1000 | 1000 | 1250 | 250 | WAC 246-272A-0232 Table VII (<=4 BR 1000 gal; +250 gal each additional bedroom) |
Source: State onsite-wastewater rules as cited per row (25 Pa. Code 73.31, 10 NYCRR App. 75-A, WAC 246-272A-0232, Minn. R. 7080.1930, OAC 3701-29-12, 30 TAC 285.91, 310 CMR 15.223, N.J.A.C. 7:9A-8.2, 15A NCAC 18E .0801, FAC 64E-6, UPC App. H). Rows marked typical are the common adopted pattern, not a citation. The locally adopted code and local health department are authoritative.
Frequently asked questions
What size septic tank for a 3 bedroom house?
1,000 gallons under most state codes — the typical adopted minimum, and the published value in New York, Minnesota, Texas, Washington, New Jersey, North Carolina and the UPC table. Outliers exist in both directions: Pennsylvania publishes 900 gallons through 3 bedrooms, Ohio requires 1,500 in two tanks or compartments, and Massachusetts has a 1,500-gallon floor for everything.
Does a garbage disposal change the required tank size?
Usually, yes. Ground food waste raises solids loading, so many codes step the tank up — New York counts a grinder as one additional bedroom (exactly what this calculator does), Minnesota requires 1.5x the base capacity, Massachusetts requires a two-compartment tank, while Pennsylvania already includes grinders in its base design flow. Check your code; the +1-bedroom step here is the common middle ground.
Is the septic tank the same as the drain field?
No — they are sized separately. The tank is sized from bedrooms (this page); the drainfield/soil treatment area is sized from design flow, soil type and percolation testing, and is usually the part that actually limits a site. A bigger tank never shrinks the required drainfield.
Is a bigger septic tank always better?
Mildly bigger is cheap insurance — more detention, more sludge storage, longer pumping intervals. But oversizing does not fix a failing drainfield, can cost more in tank and excavation than it returns, and some health departments cap or review unusual sizes. Size to the code minimum or one step above; spend the rest on the soil treatment system.
How does the daily-flow method work?
Estimate design flow as occupants x gal/person/day (codes assume 2 occupants per bedroom and roughly 60-75 gal/person/day; 70 is a common middle value), then provide about two days of detention: minimum tank = 2 x daily flow. For a 3-bedroom house that is 6 x 70 x 2 = 840 gallons — which is why the 1,000-gallon table minimum still controls.
Where do these minimums come from?
Each state row cites its onsite-wastewater rule (e.g. 25 Pa. Code 73.31, 10 NYCRR Appendix 75-A, WAC 246-272A-0232, Minn. R. 7080.1930, 30 TAC 285.91, 310 CMR 15.223). Your state’s adopted code — and the local health department that administers it — is authoritative; counties can and do amend. Verify locally before design.
Method & assumptions
- Table values are published state minimums for single-family dwellings on conventional gravity tanks, transcribed from the cited rules and verified 2026-06-11; rows marked “typical adopted minimum” are the common pattern, not a citation. State/local code is authoritative — counties amend, and several states size by design flow beyond the published columns.
- Garbage disposal is one bedroom-equivalent step (New York’s rule). Minnesota instead requires 1.5× capacity, Massachusetts a two-compartment tank, Pennsylvania includes grinders in its base flow — the per-state notes carry these.
- The daily-flow method is 2 × (occupants × gal/person/day), about 48 h detention. It is a cross-check; it never reduces the answer below the table minimum.
- Tank liquid capacity only: compartment splits, effluent filters, risers, inlet/outlet baffles, materials, burial depth and traffic rating come from the code and the tank manufacturer.
- The drainfield / soil treatment area is sized separately from soil evaluation and percolation testing — a larger tank does not shrink it. Permits, setbacks and inspections run through the local health department; verify there before design.