Septic Tank Size Calculator
Find the septic tank capacity your home needs by bedroom count.
What you'll need to order
- septic tank
- distribution box
- perforated leach pipe
- drain rock
- filter fabric
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
tank gallons = 750 + (bedrooms − 2) × 250
Local health codes vary considerably and always override this figure. The drain field calculation depends entirely on a percolation test — soil type changes the required area by a factor of four.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- US EPA Onsite Wastewater Treatment Systems Manual 2002 — tank capacity guidance by bedroom count; local health codes override it
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With number of bedrooms 3 , daily flow per bedroom 150 , soil loading rate 0.5 , this calculator returns 1000 gal.
Tank size and field area by house size
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Tank capacity needed |
|---|---|---|
| Two-bedroom cottage | 2 × 150 × 0.5 | 750.0 gal |
| Three-bedroom house | 3 × 150 × 0.5 | 1,000 gal |
| Four-bedroom house | 4 × 150 × 0.5 | 1,250 gal |
| Same house on heavy clay Slow soil needs far more field |
4 × 150 × 0.2 | 1,250 gal |
| Same house on sand | 4 × 150 × 1 | 1,250 gal |
Local health codes override every figure here, and the field area depends on a percolation test you have to actually run.
Frequently asked questions
What size septic tank do I need for a 3 bedroom house?
One thousand gallons is the common code minimum. Four bedrooms takes 1,250 and five takes 1,500. Your local health department sets the binding figure.
Why are septic systems sized by bedrooms?
Bedrooms indicate potential occupancy rather than current occupancy. A system sized for two people in a four bedroom house would fail the moment the property changed hands.
How big does the drain field need to be?
It depends entirely on soil percolation. The same 450 gallon daily flow needs 560 sq ft in sand but 2,250 sq ft in clay. Only a percolation test can determine it.
How often does a septic tank need pumping?
Every three to five years for a typical household. Longer intervals let solids carry over into the drain field, and a failed field costs many times what a lifetime of pumping does.