Septic Tank Size Calculator

A septic tank is a holding-and-settling pond sized by how much wastewater a household generates and how long that water must rest. Too small, and solids ride out to the drain field; so codes anchor the size to bedrooms and occupancy.

Septic Tank Size Calculator estimates minimum capacity from bedrooms, people, and daily water use, then suggests a working volume and dimensions.

• Bedroom rule: The code-style capacity shortcut.
• Occupancy check: People x gallons per day x retention.
• Three units: Gallons, liters, cubic feet and m3.
• Dimension range: A preliminary tank footprint.
• Standard sizes: Sees nearby commercial tanks.
• Export: Copy or download the report.

1. Enter the number of bedrooms (drives the code rule).
2. Adjust occupants and gallons-per-person if known.
3. Set the retention (1 day is standard residential).
4. Read the recommended capacity and a preliminary dimension.

Example - 3-bedroom, 4 people. Bedroom rule: 1250 gal. Flow method: 4 x 70 x 1 x 2 (settling factor) = 560 gal. The bedroom rule wins, so plan on a 1250 gal tank or the nearest stock size.

• Code-style number fast: bedrooms in, gallons out.
• Two methods agree: a built-in sanity check.
• Dimensions included: rough footing for tank placement.
• Free and private: no uploads, no logging.