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Septic tank size calculator

Capacity is set by design flow and a two-day retention requirement, then raised to whichever standard tank size clears your state's minimum.

Tank size calculatorStep 1 of 1

Sets the design flow and the legal minimum.

Code sizes on bedrooms, not people.

Used to size the matching drainfield. Unsure? Sandy loam is the common default.

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How the number is worked out

Three steps, in this order.

  1. Design flow. Bedrooms times the gallons per bedroom per day your state allows — 150 in most, 110 in Massachusetts, 120 in North Carolina and Washington. Codes count bedrooms rather than people because a house outlives its household.
  2. Working volume. Two days of design flow, so solids have time to settle and grease has time to rise. A garbage disposal adds about 250 gallons; a whirlpool tub or a water softener discharging to the tank adds a similar amount.
  3. Code floor. Whichever is larger — the calculated volume or the state minimum — rounded up to the next tank actually manufactured: 750, 1,000, 1,250, 1,500, 2,000 gallons and up.

What this does not include

Counties and health districts routinely set stricter rules than the state, and a soil evaluation can change the design entirely. Treat the result as the number to walk into that conversation with, not the number to order a tank against.

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