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Septic tank size calculator

Enter the bedrooms and your state's design flow. You get the daily flow, the tank size by the common bedroom table and by two days of storage, and the standard tank to buy.

Sized by bedrooms, not bathroomsTwo days of storageYour state's rules govern

Inputs

Bedrooms, not people or bathrooms. Septic rules size by bedrooms.
gal/day
150 in many states, 120 in some, such as Missouri. Your health department sets it.
days
The tank should hold about two days of flow.
gal
Most states will not permit smaller than 1,000 gallons for a home.

Result

You need
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Septic tank size by bedrooms

The common bedroom table, as published by University of Missouri Extension, beside two days of flow at 150 gallons per bedroom.

BedroomsBedroom tableFlow, gal/day2 days of flowTank to buy

How a septic tank is sized

Septic systems are sized by bedrooms, because bedrooms set how many people a house can hold over its life. The design flow per bedroom comes from the state or county: 150 gallons a day is common, and some states use 120.

Daily flow = bedrooms x gallons per bedroom per day Tank = the larger of the bedroom table and daily flow x 2 days Then round up to a standard tank size

University of Missouri Extension gives 1,000 gallons for three bedrooms, 1,250 for four and 1,500 for five, and says the tank should hold two days of wastewater so solids have time to settle. At 150 gallons a bedroom, a four-bedroom house makes 600 gallons a day, and two days of that is 1,200 gallons: a 1,250 gallon tank.

Your health department decides. Septic systems are permitted by the state or county, not the plumbing code, and the rules differ: some add capacity for a garbage disposal, a large tub or a home business, some require two compartments or a second tank, and all of them size the drain field from a soil test. Use this to plan and budget, then confirm with the permit office.

Questions

What size septic tank do I need for a 3 bedroom house?

1,000 gallons in most states. At 150 gallons per bedroom per day, three bedrooms make 450 gallons a day, and two days of storage is 900 gallons, so the 1,000 gallon minimum governs.

What size septic tank for a 4 bedroom house?

Usually 1,250 gallons. Four bedrooms at 150 gallons a day is 600 gallons a day, and two days of storage is 1,200.

What size septic tank for a 5 bedroom house?

Usually 1,500 gallons, and 1,500 to 2,000 where the state uses higher flows or requires more for a garbage disposal.

Is septic size based on bedrooms or bathrooms?

Bedrooms. Bedrooms set how many people the house can hold, and that sets the wastewater flow. Bathrooms do not change the required size in most states.

How big is a 1,000 gallon septic tank?

Roughly 5 ft wide, 8 ft long and 5 ft high on the outside for a typical concrete tank, though shapes vary by maker. The rated capacity is the liquid volume below the outlet.

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