Septic Tank Size Calculator

Estimate the septic tank size a house needs from its bedroom count, plus the design daily flow, drain field absorption area and trench length for your soil type.

By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Septic tank size is set by bedroom count using a design flow of 150 gallons per bedroom per day, and the drain field is sized by dividing that flow by the soil absorption rate. A 3-bedroom house with no disposal in loam needs a 1,000 gal tank, has a 450 gal per day design flow, and needs 750 sq ft of drain field, or 250 ft of 3 ft wide trench.

Your measurements

A rectangular volume: length times width times depth.

BedroomsCodes size by bedrooms, not occupants, because the house may be sold to a larger family. Count every room the county could call a bedroom: an office with a closet and an egress window counts, and so does a basement you plan to finish.Design flow per bedroom (gal per day)Most state codes use 150 gal per bedroom per day: 2 people at 75 gal each. Some use 120.Garbage disposalA disposal roughly doubles the rate sludge builds. Most counties add 250 gal or require the next size, and many also require a two-compartment tank and an effluent filter.Drain field soilUse the county soil evaluation or perc test, not a guess from the surface. Sand percs under 5 minutes per inch, loam 5 to 30, silty clay 30 to 60, and clay over 60, which fails a conventional field in most states.

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Septic tank size1,000 galAll results

Results

Septic tank size: 1,000 gal; Common minimum for 3 bedrooms; holds about 2.2 days of flow
Design daily flow: 450 gal; 3 bedrooms x 150 gal per day
Drain field absorption area: 750 sq ft; Daily flow / 0.6 gal per sq ft per day for loam
Trench length at 3 ft wide: 250 ft; Absorption area / trench width; split into runs of 100 ft or less, 6 ft on center
Permit and design: Planning estimate only; The county health department issues the permit from a soil evaluation or perc test, sets the tank, field and setbacks, and inspects the system open before backfill

Septic Tank Size formula

Design flow (gal/day) = Bedrooms x 150
Tank size = 1,000 gal for 1 to 3 bedrooms, 1,250 for 4, 1,500 for 5, 1,750 for 6; next size up with a disposal
Absorption area (sq ft) = Design flow / soil rate  (sand 1.2, loam 0.6, silty clay 0.3, clay 0.2 gal/sq ft/day)
Trench length (ft) = Absorption area / trench width (3 ft)

Worked example

A 4-bedroom house with a garbage disposal on sandy soil: design flow is 4 x 150 = 600 gal per day. The minimum tank for 4 bedrooms is 1,250 gal, and the disposal moves it up one size to 1,500 gal. Absorption area is 600 / 1.2 = 500 sq ft, which at 3 ft wide is 500 / 3 = 167 ft of trench, laid as three runs of about 56 ft.

The same house on clay at 0.2 gal per sq ft per day needs 600 / 0.2 = 3,000 sq ft of absorption area and 1,000 ft of trench. That is why clay sites usually end up with a mound, chamber or aerobic system instead of a conventional drain field.

Quick reference

Septic tank and drain field by bedrooms (150 gal per bedroom, no disposal, loam at 0.6 gal per sq ft per day)
Bedrooms	Design flow	Tank size	Absorption area	Trench at 3 ft wide
1	150 gal/day	1,000 gal	250 sq ft	83 ft
2	300 gal/day	1,000 gal	500 sq ft	167 ft
3	450 gal/day	1,000 gal	750 sq ft	250 ft
4	600 gal/day	1,250 gal	1,000 sq ft	333 ft
5	750 gal/day	1,500 gal	1,250 sq ft	417 ft
6	900 gal/day	1,750 gal	1,500 sq ft	500 ft

How septic tank size is decided

Health departments size a septic system by the number of bedrooms, not by who lives there today, because a 3-bedroom house can hold a family of six tomorrow. Each bedroom is assigned 150 gallons per day, and the tank must hold roughly two days of that flow so solids have time to settle and grease can float. That is where the 1,000 gallon minimum for up to 3 bedrooms comes from, stepping up 250 gallons per bedroom beyond that.

A garbage disposal roughly doubles the rate sludge builds. Most counties require the next tank size up or 50 percent more capacity, and many require a two-compartment tank with an effluent filter in the outlet tee. A 1,250 gal precast tank typically costs $200 to $400 more than a 1,000 gal tank, so oversizing is cheap. A precast tank weighs 8,000 to 10,000 lb, and the boom truck has to park within about 15 ft of the hole. Pump every 3 to 5 years, sooner with a disposal, and whenever the scum and sludge fill a third of the depth. Only toilet paper and what passes through you belong in it: wipes, grease, coffee grounds, paint, solvents and drain cleaner clog the outlet tee or kill the bacteria.

Drain field and the perc test

The drain field, not the tank, is usually what limits a lot. Effluent leaves the tank and must soak into the soil, and each soil type can only absorb so many gallons per square foot per day. Sandy soil takes 1.2, loam about 0.6, and clay only 0.2, so the same house needs six times the trench length on clay as on sand. The county requires a soil evaluation or percolation test by a licensed evaluator before it issues the permit, and it assigns the rate for your site. Figure $300 to $1,500 for the test and $200 to $1,000 for the permit, and some counties only accept tests run in the wet season.

Setbacks decide where the field can go. Typical numbers are 10 ft from the house and property lines and 10 ft from water lines. A well needs 50 ft to the tank and 100 ft to the field, and a stream or lake 50 ft; the county sets the exact figures. The trench bottom needs 1 to 3 ft of unsaturated soil above the seasonal high water table or bedrock. That is what pushes clay and wet sites to a mound, sand filter, drip or aerobic unit, typically $15,000 to $40,000 installed against $5,000 to $15,000 for a conventional system. The inspector looks at the tank, distribution box and trenches open before backfill and files the as-built you will need at sale.

What this calculator leaves out

The tank delivery and set, risers and locking lids, inlet and outlet tees, the effluent filter and the distribution box. The field itself: 4 in perforated pipe with washed stone and geotextile, or plastic chambers. A pump chamber with a control panel and alarm if the field sits uphill, and the electrical circuit for it. The soil evaluation, design and permit fees, and the reserve area for a replacement field that most counties require on the lot. It does not size an existing system by occupants; a 1,000 gal tank serving eight people needs pumping every year or two.

Code limits that apply

NY Appendix 75-Achecked
Conventional absorption field only where the soil percs at 60 minutes per inch or faster
If you miss it: A field laid in slow clay surfaces effluent in the yard the first wet spring and the county orders a mound at $15,000 or more
WAC 246-272A-0232
Tank of 1,000 gal for up to 3 bedrooms, rising 250 gal for each bedroom above that
If you miss it: An undersized tank passes solids before they settle and the drain field clogs years early
WAC 246-272A
1 to 3 ft of unsaturated soil between the trench bottom and the seasonal water table or bedrock
If you miss it: Trenches cut into the water table drown in spring, effluent surfaces on the lawn and the system is condemned
NY Appendix 75-A
Well 50 ft from the tank and 100 ft from the field; 10 ft from the house and lot lines
If you miss it: A field inside 100 ft of the well shows coliform on the well test and the permit is denied until the field moves
EPA septic guidance
Inspect the tank every 3 years and pump every 3 to 5 years, sooner with a garbage disposal
If you miss it: Sludge reaches the outlet tee and washes into the field; replacing a clogged field costs 20 times a pumping

Provision	Limit	If you miss it
NY Appendix 75-Achecked	Conventional absorption field only where the soil percs at 60 minutes per inch or faster	A field laid in slow clay surfaces effluent in the yard the first wet spring and the county orders a mound at $15,000 or more
WAC 246-272A-0232	Tank of 1,000 gal for up to 3 bedrooms, rising 250 gal for each bedroom above that	An undersized tank passes solids before they settle and the drain field clogs years early
WAC 246-272A	1 to 3 ft of unsaturated soil between the trench bottom and the seasonal water table or bedrock	Trenches cut into the water table drown in spring, effluent surfaces on the lawn and the system is condemned
NY Appendix 75-A	Well 50 ft from the tank and 100 ft from the field; 10 ft from the house and lot lines	A field inside 100 ft of the well shows coliform on the well test and the permit is denied until the field moves
EPA septic guidance	Inspect the tank every 3 years and pump every 3 to 5 years, sooner with a garbage disposal	Sludge reaches the outlet tee and washes into the field; replacing a clogged field costs 20 times a pumping

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Setting a plastic tank empty in a wet hole: it floats and shears the inlet; fill it with water as you backfill, or use precast.
Lids buried with no risers: every pumping starts with a $150 to $300 dig; set risers to grade on both lids with locking covers.
No effluent filter in the outlet tee: solids reach the field and clog it; a $50 filter cleaned at each pumping prevents that.
Driving the concrete truck across the field during construction: crushed laterals fail years later; fence it off before the first delivery.
Downspouts and the sump discharge aimed at the drain field: a saturated field surfaces effluent; pipe roof water at least 20 ft past it.
Flushable wipes, grease and paint: wipes mat in the outlet tee within months and paint kills the bacteria; only waste and toilet paper go in.

Key facts

Most state codes size septic systems at 150 gallons per bedroom per day, which assumes 2 occupants per bedroom at 75 gallons each.
A 1,000 gallon tank is the most common minimum in the US; a 3-bedroom house on 450 gallons a day gets about 2.2 days of retention in it.
The EPA recommends inspecting a septic tank every 3 years and pumping it every 3 to 5 years, more often with a garbage disposal.
Soils that percolate slower than 60 minutes per inch generally fail a perc test for a conventional drain field.
Conventional drain field trenches are 1 to 3 ft wide, 18 to 36 in deep, and limited to about 100 ft per run with at least 6 ft between trench centers.

Frequently asked questions

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

1,000 gallons is the common minimum for 3 bedrooms, based on a design flow of 450 gallons per day. Add a garbage disposal and most codes move you up to 1,250 gallons.

How big does a drain field need to be?

Divide the design daily flow by the soil absorption rate. A 3-bedroom house at 450 gallons a day on loam at 0.6 gal per sq ft per day needs 750 sq ft, which is 250 ft of 3 ft wide trench.

How far does a septic system have to be from the house and the well?

Typical county setbacks are 10 ft from the house to the tank and field, and 10 ft from property lines. A well needs 50 ft to the tank and 100 ft to the field, and a stream or lake 50 ft. Water lines crossing the field need 10 ft of separation or a sleeve. The county health code sets the exact numbers, and most counties also require a reserve area for a second field before they approve the first.

How often should a septic tank be pumped?

Every 3 to 5 years for a typical household, per EPA guidance. Homes with a disposal or a tank that is undersized for the occupants should pump closer to every 2 to 3 years.

Sources and references

How Septic Systems Work (EPA)
Appendix 75-A.6, Septic Tanks and Enhanced Treatment Units (New York State Department of Health)
WAC 246-272A-0232, Septic tank sizing (Washington State Department of Health)
Soil Evaluation for Septic System (University of Illinois Extension)

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