A septic tank is an underground chamber made of concrete, fiberglass, or plastic through which domestic wastewater (sewage) flows for basic treatment. Settling and anaerobic processes reduce solids and organics, but the treatment efficiency is only moderate (referred to as “primary treatment”). 

Septic tank systems are a type of simple onsite sewage facility (OSSF). They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed of in a septic drain field which provides the further treatment. However, groundwater pollution may occur and can be a problem.

An illustration with text descriptions of a Septic Tank using a section view to detail the inner process and components.

Table of Required Septic Tank Size Based on Daily Waste-Water Volume in Gallons

Average Sewage Wastewater
Flow – Gallons Per Day
Minimum Septic Tank Size in Gallons
of Effective Capacity Needed
0-500 900
601-700 1200
801-900 1500
1001-1240 1900
2001-2500 3200
4501-5000 5800

Septic Tank in Gallons Size Based on Number of Bedrooms

Number of bedrooms Minimum Septic Tank Capacity (Gallons) Minimum Septic Tank Liquid Surface Area (sq.ft.)
0 bedrooms 750 gal. (2) – obsolete in NYS
1, 2, or 3 bedrooms 1,000 gallons 27 sq. ft.
4 bedrooms 1,200 gallons 34 sq. ft.
5 bedrooms 1,500 gallons 40 sq. ft.
6 bedrooms 1,750 gallons 47 sq. ft.

Notes to the septic tank sizing table above

Original source: New York State NYS75-A. Wastewater Treatment Standards

How to Calculate the Septic Tank Capacity in Gallons

Round Septic Tanks 3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity.
Rectangular Septic Tanks Length x Width x Depth in feet x 7.5 = gallons
Rectangular Septic Tanks
(alternative method 1)
Length x width in inches / 231 = gallons per inch of septic tank depth. Multiply this number by septic tank depth in inches to get gallons
Rectangular Septic Tanks
(alternative method 2)
Length x Width x Depth in feet / .1337 = gallons

COMPUTING EXAMPLE – An Example of Computing Septic Tank Size, Capacity or Volume in Gallons

One gallon of water has a volume of .1337 cubic feet. For a rectangular septic tank, multiply depth (or inside “height”) in feet times width times length. Divide this figure by .1337 to establish the number of gallons in the septic tank

Example 1

How many gallons is held in a 4ft. deep x 5ft. wide x 8 ft. long septic tank?

If the tank dimensions were 4ft. x 5ft. x 8ft. = 160 cubic feet. Using the conversion factor to convert cubic feet to gallons, 160 / .1337 = 1196 – or about a 1200-gallon tank. One cubic foot of volume can contain 7.481 gallons of liquid. So a second approach to calculating septic tank actual size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant, which we round up to 7.5 gallons/cubic foot.

Click Here To How To Read Septic Tank Drawing

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