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.
Table of Required Septic Tank Size Based on Daily WasteWater Volume in Gallons 

Average Sewage Wastewater Flow – Gallons Per Day 
Minimum Septic Tank Size in Gallons of Effective Capacity Needed 
0500  900 
601700  1200 
801900  1500 
10011240  1900 
20012500  3200 
45015000  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 NYS75A. 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 1200gallon 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.