A septic tank is an underground chamber made of concrete, fibreglass 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 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.
Required Septic Tank Size for Daily Water
Usage Volume in Gallons
Average Sewage            Minimum Septic Tank Size In Gallons
Wastewater                    Of  Effective Capacity Needed (1)
Flow – Gallons Per
Day
0-501-700                          1200
801-900                          1500
1001-1240                      1900
2001-2500                      3200
4501-5000                      5800
Septic Tank in Gallons Size Based on Number of Bedrooms
0-2 bedrooms 750 gals. (2)

3 bedrooms 1000 gal.
4 bedrooms 1200 gal.
5-6 bedrooms 1500 gal.

Calculating 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: 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