How to determine the required size of a septic tank

How to determine the required size of a septic tank

In this article, we will explain how to determine the required size of a septic tank. The average residential septic tank size required for a normal daily sewage wastewater flow is provided in a table of septic tank sizes (in gallons). This article also explains how to calculate septic tank volume based on a septic tank inside dimensions measured in feet.
Required Septic Tank Size for Daily Water Usage Volume in Gallons
Average Sewage Wastewater
Flow in Gallons/Day
Minimum Septic Tank Size in Gallons
(Effective Capacity Needed)
Septic Tank in Gallons (Based on Number of Bedrooms) 
0 bedrooms = 750 gal. 
1-3 bedrooms = 1000 gal.
 4 bedrooms = 1200 gal. 
5 bedrooms = 1500 gal. 
6 bedrooms = 1750 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 
Rectangular Septic Tanks (alternative method 2): Length x Width x Depth in feet / .1337 = gallons

Example for Computing Septic Tank Size, Capacity or Volume in Gallons
One gallon of water = .1337 cubic feet volume. For a rectangular septic tank, multiply depth in feet by width by length. Divide this by .1337 to solve for the number of gallons in
the rectangular septic tank.
Example: How many gallons is held in a 4 foot deep by 5 foot wide by 8 foot long septic tank? 4 x 5 x 8 = 160 cubic feet. So, using the conversion factor to convert cubic feet to
gallons, 160 / .1337 = 1196 – or about a 1200-gallon tank. One cubic foot of volume contains liquid in the amount of 7.481 gallons. Therefore, a second approach to calculating septic
tank size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant which we can round up to 7.5 gallons/cubic foot.


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