In this article, we will explain how to determine the required size of a septic tank. The typical residential septic tank size required for a given average daily sewage wastewater flow in gallons is provided in a table of septic tank sizes. 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 Minimum Septic Tank Size In Gallons

Wastewater Of Effective Capacity Needed (1)

Flow – Gallons Per

Day

Wastewater Of Effective Capacity Needed (1)

Flow – Gallons Per

Day

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**

3 bedrooms 1000 gal.

4 bedrooms 1200 gal.

5-6 bedrooms 1500 gal.

**Calculating Septic Tank Capacity in Gallons**

**Round SepticTanks**

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.

Plumbing - Calculation of Septic Tank Size

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