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.

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Average Sewage WastewaterFlow – Gallons Per Day | Minimum Septic Tank Size in Gallonsof Effective Capacity Needed |

0-500 | 900 |

601-700 | 1200 |

801-900 | 1500 |

1001-1240 | 1900 |

2001-2500 | 3200 |

4501-5000 | 5800 |

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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

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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.