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Septic Tank Size Calculator

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When planning for the installation of a septic system, one crucial aspect to consider is determining the correct size of the septic tank. The Septic Tank Size Calculator is an invaluable tool designed to simplify this process. By inputting a few key pieces of information, such as the number of bedrooms in your home, this calculator provides an estimate of the tank size needed to efficiently handle your household's wastewater.

Purpose and Functionality

The primary goal of the Septic Tank Size Calculator is to ensure that your septic tank has sufficient capacity to manage the wastewater generated by your household without overloading the system. A correctly sized septic tank allows solid waste to settle properly and undergo preliminary decomposition, reducing the risk of system backups and extending the lifespan of your septic system.

The functionality of the calculator is based on a few key variables and constants:

  • Variables:
    • B: Number of bedrooms
    • P: Average number of people per bedroom
    • W: Average daily wastewater generation per person
    • T: Required tank capacity in gallons
    • R: Retention time in the tank
  • Constants:
    • C: Conversion factor to adjust tank size based on occupancy and other considerations

Formula and Calculation

The calculation follows three main steps:

  1. Calculate the total daily wastewater generation (D):
    • D=P×B×W This step estimates the total volume of wastewater your household generates each day.
  2. Calculate the basic required tank capacity (Tbasic​):
    • Tbasic​=D×R This step estimates the tank capacity needed based on the daily wastewater volume and the desired retention time.
  3. Apply the conversion factor to find the final required tank capacity (T):
    • T=Tbasic​×C This final step adjusts the basic tank capacity to account for safety and efficiency considerations, providing the final tank size recommendation.

Step-by-Step Example

Consider a home with 4 bedrooms, assuming the standard values for P, W, R, and C.

  1. Daily Wastewater Generation:
    • 2×4×75=600D=2×4×75=600 gallons per day
  2. Basic Required Tank Capacity:
    • 600×2=1200Tbasic​=600×2=1200 gallons
  3. Final Required Tank Capacity:
    • 1200×1.5=1800T=1200×1.5=1800 gallons

Therefore, a home with 4 bedrooms requires a septic tank with at least 1800 gallons capacity.

Information Table

Here's a simple table summarizing how different numbers of bedrooms affect the required septic tank size:

Number of BedroomsDaily Wastewater Generation (gallons)Basic Tank Capacity (gallons)Final Tank Capacity (gallons)


The Septic Tank Size Calculator is a straightforward yet essential tool for homeowners and builders alike. By ensuring your septic tank is adequately sized, you not only comply with local regulations but also protect the environment and extend the longevity of your septic system. Remember, these calculations serve as estimates; it's always best to consult with a professional to account for specific household needs and local guidelines.

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