How to Calculate the Size and Capacity of Septic Tank | Formula

How to Calculate the Size and Capacity of Septic Tank | Formula

Calculating the size and capacity of a septic tank involves estimating the required volume based on the wastewater generation and retention time. The size of the septic tank depends on factors such as the number of users, daily water usage, and local regulations. Here’s a step-by-step guide on how to calculate the size and capacity of a septic tank:

How to Calculate the Size and Capacity of Septic Tank | Formula

Step 1: Determine the Daily Water Usage

Estimate the daily water usage for the building or household that the septic tank will serve. Consider factors such as the number of residents, average water consumption per person, and any additional water usage from appliances.

Step 2: Calculate the Total Wastewater Volume

Multiply the daily water usage (in liters or gallons) by the wastewater generation factor to calculate the total wastewater volume. The wastewater generation factor is typically assumed to be around 80% to 90% of the total water usage, as not all water used goes into the septic tank.

Total Wastewater Volume = Daily Water Usage × Wastewater Generation Factor

Step 3: Determine the Retention Time

The retention time is the duration for which the wastewater remains in the septic tank for treatment before it overflows to the soak pit or drainage field. The recommended retention time is usually 1 to 3 days, depending on the local regulations and the tank’s design.

Step 4: Calculate the Septic Tank Capacity

To calculate the required septic tank capacity, multiply the total wastewater volume (from Step 2) by the retention time (in days).

Septic Tank Capacity = Total Wastewater Volume × Retention Time

Step 5: Choose the Septic Tank Size

Select a septic tank size that is equal to or slightly larger than the calculated septic tank capacity. Standard septic tanks are available in various sizes, and it’s essential to choose a size that complies with local regulations and can accommodate the required capacity.

Formula for Septic Tank Capacity:

Septic Tank Capacity (in liters or gallons) = Daily Water Usage (in liters or gallons) × Wastewater Generation Factor × Retention Time (in days)


Let’s assume a household with four residents, each using an average of 150 liters of water per day. The wastewater generation factor is 85%, and a retention time of 2 days is desired.

Daily Water Usage = 4 (residents) × 150 liters/person = 600 liters/day

Total Wastewater Volume = 600 liters/day × 0.85 (Wastewater Generation Factor) = 510 liters/day

Septic Tank Capacity = 510 liters/day × 2 (Retention Time) = 1020 liters

In this example, a septic tank with a capacity of approximately 1020 liters would be required to serve the household.


Calculating the size and capacity of a septic tank involves estimating the daily water usage, calculating the total wastewater volume, and determining the required retention time. By following these steps and considering local regulations, you can select an appropriate septic tank size that effectively manages wastewater and ensures proper treatment and disposal. It’s crucial to consult with a qualified engineer or septic system expert to design and install a septic tank that meets the specific needs of the building and complies with local codes and standards.

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