Septic tank size would depend on the the number of rooms in your household and/or household size.
Generally, a smaller household of, say, 3 bedrooms, will need a smaller septic tank compared to a household with 5 bedrooms. Below is a table to help you determine septic tank size (or capacity).
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A tool for determining your septic system requirements is here:
The Quickest Way to Your Septic System Requirements