How do I determine the size of my septic tank?

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).

calculation table for determining septic tank capacity

Source: Western Australia Health Act 1911 - Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974

A tool for determining your septic system requirements is here: 
The Quickest Way to Your Septic System Requirements