What size septic tank do you need for a 7 bedroom house?

A soils test should be done first to determine drainfield characteristics/size. A general rule is to have 300-500 gal (1.1-1.9 m^3) plus at least 30 gal (0.1 m^3) of storage per person served over 5 by the tank (this general rule is from the Civil Engineering Reference Manual for the PE Exam, Tenth Edition. However, minimum size of septic tanks are usually 1,000 gal and your locality may have more stringent specifications. I am the President/Owner of McKeon Engineering & Associates, LLC. Please visit www.mckeonengineering.com for more information.