What size septic tank do I need? Categories: Precast Concrete (1), Septic Systems (8) Share this article, choose your platform! A What size septic tank do I need? The size depends on the number of bedrooms it supports. The most common size used in NH is 1,250 gallons, which is appropriate for up to 4-bedrooms. Share this article, choose your platform! About the Author: Jon Cardinal My father started the Andrew J. Foss, Inc. precast concrete company in 1963 at the age of 19. My precast education started at a very young age. He taught me everything I know about creating quality precast concrete products, from septic tanks to concrete headwalls. He also taught me to succeed in business you need to offer an exceptional product and treat your customers as you would expect to be treated. Now, as an owner of the company along with my brother Matthew, we've taken everything we've learned from our father to start the next generation of our family-owned precast concrete company.