Ask the internet what size septic tank a three-bedroom house needs and you will get one confident national chart. The trouble is that septic sizing in the US is not national. It is set state by state, in each state's own code — and the states do not agree.
The most common three-bedroom minimum is 1,000 gallons. Ohio requires 1,500. Same house, same bedrooms, a tank half again as big, because the rule was written in Columbus and not in Washington. Some states size by bedrooms, some bring in living area or fixture counts, and some relax the requirement if the house has water-saving fixtures throughout.
SepticSizeChart exists because those differences are exactly the part the national charts skip. Every answer on the site comes from the state's own published rule, with the citation and a link to the official text — ten states read from the official code so far, more added as they are verified, and no state page published until its rule has been checked line by line. The site also prices a typical system from the same state-level view: a new three-bedroom system in a typical-cost state like North Carolina runs about $6,900 to $11,100, itemised line by line.
If you are planning a build or a replacement, start with your state's rule, not the national average. The average is a tank that may not be legal where you live.