Residential septic tanks, as well as leaching areas, are sized according to the number of bedrooms in the house.
   The basis for this is a potential "flow rate" of 75 gallons per day per person, and 2 people per bedroom. So a three bedroom system might receive 75 X 6 = 450 gallons per day. 

   A home with 1 to 3 bedrooms requires a 1,000 gallon tank.

   A home with 4 or 5 bedrooms requires  a 1250 gallon tank.
   A home with 6 or 7 bedrooms requires a 1500 gallon tank.

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Notes:
   Smaller tanks, commonly 750 gallons, date from the 1950's or before.
   From 1971 to Jan. 1, 2018 the required tank sizes were:
   1 - 3 bedrooms: 1,000 gallon tank.
   4 bedrooms:     1,250 gallon tank.
   5 bedrooms:     1500 gallon tank.
   6 bedrooms:     2,000 gallon tank.

   Large bathtubs, with a capacity of 100 to 200 gallons, require an additional 250 gallons of tank capacity; over 200 gallons, add 500 gallons of tank capacity. Also, if you're planning to install a large bathtub have the literature from the manufacturer available for review by the local health department.

   Garbage grinders are prohibited or strongly disfavored by local health departments, as the greatly increased amount of suspended solids accelerates biomat formation, slowing the soak-down capability of the leaching area and shortening its useful life. Also, if you have a newer tank with an outlet filter, much more frequent cleaning might be needed.
   The state code says, "Garbage grinders are not recommended for use with subsurface sewage disposal systems."

   Officially though, for design purposes according to state code, add 250 gallons to the proposed tank size if a garbage grinder is nevertheless  installed.

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