Depends on the year which you fail to list.
12 gallons.
A 2002 Buick Century can hold up to 15.5 gallons of gas. The 2002 Buick Century gets 21 mpg on both highway and city roads.
17
18 gallons
A little over 17 gallons.
Car not getting gas Buick century 1994?
20 gallons
It depends on where you are
how to replace the tether gas cap on a 1998 Buick Century?
18 gallons
17.43535537 gallons go here for info: http://www.internetautoguide.com/car-specifications/99-int/2000/buick/century/custom-sedan/526/index.html