the correct way is ground and first floor are the same. so basically the american way. thought experiment. you need to tile 100% of the walking surface in your house for some reason. the house is 800 square feet. how much tile do you need? english way 800*1 because there's one floor (the first floor). american way 800 * 2 because there's two floors (the first and second floor). 1600 is the correct answer.
United States