hallo guys can anyone explain me how to calculate tire and rim size thanks fore the information

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=7&

Tire size = 275/60/15

275 is mm and is divided by 25.4 to convert to inches =(10.8")
60 is the aspect so 10.8 multiplied by .60 =(6.5")
2 sides so add together =(12.99")
now add the rim diameter =(27.99")

So that tire would be 10.8" wide (just over 10 3/4) by 28" tall.