text: if my car trade in value is $21,000 now, is it valid to depreciate by 0.9875 every month? i am figuring that a car depreciates 15% per yr. it is a 2002 model yr audi. so i did 15%/12 = 1.25%.   1.0-.0125=0.9875. try .9578 and divide by 13 then add 2002 and divide by your birthyear. add the current temperature and subtract your iq from the quotient.