ride train to Zurich main station. Catch tram to Bellevue or Bürkliplatz stop, or S-train to Stadelhofen and walk.
If you consider the Banhofstrasse to be the 'center' of Zurich, then it is about 11.9 km by car from the airport to downtown, according to Google maps
267 metres (875 feet and 11.81 inches)
One major international Airport - Klotten. About 10 miles aways from Zürich.
Kloten
221.19 kilometres (137.44 miles)