How much is the airport tax?

Ans: A Passenger Service Charge of SGD 21 should be incorporated in the air ticket (including tickets issued outside of Singapore). If this has not been done, you may be required to pay the SGD 21 during check-in. In certain cases, airlines may absorb the service charge.
Passengers who are in transit for less than 24 hours may leave the airport without having to pay the service charge upon departure from Singapore.
 
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Travellers' Essentials
Singapore Immigration and Customs 
Singapore Changi Airport