This estimate is based on data compiled by the National Taxpayers Union (NTU).

Donald Trump flew from the Andrews Air Force Base in Washington D.C at 12:30 a.m. Tuesday and arrived in Hanoi at approximately 9 p.m., which means the flight took approximately 21 hours, the Global News reported.

An hour in air on Air Force One was estimated to cost around $140,000 in 2017, covering fuel, reparation and maintenance costs, according to the NTU.

Therefore, the Washington D.C-Hanoi flight of the Air Force One Tuesday would have cost approximately $3 million.

Air Force One is the name designated to any aircraft used to carry a president of the United States, to avoid confusion with other aircrafts. The current Air Force One is model VC-25A, a special version developed from the Boeing 747-200B. It has two floors, with a total floor area of 370 square meters, including conference rooms, offices and medical rooms. The president of the United States would rest in the VC-25A’s first compartment, nicknamed the "White House," fully equipped with bedroom and bathrooms.

The VC-25A can carry 76 passengers and 26 crew members. It can reach speeds up to 1,015kph at an altitude of 13km.

Trump is in Hanoi for his second summit with North Korean leader Kim Jong-un. The two-day summit starts Wednesday.