The only international airport in the country, Paro airport is about 6km south of Paro town.
You can book a taxi outside Paro airport at a fixed rate of Nu 1000 to Thimphu. A taxi to the airport from Thimphu's long-distance bus station can cost half this rate. It is 53km from the airport to Thimphu; the journey takes 90 minutes.
If you have an early-morning flight from Paro (and most are), it's best to spend the night in Paro.