Flight Incheon International Airport departs from Seoul Incheon International Airport at 18:40 and arrives at Singapore Changi Airport at 23:40.
The flighttime is about 6h 00m. The distance travelled ("as the bird flies"): 2923 Km. So the average speed is roughly 487 Km.
There is an 1 hour time difference between Seoul and Singapore
The best day to arrive at Seoul is Thursday and it would be best to avoid Sunday, as prices are on average 112.4% higher than on Thursday. (Average prices, based on 266623 datapoints.)
We have a lot more statistics for flights KE 645 and between Seoul and Singapore.