If you're travelling to Bangkok on an international flight and have an onwards domestic flight, don't forget that Nok Air, AirAsia and Orient Thai all fly from Don Muang Airport. International flights land at Suvarnabhumi Airport so if your next flight leaves from Don Muang you need to leave plenty of time to get there.
You will need to leave enough time to collect your luggage and travel from Suvarnabhumi International Airport to Don Muang (or vice-versa if you are leaving the country).
The alternative is to use the Thai Airways or Bangkok Air on your domestic leg, as their domestic flights use Suvarnabhumi), although fares are usually more expensive.
Between the hours of 4am to 10pm there are buses every 15 minutes. The service is point-to-point so does not stop along the way.
Passengers can board buses at the first floor of Don Mueang Airport’s Terminal 1 and arrive at the second floor of Suvarnabhumi’s Terminal Building, Gate 3.
The estimated travel time is 50 minutes, but could be longer if the traffic is particularly bad.
From Suvarnabhumi, catch the bus from the second floor of the terminal building near gate 3. From Don Muang, the stop is at Terminal 1.
Alternatively, the 555 public bus service goes the same route but takes longer as it picks up and drops off along the way.
Taxis are abundant at both airports. Remember to pay extra for the toll roads to avoid congested roads in the capital. Journey time could be as low as 45 minutes if using the toll roads, but we suggest allowing at least double that as traffic can be unpredictable.
Passengers also have an alternative of hopping on a public van available at Gate 8 on the first floor from 6am - 6pm. However, the van will not go inside Don Mueang airport. It only stops at the bus stop at the opposite side, which seems like a less convenient option.
An interesting discussion on the travel options can also be found on TripAdvisor.