Check what kind of connection you actually have
If both flights are on one ticket, the airline owns the problem and will have built the transfer into the itinerary. If you booked two separate tickets, which is common when a long-haul arrival meets a low-cost regional hop, then a missed second flight is entirely your cost. That distinction should drive how much buffer you leave.
Read the airport codes on both bookings. BKK is Suvarnabhumi, DMK is Don Mueang, and travellers regularly discover the difference at the wrong moment.
The free shuttle bus
Airports of Thailand publishes a free airport shuttle between the two airports for passengers with air tickets. Its Don Mueang page lists the service from 05:00 to 24:00 at roughly half-hourly intervals, departing from Gate 3, Terminal 1, Floor 1 arrivals, and Gate 14, Terminal 2, Floor 1 arrivals, with a contact number for the service.
What the published pages do not give is a journey time, and that is the number you would most like to have. The road crosses the whole city, so the honest planning assumption is that this leg is unpredictable rather than fixed, and that a mid-afternoon crossing behaves differently from a dawn one.
The alternatives
A metered taxi is the flexible option and runs 24 hours from both airports, with the 50 baht airport surcharge plus tolls. It is also the only one that works outside the shuttle's published hours.
Public bus route 554, listed on Suvarnabhumi's bus page at roughly 35 baht, runs between Suvarnabhumi and Don Mueang and on to Rangsit, with approximate hours from around 05:00 to 21:45. It is a genuine budget option and a poor connection option, for the same reason: it stops where city buses stop.
| Option | Suits | What you give up |
|---|---|---|
| Free AOT shuttle | Ticketed passengers inside its hours | Unknown frequency and journey time |
| Metered taxi | Tight timing, late hours, luggage | Fare plus surcharge and tolls |
| Public bus 554 | Budget travel with time to spare | Speed and predictability |
How much time to leave
Work backwards from the second airline's check-in and bag-drop cut-off rather than from the departure time, and add the immigration and baggage reclaim you face on arrival. Then add the crossing itself, then add the queue at check-in. On separate tickets, a buffer that feels absurd on paper is the thing that stops one delayed inbound flight from costing you a second fare.
If the gap is genuinely large, storing your bags and going into the city is a better use of the hours than sitting in a terminal. If it is small, stay where you are.
Before you book the second flight
Confirm which airport each flight uses, whether the two are on one ticket, and whether your arrival lands inside the shuttle's published hours. Those three answers decide whether this is a routine transfer or a gamble.