From Bangkok, there are many ways to get to Koh Chang. But all involve making your way to Trat province, and taking a ferry from there to the island.
One of the most economical and practical ways to make the four- to five-hour trip to Trat is by bus, and there are many options available from Bangkok’s three bus terminals. The Ekamai Bus Terminal offers the most options, with busses leaving almost hourly, seven days a week. There are several companies that offer air conditioned busses—Cherdchai Tour has the most modern—though departure times differ for each company. Choose whatever suits your needs, but keep in mind that the last ferry departs at 5 p.m. and time your trip accordingly.
You can also depart from the Northern Bus Terminal, called New Mo Chit, which has two bus companies, or the Public Transportation Center at Suvarnabhumi Airport, which has government-run busses. Look for The Transport Co., Ltd.
Expect to pay about 300 baht for a one-way ticket and 500 baht for a round-trip. Most busses will drop you in Trat town, and you can take a pickup-truck taxi to one of the three piers where ferries depart. (Note that if you want to go directly to the piers, the 99 Tour bus will do that). A taxi will cost about 100 baht if it is full, and as much as four times that for a private ride.
There are, of course, alternatives to the bus. You can drive yourself or take a van, which depart daily from Khao San Road. Fares vary by season, but expect to pay about 500 baht for a one-way trip. Larger groups can arrange transportation through their hotels—either private cars or minivans—with the cost depending on how many people travel. For a one-way fare, expect to pay about 4,800 baht for up to five passengers and 5,200 baht for up to nine.
There are also flights available from Bangkok Airways, which depart from Suvarnabhumi Airport and land in Trat (there is no airport on Koh Chang Island). The flight takes about 40 minutes.
Once in Trat, you’ll have to make your way to one of the three ferry piers—Ao Thammachat, Center Point or Lam Ngob—and buy tickets in the stalls. Ferries from Ao Thammachat will save you the most time. The converted fishing boats on Lam Ngob will slow you down. Expect to pay 50 baht per person and 150 baht for a car, one-way. The ferry trip takes about 25 minutes from Ao Thammachat and almost twice that from Centre Point.
If you miss the last ferry, private boats are available, but the cost is in the neighborhood of 5,000 baht. The alternative is to stay in Trat and catch the ferry the next day.
Of course, when you get to Koh Chang, you’ll have to arrange transportation—a taxi, if nothing else—to get where you are going.