International flights: Check-in counters open three hours before scheduled departure time and close 30 minutes before the scheduled departure time.
Domestic flights: Check-in counters open two hours before the scheduled departure time and close 30 minutes before the scheduled departure time. 
Please be aware that during morning and afternoon rush hours, it can often take two hours to reach the airport from a riverside hotel. Plan accordingly.

First Class and Executive check-in counters are located at Row 4 in the Departure Hall of Terminal 1
Royal Orchid Plus Gold Card members are entitled to use the Royal Executive check-in counters.
Under THAI's new Common Check-in policy, Economy Class passengers can check in at any THAI counter in Row 1-4 in the Departure Hall.
For Domestic terminal, passengers can check-in at any THAI counter.

For international flights, your passport with visa (if required at the destination), and your ticket.
For domestic flights, only your ticket and I.D are required.
Also, if you want credit for frequent flyer miles, you should also present your Royal Orchid Plus card.

The Airports Authority of Thailand assesses a 500-baht tax for international flights. After checking in, purchase your ticket from an automated machine or a teller located next to the doors into the Immigration Hall.
The 50-baht fee for domestic flights is included in the ticket

The luggage allowance for Royal First Class is 40 kg. (88 lbs.); for Royal Silk Class passengers: 30 kg. (66 lbs.); and for Economy Class passengers: 20 kg. (44 lb.).
Royal Orchid Plus Gold Card members are entitled to check an additional 20 kg. (44 lbs.). Royal Orchid Plus Silver Card members can check an additional 10 kg. (22 lbs.) of luggage.
Infants (not occupying a seat) are allowed 10 kg. (22 lbs.) excluding one fully-collapsible child stroller, pushchair, or infant-carrying basket.
For travel To/from Canada and the U.S.A., the following regulations apply:
Passengers in all classes of service, from First to Economy, are allowed two pieces of luggage. The sum of the three dimensions (length + height + width) for each piece cannot exceed 158 cm. (62 inches) and the weight of each piece cannot exceed 32 kgs. (70 lbs).
A Child (2-12 years) occupying a seat is entitled to the same free luggage allowance as an adult passenger.
An Infant (not occupying a seat) is entitled to one piece of checked luggage, the sum of whose three dimensions does not exceed 115 cm. (45 inches), plus one fully-collapsible child stroller or pushchair or infant-carrying basket.

Each passengers may hand-carry one piece luggage the sum of whose dimensions (length+ height + width) does not exceed 115 cm. ( 56 + 46 + 25 cm) (44.85 inches, i.e. 21.5 x 18 x 9.75 inches) and which does not weigh more than seven kg. (15.4 lbs). Pieces exceeding these dimensions must be checked in at the Check-in counter prior to departure. A woman is allowed to carry a purse as well.

You can find our aircraft seat plan on Aircraft Seatmaps.

A children, 5-11 years of age, travelling alone is designated an Unaccompanied Minor and will be escorted by THAI personnel, looked after during the flight, and met on arrival provided the parent has completed the necessary documents prior to the flight and a copy of these documents is required to present at check-in counter. (See  Before You Fly FAQs).

International law prohibits the placing of magnetised, inflammable, corrosive, radioactive, oxidising, or explosive articles; pressurised gasses, toxic substances, oxygen cylinders, paint, mercury, munitions, and fireworks in checked or unchecked (carry-on) luggage. In many countries, strong-smelling materials, lighter fuel and refills, fresh food or fruit, infected live animals, or weapons must not be taken aboard the aircraft. Such items are subject to confiscation and the owner may be subject to a fine, depending on the country. If you are not certain if an item you are carrying is prohibited, please ask a THAI agent before you arrive at check-in.

THAI's Special Unit Officers are stationed at various points around the Departure Hall to aid passengers needing special assistance. Wheelchairs are available. THAI also provides escorts to the airplane for passengers in need of assistance such as, Unaccompanied Minors, young passengers, medical cases.

The Duty Free items you purchased can be picked up at the Duty Free Counter in the Departure lounge.