Rating, Tare Mass and Payload of Containers

Rating
Rating is the maximum gross mass (or weight), that is, the maximum permissible weight of a container plus its contents.
20' - 24000 kgs (52900 lbs)
40' - 30480 kgs (67200 lbs)

Tare Mass
Tare mass is the mass (or weight) of empty container.
20' - 1800 - 2400 kgs
40' - 2800 - 4000 kgs
40'HC - 3900 - 4200 kgs

Payload
Payload = Rating - Tare Mass
In exporting, it is common to encounter a payload of 17,500 kgs. or less in the 20' container, and 24,000 kgs. or less in the 40' container.
Max payload:
20' - 21727 kgs (47999 lbs)
40' - 26780 kgs (59040 lbs)
40'HC - 26512 kgs (58450 lbs)

Source: http://www.export911.com/e911/ship/dimen.htm#xDimension