A unit of cargo capacity equal to 100 cubic feet (about 2.832 cubic meters); see ton [3]. The symbol RT seems to be in wide use for this unit, but it is also used for the refrigeration ton (see ton [7]) and for the revenue ton (see below)
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}