The description belongs to selecting a transport block size (TBS) in a communication system. One aspect is directed to an apparatus for selecting a transport block size (TBS) for a transmitter of a communication system. The communication system supports variable transmission time interval (tti) lengths with respect to the number of symbols to be transmitted in a tti. the apparatus is configured to receive first information indicative of a length of tti, to receive second information indicative of a number of reference symbols included in tti, to decode from the first information the length of tti, and to decode from second information, the number of reference symbols included in tti. the apparatus is further configured to calculate, based on the length of tti and based on the number of reference symbols included in tti, a number of data symbols included in tti, calculate, based on an allocation bandwidth and with based on the number of data symbols included in tti, a number of data resource elements (res) in tti, and select tbs from a data structure based on the calculated number of data ress. Additional aspects of the description pertain to a base station, a eu, methods, computer programs, computer readable recording media and a system.