A disclosure of this specification provides a method performed by a wireless device for determining a transport block size (TBS). The method may include the steps of determining a number of resource elements (REs) within a slot, calculating a value related to the TBS, based on the determined number of REs and a code rate, comparing the calculated value with a pre-determined threshold value, determining the TBS according to the comparison, wherein, if the calculated value is smaller than or equal to the pre-determined threshold value, the TBS may be determined using a predetermined table.