Embodiments of this application provide a method and apparatus for determining a transport block size. The method includes: obtaining a target overhead value based on a target time-frequency resource and one or more of at least two overhead values, where the target time-frequency resource is used for uplink data transmission or downlink data transmission and includes one or more time-frequency resource units, and each overhead value is corresponding to at least one time-frequency resource unit; and determining a transport block size based on the target overhead value. It can be learned that, according to the method and apparatus for determining a transport block size provided in the embodiments of this application, a corresponding target overhead value may be determined for each of different target time-frequency resources. This is not only applicable to a scenario in which a dynamic time-frequency resource is scheduled, but also improves accuracy of determining the transport block size. Therefore, transmission performance is improved.