Calculate Transport Block Size from MCS index, PRBs, and MIMO layers — based on 3GPP TS 38.214.
Transport Block Size (bits)
Modulation order (Qm)
Code rate R
N_RE (useful REs per PRB slot)
N_info (information bits before quantization)
TBS (bits)
TBS (bytes)
Approx. data rate at 1000 slots/s
Frequently Asked Questions
What is the 5G NR Transport Block Size (TBS)?
The Transport Block Size (TBS) is the number of information bits delivered in a single transmission time interval (TTI) on the physical downlink or uplink shared channel. It represents the actual user data payload, excluding coding overhead. In 5G NR, TBS is determined by the MCS (which sets modulation order and code rate), the number of allocated PRBs, MIMO layers, and OFDM symbols per slot.
How is TBS calculated in 5G NR?
TBS calculation in 5G NR follows 3GPP TS 38.214 Section 5.1.3.2. The process has three steps: (1) Calculate N_RE — the number of useful resource elements per PRB per slot, subtracting DMRS and overhead REs, capped at 156 per PRB; (2) Calculate N_info = N_RE × R × Qm × layers; (3) Quantize N_info to the nearest valid TBS. For N_info ≤ 3824 bits, a quantization to powers of 2 is used; for larger values, a separate rounding formula applies.
What is MCS in 5G NR?
MCS stands for Modulation and Coding Scheme. In 5G NR, the MCS index determines both the modulation order (Qm) and the target code rate (R). 3GPP defines three MCS tables: Table 1 supports up to 64QAM, Table 2 supports up to 256QAM for higher throughput in good channel conditions, and Table 3 is a low spectral efficiency table for coverage-limited scenarios. The gNB selects the MCS based on the channel quality reported by the UE (CQI).