The answer is at the end of this article. Physical Resource Block (PRB) = 2*Resource Block = 12 Subcarriers x 14 OFDM in 1ms . RE can deliver 2, 4, 6, 8 bits in QPSK, 16QAM, 64QAM, 256QAM respectively. If we calculate for 64QAM as 6bits since 256QAM not widely spread (check Samsung S7 in T-Mobile).