You can calculate video memory bandwidth by multiplying the memory speed by bus width. For example a R9 380 (256-bit memory bus) with a 1.5Ghz (6Gbps) memory clock has a bandwidth of 256/8 * 6 = 192GB/s peak memory bandwidth. Note that you must read at least 2 cache lines per page to achieve the full memory bandwidth. 64-byte random reads would be limited to half the peak bandwidth.
