Convert Blu-ray disc (double layer) to petabyte

The SI prefixes count in thousands: 1 kB = 1,000 bytes, 1 GB = 10⁹ bytes. Computers count in powers of two, and for those the IEC prefixes exist: 1 KiB = 1,024 bytes, 1 GiB = 2³⁰ = 1,073,741,824 bytes, about 7 % more. Disk and memory-card makers use the decimal units, most operating systems display the binary ones (Windows labels them 'GB' regardless), which is why a '1 TB' drive shows as 931 GB: 10¹² bytes is 931 GiB. Nothing is missing. RAM is the exception and is always sold in binary sizes: 16 GB of RAM really is 16 GiB.

A CD holds 650 MB of data (74 minutes of audio) or 700 MB (80 minutes). DVDs and Blu-ray discs are sold by nominal capacity: 4.7 GB for a single-layer DVD-5, 8.5 GB for the dual-layer DVD-9, double that for the double-sided DVD-10 and DVD-18, and 25 GB per layer for Blu-ray, so 50 GB for a dual-layer disc. As with hard drives, packaging counts in decimal gigabytes while operating systems display binary ones, so a disc always looks smaller on screen than on the box.