File size from bitrate
Bitrate is quoted in bits per second and file size in bytes, and the factor of eight between them is the single most common slip in this calculation. An 8,000 kbps stream is 1,000 kilobytes a second, not 8,000 — so 45 minutes is about 2.6 GB rather than 21. The same confusion is why a "100 megabit" connection downloads at about 12 megabytes a second.
File size is bitrate times duration divided by eight. Forty-five minutes at 8,000 kbps is about 2,575 MB — 2.5 GB. Dividing by eight converts bits to bytes, which is where most errors in this calculation come from.
How to work out file size
There is a second, smaller ambiguity underneath the bits-and-bytes one: whether a megabyte is 1,000,000 bytes or 1,048,576. Storage manufacturers use the decimal figure, operating systems have historically used the binary one, and the gap is about 5% at the megabyte level and 7% at the gigabyte. This calculator uses binary megabytes, which matches what a file manager will report — so a file that shows as 2.5 GB on disk is about 2.68 decimal gigabytes on a drive label.
Questions
Multiply bitrate by duration in seconds, then divide by eight for bytes.