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I'm asking this question for a client and unfortunately have limited information.

Let's assume I have an HD video that I am transmitting online to someone else at 26.5 Mpbs (megabits per second). The client (for whatever reason) doesn't know how much space the movie occupies, he only knows that it is 100 minutes long.

How many GB (gigabytes) would that take up on a client's computer when legally downloaded/streamed?

With the calculations below I get 19.9 GB which seems REALLY high to me, I was expecting more like 2-3 GB. Did I do something wrong? Something I thought of (but have no experience with) is how much of that 26.5 Mbps is actually data from the movie? Some of it might be overhead etc?

// Calculations and notes

Using a bandwith tool at http://web.forret.com/tools/filesize.asp?speed=26%2C5&unit=Mbps&dur=6000 26.5 Mbps * 60 seconds * 100 min = 19.9 GB data

26.5 Mbps (that's megabits per second not megabytes per second) = 0.0033125 GBps (Gigabytes per second) 0.0033125 GBps * 60 seconds/min = 0.19875 GB/min 0.19875 GB/min * 100 min = 19.875 GB

Note that Mbps is in metric form (1Mb = 1,000,000 bits) but Gigabyte is in binary form (1 MB = 2^20 = 1,048,576 bytes and 1 GB = 2^30 = 1073741824 bytes) and that there are 8 bits in one byte.

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This seems right to me. 26.5mbit/sec. ~~ 1590mbit/min ~~ 159000mbit/100min ~~ 19.875Mbyte/100 min. This is assuming that the download speed stays constant. –  Hennes Apr 11 '13 at 19:54
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you are totally ignoring the type of video coming thru (encoding-wise) and any compression to this stream done by the networking gear. 26.5 Mbps doesn't necessarily mean adding 26.5 M-bits of data appended to the end of the stored file every second. As you said, there is not enough data to answer this question assuredly. Your approximation is as good as you can get, but if I were you, i wouldn't write my books on that. –  Mel_Burslan Apr 11 '13 at 19:56
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2 Answers

up vote 1 down vote accepted

There's no way to answer without knowing what format the client stores the data in. Your calculation is correct that you are sending 19.875GB, but that doesn't mean the client necessarily stores all of that.

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thanks, that's what I was wondering. –  Anthony Elliott Apr 11 '13 at 20:11
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Wikipedia has a table that lists streaming bit rates of common video resolutions:

http://en.wikipedia.org/wiki/High-definition_video#HD_on_the_World_Wide_Web.2FHD_Streaming

It seems to me that this indicates that your 26.5 Mbps is not the actual bit-rate of the movie transmitted. After all, most streaming systems do not stream at the bit rate of the film, but cache an amount beyond the actual frames being played at that instant. Also, streaming video formats use compression. So the 26.5 Mbps seems to me to indicate the uncompressed HD stream, which almost nobody actually uses except for archival purposes. (See here: http://stackoverflow.com/questions/701991/h-264-file-size-for-1-hr-of-hd-video )

Format and other things will also affect the size of the file corresponding to a given bit rate, though nowadays, file sizes are relatively similar, moreso the larger the file is.

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