Data storage capacity of teletype paper tape

Paper tape for pre-ASCII Teletype (re)perforators came on rolls 11/16" wide, with a 2" inner diameter and an 8" outer diameter. The tape is 0.1mm thick. This gives a length of around 940 feet. 5-bit wide characters are punched 10 per inch, so one roll will store 112800 characters.

However, these are 5 bit wide ITA-2 characters. If we want to store 8 bit bytes, we need to encode. To remain compatible with teletype equipment, we can't use base-32 encoding because while there are 32 possible values that can be punched, some are blanks, shifts, etc which wouldn't print correctly or would confuse the machine.

So instead we can base-16 encode, which allows us to use only characters that are printable uppercase letters, which produces much cleaner output on a reperforator that also prints the contents on the tape while punching it. So now we use two printed/punched characters to represent each 8-bit byte, and a full roll of tape holds 56400 bytes.

• torrented movie (4.5 GBytes): 15,250 miles of tape on 85670 rolls.
• linux kernel (2.3 MBytes): 15.5 miles of tape on 87 rolls.
• phonecam selfie (130 KBytes): 1767 feet of tape on 2 rolls.
• ssh private key (900 bytes): 15 feet of tape.

On the other hand, the paper tape will still be readable in a thousand years if nobody sets it on fire, so there's that.

1.2 megabytes of paper tape

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