Just what does a petabyte look like? The US Library of Congress is often invoked when imagining a petabyte and its kin – the (smaller) terabyte and the larger zettabyte and exabyte.

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Data experts at Deloitte, McKinsey, IBM, Gartner and mobile advertising firm Adfonic recently had their arms twisted to help Computer Weekly readers envisage a petabyte.

And so, Michael Chui, principal at McKinsey, says that the US Library of Congress “had collected 235 terabytes of data by April 2011 and a petabyte is more than four times that”.

TechTarget’s own WhatIs site offers a useful point of departure for thinking about how big a petabyte is: “A petabyte is a measure of memory or storage capacity and is 2 to the power of 50 bytes or, in decimals, approximately a thousand terabytes.”

And a terabyte?

“A terabyte (TB) is a measure of computer storage capacity that is 2 to the power of 40, or approximately a trillion bytes (that is, a thousand gigabytes).”

According to futurist Raymond Kurzweil, continues WhatIs, in The singularity is near, the capacity of a human being’s functional memory is estimated to be 1.25 terabytes. This means the memories of 800 human beings fit into one petabyte.

If this seems too speculative, Wes Biggs, chief technology officer at Adfonic, ventures the following more grounded measures:

If the average MP3 encoding for mobile is around 1MB per minute, and the average song lasts about four minutes, then a petabyte of songs would last over 2,000 years playing continuously.

If the average smartphone camera photo is 3MB in size and the average printed photo is 8.5 inches wide, then the assembled petabyte of photos placed side by side would be over 48,000 miles long – almost long enough to wrap around the equator twice.

One petabyte is enough to store the DNA of the entire population of the US – and then clone them, twice.