One petabyte is 1,000 terabytes decimally, so a terabyte is 0.001 PB. This is the scale at which individual drives stop being the unit of thought and arrays, data centres and archives take over.
About the units
The petabyte is 10¹⁵ bytes under the SI convention and 2⁵⁰ bytes — a pebibyte, 1,125,899,906,842,624 — under the binary one. The gap at this prefix has grown to 12.6 per cent, large enough that a storage contract that does not specify the convention is a genuine commercial risk. The prefix peta comes from the Greek for five, since it is the fifth power of a thousand. Above it sit the exabyte, zettabyte and yottabyte, all of which now appear in real-world figures for global network traffic rather than as theoretical curiosities.
The exact factor
The factor is exactly 1,000 decimal. Some scale anchors: a petabyte is roughly 250 million high-resolution photographs, or about 20 million four-drawer filing cabinets of text. A single modern hard drive holds tens of terabytes, so a petabyte is a rack of them. Global internet traffic passed the zettabyte mark per year in the late 2010s, and a zettabyte is a million petabytes.
Where you meet this conversion
Data centre capacity planning, scientific computing and media archives. Radio astronomy, genomics and particle physics all generate data at rates measured in petabytes per year, and their storage budgets are written in these units. Cloud providers price object storage per gigabyte-month while quoting total platform capacity in exabytes, so the same service description spans six orders of magnitude. Long-term archival planning uses petabytes because the unit matches the lifetime of a storage system rather than the size of a purchase.