How many feet of DNA are packed into a single human cell?

How to get close to this number

Reason from thinness rather than from length. DNA is a molecule a couple of nanometres wide, and the nucleus is a few micrometres across — so the ratio between the container and the strand is enormous, which means an enormous length can fit inside something invisible. Whenever something extremely thin is packed into something small, expect a total length that sounds impossible.

The mental block is that nobody has an intuition for how much can be coiled into a microscopic space. A useful comparison is thread on a spool: a bobbin the size of a thumb holds a length of cotton you would struggle to guess, and DNA is many orders of magnitude finer than cotton. The packing is not loose either — it is wound around proteins in a hierarchy of coils, which is what makes the fit possible.

The whole-body figure in the explanation multiplies this by the number of cells, and it is worth treating that as illustrative rather than precise. Cell counts for a human body are themselves estimates with wide error bars, and not every cell carries a full nucleus. The per-cell figure is solid; the astronomical total built on top of it is a way of conveying scale, and should be read as such.