Enrico Fermi was an Italian nuclear physicist in the first half
of the twentieth century.
He was well-known for being excellent
at scientific estimation, and gave his name to the "Fermi method".
The idea is to break an estimation problem up into a combination of
smaller sub-problems that we can estimate with more confidence
(either they connect with our general knowledge, or our experience,
or our common sense).
We make an order of magnitude estimate for each,
and hope that any errors we make will roughly cancel each other out
when the estimates are combined to give an estimate for the original
problem.
This method often produces surprisingly accurate estimates,
although may need some practice!
For a bit of practice, go here: Fermi-estimate annual passenger numbers at Leeds station .
Links:
Discussion.
Playing the Fermi game!
The piano tuner problem