A ‘back of the napkin’ estimate that can be quite accurate. By estimating multiple variables somewhat accurately and both underestimating and overestimating, the variables tend to balance out and result in accuracy within an order of magnitude. This is usually enough to get a general idea of the problem and solution which is useful, similar to dimensional analysis like Big O Notation.

See also:

- A famous Fermi estimation is of the Drake equation known as the Fermi paradox
- An estimation aids in understanding similar to projections onto different dimensions