Decompose an unknown quantity into factors you can estimate or bound, and combine them into a useful order-of-magnitude answer.
Fermi's students estimated the number of piano tuners in Chicago from population, households, pianos per household and tunings per year. Combining several rough estimates can land within an order of magnitude, particularly where the errors are not all biased in the same direction.
In practice
- A team asked how many support tickets a new market will generate estimates from population, adoption rate and contact rate, and lands within a factor of two.
- A planner sanity-checks a £40m forecast by asking how many customers that implies per day, and finds it exceeds the town's population.