For instance, a computer’s CPU is measured in GHz, which is a proxy for the number of calculations the CPU can run per second. So it is about one billion (109) calculations per second. Now let’s suppose the number of calculations your program needs to run is 106, then you can make a Fermi estimation about the program’s run time as 106109=10−3, which is millisecond. Usually we would expect the actual run time will be within an order of magnitude of this estimation.
For instance, a computer’s CPU is measured in GHz, which is a proxy for the number of calculations the CPU can run per second. So it is about one billion (109) calculations per second. Now let’s suppose the number of calculations your program needs to run is 106, then you can make a Fermi estimation about the program’s run time as 106109=10−3, which is millisecond. Usually we would expect the actual run time will be within an order of magnitude of this estimation.